Philosophical Geek

A WPF Numeric Entry Control with Pop-up Calculator

Ben — Mon, 31 Jan 2011 18:00:00 +0000

Part 2 – This article

When WPF first shipped, there was a noticeable lack of certain controls we’ve become used to in Win32 and WinForms: Calendar, DateTimePicker, and NumericUpDown. WPF 4 adds Calendar and DatePicker, but not anything for numeric entry.

I described how to create a numeric entry control in a previous article. In this article, I will follow that up by adding a popup calculator to allow you to quickly manipulate the number.

If you find this article useful, you will love my book, C# 4.0 How-To, which is filled with tips such as this.

Adding a Popup Calculator

Now that we have a basic numeric entry control, let’s extend it by adding the power of a keyboard-driven calculator popup window.

If you’ve used Quicken or the now-sadly-defunct Microsoft Money, you’ve seen a feature that pops up a calculator window whenever you hit +, –, *, or / in a number field.

Here are some further requirements for such a control:

It should be activated when the user types +, –, *, or /
It should be dismissed if the user hits Escape, Enter, or clicks anywhere outside the calculator (which is considered equivalent to Enter, not escape)
Both mouse and keyboard input should be handled
This version only handles integers
It should handle Backspace key to erase previously-input character
It should be visually compact
It should allow the user to string computations together. A + B – C / D * E

Extend the Numeric Entry Control

We first need to come up with a way to trigger the calculator in our numeric entry control. This is going to simply be if the user hits any math-operation character, such as +, –, *, or /.

First, we’ll need to define the allowed operations and a way to associate keyboard keys with math operations:

public enum MathOperation

    None = 0,

    Add,

    Subtract,

    Multiply,

    Divide

};

Now, let’s subclass NumericEntryControl with a new class that adds some additional keyboard handling.

class NumericEntryWithCalcControl : NumericEntryControl

    static Dictionary _keyToOp =

        new Dictionary();

    static NumericEntryWithCalcControl()

        //associate keyboard keys with a math operation

        _keyToOp[Key.Add] = MathOperation.Add;

        _keyToOp[Key.Subtract] = MathOperation.Subtract;

        _keyToOp[Key.Multiply] = MathOperation.Multiply;

        _keyToOp[Key.Divide] = MathOperation.Divide;

#if DEBUG

        //easier keys on a laptop

        _keyToOp[Key.A] = MathOperation.Add;

        _keyToOp[Key.S] = MathOperation.Subtract;

        _keyToOp[Key.M] = MathOperation.Multiply;

        _keyToOp[Key.D] = MathOperation.Divide;

#endif

With this bookkeeping down, we can intercept the keystrokes and show our calculator window (which we’ll define later) in the right spot. We also should handle when the window closes.

private CalcWindow _calc = null;

protected override void OnKeyDown(KeyEventArgs e)

    MathOperation op = MathOperation.None;

    if (_keyToOp.TryGetValue(e.Key, out op))

        e.Handled = true;

        HandleOperation(op);

    else

        e.Handled = false;

        base.OnKeyDown(e);

private void HandleOperation(MathOperation mathOperation)

    //show the calc dialog and initialize with Value and Op

    if (mathOperation != MathOperation.None)

        _calc = new CalcWindow(Value, mathOperation);

        _calc.Closed += new EventHandler(calc_Closed);

        Point point = this.PointToScreen(new Point(0, 0));

        //adjust location to correct for DPI settings

        PresentationSource source = PresentationSource.FromVisual(this);

        double dpiX = 96.0 * source.CompositionTarget.TransformToDevice.M11;

        double dpiY = 96.0 * source.CompositionTarget.TransformToDevice.M22;

        _calc.Left = point.X * 96.0 / dpiX;

        _calc.Top = point.Y * 96.0 / dpiY;

        _calc.Show();

private void calc_Closed(object sender, EventArgs e)

    if (_calc != null)

        this.Value = _calc.CurrentValue;

        _calc.Closed -= new EventHandler(calc_Closed);

        _calc = null;

Notice in ProcessOperation that the location of the calculator window is set to the upper left corner of the NumericEntryControl and it takes into account the current DPI. This is important if you’re using anything other than the standard 96 DPI.

Create the Calculator

The actual calculator is a WPF window with no border (or a thin one), a textbox, and buttons representing a standard calculator layout.

The screenshot to the right shows the design layout of this window. The C button clears the input and sets the current value to 0. The left arrow erases the last character you input.

Since this window is designed to popup over other controls, I wanted to keep it as compact as possible.

The XAML code looks like this:

"NumericEntryDemo.CalcWindow"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        xmlns:my="clr-namespace:NumericEntryDemo"

        Title="CalcWindow" WindowStyle="None"

        SizeToContent="WidthAndHeight"

        WindowStartupLocation="Manual"

        ResizeMode="NoResize"

        BorderThickness="1"

        "_textbox"

                 DockPanel.Dock="Top"

                 Margin="2"

                 IsReadOnly="True"

                 Text="{Binding RelativeSource={RelativeSource FindAncestor,

                    AncestorType=my:CalcWindow, AncestorLevel=1}, Path=CurrentValue}"

                 TextAlignment="Right" />

        "4" Rows="5" Width="100" Height="100" Margin="2">

        "0.5,1" StartPoint="0.5,0">

            "White" Offset="0" />

            "#FFDBDBDB" Offset="1" />

All of the click handlers and binding targets will be filled out in the code-behind.

To start with, we need a few housekeeping variables:

The original value upon starting the calculator, just in case after all the calculations the user just hits Escape
A way to track whether we’ve canceled the whole thing
The current operation
The current input, in both string and number form
The previously calculated value, allowing you to string together calculations like “123 + 456 / 2 * 29”
Map buttons and keystrokes to numbers, math operations, and the other commands.

With that, we can see the beginning of our class:

public partial class CalcWindow : Window

    private int _initialValue = 0;

    private bool _canceled = false;

    private MathOperation _currentOp = MathOperation.None;

    //value of current input

    public static readonly DependencyProperty CurrentValueProperty =

        DependencyProperty.Register("CurrentValue", typeof(int),

        typeof(CalcWindow), new PropertyMetadata(0));

    //current input in string form

    public static readonly DependencyProperty CurrentInputProperty =

        DependencyProperty.Register("CurrentInput", typeof(string),

        typeof(CalcWindow), new PropertyMetadata(""));

    //previously calculated value

    public static readonly DependencyProperty PreviousValueProperty =

        DependencyProperty.Register("PreviousValue", typeof(int),

        typeof(CalcWindow), new PropertyMetadata(0));

    private static readonly Dictionaryint> _keyDigits =

        new Dictionaryint>();

    private static readonly Dictionary _keyOps =

        new Dictionary();

    private readonly Dictionaryint> _buttonDigits =

        new Dictionaryint>();

    private readonly Dictionary _buttonOps =

        new Dictionary();

    static CalcWindow()

        _keyDigits[Key.D0] = 0;

        _keyDigits[Key.D1] = 1;

        _keyDigits[Key.D2] = 2;

        _keyDigits[Key.D3] = 3;

        _keyDigits[Key.D4] = 4;

        _keyDigits[Key.D5] = 5;

        _keyDigits[Key.D6] = 6;

        _keyDigits[Key.D7] = 7;

        _keyDigits[Key.D8] = 8;

        _keyDigits[Key.D9] = 9;

        _keyDigits[Key.NumPad0] = 0;

        _keyDigits[Key.NumPad1] = 1;

        _keyDigits[Key.NumPad2] = 2;

        _keyDigits[Key.NumPad3] = 3;

        _keyDigits[Key.NumPad4] = 4;

        _keyDigits[Key.NumPad5] = 5;

        _keyDigits[Key.NumPad6] = 6;

        _keyDigits[Key.NumPad7] = 7;

        _keyDigits[Key.NumPad8] = 8;

        _keyDigits[Key.NumPad9] = 9;

        _keyOps[Key.Add] = MathOperation.Add;

        _keyOps[Key.Subtract] = MathOperation.Subtract;

        _keyOps[Key.Multiply] = MathOperation.Multiply;

        _keyOps[Key.Divide] = MathOperation.Divide;

#if DEBUG

        _keyOps[Key.A] = MathOperation.Add;

        _keyOps[Key.S] = MathOperation.Subtract;

        _keyOps[Key.M] = MathOperation.Multiply;

        _keyOps[Key.D] = MathOperation.Divide;

#endif

    public int CurrentValue

get

            return (Int32)GetValue(CurrentValueProperty);

set

            SetValue(CurrentValueProperty, value);

    public string CurrentInput

get

            return GetValue(CurrentInputProperty) as string;

set

            SetValue(CurrentInputProperty, value);

    public int PreviousValue

get

            return (Int32)GetValue(PreviousValueProperty);

set

            SetValue(PreviousValueProperty, value);

    public CalcWindow(int initialValue, MathOperation operation)

        _initialValue = initialValue;

        _currentOp = operation;

        CurrentValue = _initialValue;

        PreviousValue = CurrentValue;

        InitializeComponent();

        InitializeButtons();

    private void InitializeButtons()

        _buttonDigits[_button0] = 0;

        _buttonDigits[_button1] = 1;

        _buttonDigits[_button2] = 2;

        _buttonDigits[_button3] = 3;

        _buttonDigits[_button4] = 4;

        _buttonDigits[_button5] = 5;

        _buttonDigits[_button6] = 6;

        _buttonDigits[_button7] = 7;

        _buttonDigits[_button8] = 8;

        _buttonDigits[_button9] = 9;

        _buttonOps[_buttonAdd] = MathOperation.Add;

        _buttonOps[_buttonSubtract] = MathOperation.Subtract;

        _buttonOps[_buttonMultiply] = MathOperation.Multiply;

        _buttonOps[_buttonDivide] = MathOperation.Divide;

All that’s left is to the actual work of handling input. Since input can be from both keyboard and button clicks, let’s first define methods to handle the general operations, and we can hook up the input later.

Generic Input Handling

First, we’ll look at the generic input handlers. These are called by the mouse and keyboard handlers and they return true so they can notify the caller of whether they actually handled the operation.

The “C” button on the calculator will not only clear the current input, but set the current value to 0 (I’m not going to bother implementing a “CE” button that clears just the current input, but it would be easy to add).

private bool HandleClear()

   CurrentInput = "";

   CurrentValue = 0;

   return true;

When the user hits escape, you want to close the window, after setting the value back to the initial one.

private bool HandleEscape()

    _canceled = true;

    CurrentValue = _initialValue;

    Close();

    return true;

We need the _canceled flag because, as we’ll see below, when the window is deactivated, under normal circumstances we want to complete the current operation, but in the canceled case we want to do nothing.

When a user hits Enter or presses the Equals button, the current operation should be completed and the window closed. In this control, since we also want the current operation to be completed when the window is deactivated, hitting Equals will just cause the window to close and we’ll handle the calculation in the deactivation code.

private bool HandleEquals()

    //The final operation will be

    //handled when the window closes

    Close();

    return true;

This calculator provides a back button to erase the previously-entered digit. If the input becomes empty, then the current value is set to 0.

private bool HandleBack()

    if (!string.IsNullOrEmpty(CurrentInput))

        CurrentInput = CurrentInput.Substring(0, CurrentInput.Length - 1);

        //this is guaranteed to succeed because we validate when we add digits

        if (!string.IsNullOrEmpty(CurrentInput))

            CurrentValue = Int32.Parse(CurrentInput);

        else

            CurrentValue = 0;

    return true;

When accepting new input, our button and keyboard handlers will ensure that the user can only enter digits, but we also need to make sure we handle integer overflow. An easy way to do this is to treat the new digit as a character and append it to the current input and then reparse it.

private bool HandleDigit(int digit)

    string newInput = CurrentInput + digit.ToString();

    int temp = 0;

    //make sure the full input is a number

    if (Int32.TryParse(newInput, out temp))

        CurrentInput = newInput;

        CurrentValue = temp;

        return true;

    return false;

Finally, when the user selects an operation we need to do one of two things. If there is no current input, then we can just change the current operation to the new one. This lets you change your mind, say if you hit ‘+’, but really wanted ‘-’.

On the other hand, if the user has already entered a number, then you know you’re finishing up the current expression and the user wants to calculate the results and start a new expression. As an example, consider the expression A + B. A is the initial value of the NumericEntryControl, the + is what triggered the calculator to show up, and B is the number the user entered after the calculator was visible. When the next operation is input, A + B should be resolved, placed into A, the current operation set, and the user can now enter the next B, and so on.

private bool HandleOperation(MathOperation op)

    if (CurrentInput.Length > 0)

        ProcessCurrentOperation();

    _currentOp = op;

    return true;

With that, we’ve seen all of the generic input handling. Now let’s move on to the actual math.

Doing the Math

The ProcessCurrentOperation method is the heart of this control as it updates all the values according to the user’s input. Here too we need to consider what happens if the user’s calculations cause an integer overflow. To detect this we need to use a combination of the checked keyword and some exception handling. To simplify things, if this happens, the whole calculation is canceled and the window closed. We also handle divide by zero errors by just setting the new value to 0.

(The checked keyword tells .NET to issue exceptions when integer math overflows. The default behavior depends on compilation and environment settings, but the unchecked behavior is usually what happens).

private void ProcessCurrentOperation()

    if (string.IsNullOrEmpty(CurrentInput))

        return;

    int newValue = 0;

    checked

try

            switch (_currentOp)

                case MathOperation.Add:

                    newValue = PreviousValue + CurrentValue;

                    break;

                case MathOperation.Subtract:

                    newValue = PreviousValue - CurrentValue;

                    break;

                case MathOperation.Multiply:

                    newValue = PreviousValue * CurrentValue;

                    break;

                case MathOperation.Divide:

                    if (CurrentValue != 0)

                        newValue = PreviousValue / CurrentValue;

                    else

                        newValue = CurrentValue;

                    break;

                default:

                    Debug.Assert(false, "Invalid operation. Should never happen!");

                    break;

            CurrentInput = "";

            CurrentValue = newValue;

            PreviousValue = newValue;

        catch (OverflowException)

            //will cause the window to close, so we better cancel everything first

            _canceled = true;

            CurrentValue = _initialValue;

            MessageBox.Show("The result overflowed, calculation canceled.");

Deactivation

As we said before, deaction should cause the current operation to complete, unless the user has canceled.

protected override void OnDeactivated(EventArgs e)

    if (!_canceled)

        ProcessCurrentOperation();

        if (IsVisible)

            //window is still visible if we've

            //clicked away from the window

            //before finishing calculation

            Close();

    base.OnDeactivated(e);

The only thing that remains is hooking up mouse and keyboard input to our existing methods.

Mouse Handling

These are already referenced in the XAML, and all they do is forward to the generic handlers.

void opButton_Click(object sender, RoutedEventArgs e)

    MathOperation op = _buttonOps[sender as Button];

    e.Handled = HandleOperation(op);

void digitButton_Click(object sender, RoutedEventArgs e)

    int digit = _buttonDigits[sender as Button];

    e.Handled = HandleDigit(digit);

private void clearButton_click(object sender, RoutedEventArgs e)

    e.Handled = HandleClear();

private void backButton_Click(object sender, RoutedEventArgs e)

    e.Handled = HandleBack();

private void equalsButton_Click(object sender, RoutedEventArgs e)

    e.Handled = HandleEquals();

Keyboard Handling

To handle the keyboard, we need to preempt the normal message flow by looking at the preview messages (which flow from the top of the UI hierarchy down, rather than the normal messages which bubble from the bottom to the top). If we don’t do this, then when we hit keys like Enter, they go to the button with the current focus, when instead we want it to always cause the Equals functionality.

protected override void OnPreviewKeyDown(KeyEventArgs e)

    /*take over all keyboard input

     * this is important because if a button has focus

     * and we hit enter then the button will be clicked

     * */

    int digit = 0;

    MathOperation op = MathOperation.None;

    if (_keyDigits.TryGetValue(e.Key, out digit))

        e.Handled = HandleDigit(digit);

    else if (_keyOps.TryGetValue(e.Key, out op))

        e.Handled = HandleOperation(op);

    else

        switch (e.Key)

            case Key.Enter:

                e.Handled = HandleEquals();

                break;

            case Key.Escape:

                e.Handled = HandleEscape();

                break;

            case Key.Back:

                e.Handled = HandleBack();

                break;

            case Key.C:

                e.Handled = HandleClear();

                break;

            default:

                e.Handled = false;

                break;

    base.OnPreviewKeyDown(e);

There you have it, a working pop-up calculator, usable from any other control you can subclass. As is often the case, there are a lot more features you could add to this basic implementation:

Undo
Floating-point support
Button highlights with corresponding key presses
More advanced display, showing current operator, previous values, etc.
More robust handling of overflow

Download Source Code

If you find this article useful, you will love my book, C# 4.0 How-To, which is filled with tips such as this.

Some awesome dual-piano music

Ben — Sun, 30 Jan 2011 17:47:39 +0000

I came across this awesome piano duo via Great White Snark:

Awesome Star Wars rendition:

I also loves this rendition of Rondo all Turca

You can see a ton more about Anderson and Roe at their website.

New C# 4.0 How-To Review

Ben — Thu, 04 Nov 2010 17:28:06 +0000

Of course I think you should go get my book, but so do other people. Here’s a recent, good review of the book: C# 4.0 How-To by Ben Watson.

Some excerpts:

There were a couple things about this book that really compelled me. The format (this is a first How-To book by Sams for me so I assume it goes across the board) was very much like a focused blog. It was broken up into small posts about each topic. The other thing that compelled me was the amount of code samples.

That is definitely the style I was going for and I don’t know if anyone else has picked up on that yet.

Instead, this is a book that you set on your desk and put post-its and dog ears for key sections that you use and use and use before you put the pattern to memory.

I myself use the book in that manner whenever I need a refresh on how to accomplish something.

Go get it from Amazon or B&N or other great bookstores everywhere!

Birth of Emma Watson

Ben — Sat, 11 Sep 2010 02:20:57 +0000

On August 6, 2010, our first baby, Emma, was brought into this world. It’s over a month since then, but as you can imagine, I’ve been busy. She’s a beautiful, healthy, wonderful girl. Every day is a new adventure with her, as she learns something new, does something different, changes her patterns, and explores her world. Right now, she’s just fallen asleep with no almost no effort on our part for the first time.

Of course, I had to mark the occasion of her birth with a LEGO crib!

(more at flickr, click the image)

Another C# 4.0 How-To Book Give-away

Ben — Fri, 10 Sep 2010 21:18:34 +0000

In celebration of the beginning of the school year here in the USA, I’m going to give away a few more copies of my book C# 4.0 How-To .

If you’ve ever wanted a step-by-step guide with practicable code examples for hundreds of tasks in C#, .Net, and Windows, then this is the book for you. So far, I’m very pleased with the reviews its gotten (if you already have the book and haven’t left a review, why not? )

Just leave a comment on this post, and I’ll choose two people at random. You must be in the US or Canada.

Also, for twitter users, if you retweet a link to this post using the hashtag #cs4howto, I’ll include you in the drawing as well.

The more comments and #cs4howto re-tweets there are, the more books I’ll give away!

Measure Amount of Data to Serialize with a Null Stream

Ben — Mon, 26 Jul 2010 17:00:00 +0000

If you’ve got to serialize some data, especially in a binary format, it’s common to output the length of the data. This is useful for versioning, random access, knowing when you’re done reading the records, among other reasons.

Therefore, you need to know the size of the data you’re going to serialize. There are a few ways to do this:

Measure the position you’re at, write the data, measure the new position, subtract, and that’s your length.
If you want to write the length first (which is usually better), you can write a dummy value, such as 0, then writing the data, then backing up in the stream, and writing the real value.
If you can’t back up the stream (very possible in some situations, or undesirable in others), you can measure the amount of data before you write. However, now you have to maintain that code in addition to the actual serialization.
My solution presented here, avoids having to maintain separate code by writing the data to a null stream which does not write any data, but keeps track of how much data was “written.”

class NullStream : System.IO.Stream
{
    public override bool CanRead { get { return false; } }

    public override bool CanSeek { get { return false; } }

    public override bool CanWrite {get { return true; } }

    public override void Flush() { /*do nothing*/ }

    public override long Length { get { return Position; } }

    private long _position = 0;
    public override long Position
    {
        get
        {
            return _position;
        }
        set
        {
            _position = value;
        }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        throw new InvalidOperationException();
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        Position += count;
    }

    public override long Seek(long offset, System.IO.SeekOrigin origin)
    {
        throw new InvalidOperationException();
    }

    public override void SetLength(long value)
    {
        throw new InvalidOperationException();
    }
}

You can use it like this:

long GetDataSize()
{
    using (NullStream stream = new NullStream())
    {
        if (SaveData(stream))
        {
            return stream.Position;
        }
    }
    return 0;
}

There is a downside to something like this: you’re still essentially doing a lot of the work of serialization. Sure, you’re not writing out the bytes anywhere, but if, say, you need to encode a string as bytes before writing to the stream, that’s still going to happen.

Still, this technique made sense in my case, maybe it will work for you.

Get a Free, Autographed Copy of C# 4.0 How-To!

Ben — Mon, 10 May 2010 21:04:04 +0000

To celebrate how well C# 4.0 How-To is doing, I’m going to give away two free copies of the book!

Here’s how it’s going to work:

1. Leave a comment on this post describing a project you’d like to build with C# 4.

2. I’ll pick two people from those comments at random.

(Make sure you enter your e-mail address where asked—it won’t be published to the blog, but I need it to contact you.)

I’ll leave the comments open for a while and I’ll update this post with the closing date.

Feel free to share a link to this blog post, tweet it, etc. If I get a lot of responses I may give away more.

Thanks to all those have already bought it!

UPDATE 18 May: I am going to close comments on Saturday morning (22 May) and pick the winners then. Thanks for commenting!

UPDATE 22 May: Comments are closed.

Interview with Me

Ben — Fri, 19 Mar 2010 17:05:16 +0000

InformIT has just published an interview with me where they asked me a bunch of questions related to C# 4.0 How-To. We got into the multicore future, Internet versus books, why C# programmers need to know about UAC, and a lot more. Check it out!

Blog moving hosts this weekend

Ben — Fri, 12 Mar 2010 23:43:28 +0000

I looked at my site’s settings in Google’s webmaster center and it told me that my site was in the top 5% slowest sites on the Internet. Wow. That really sucks. I’m sorry, dear reader. In any case, this was the final straw, after a string of outages and other general unhappiness. So I’m moving on…

I hope to make this change with no downtime, but who knows who well that will work.

The Book is Made Real

Ben — Wed, 10 Mar 2010 06:24:56 +0000

These arrived in the mail today.

C# 4.0 How-To Available Now!

Ben — Mon, 08 Mar 2010 21:41:15 +0000

Well, it’s finally out! Amazon no longer lists the book as available for pre-sale, and it should be shipping to purchasers today or tomorrow. If you’re a B&N shopper, you can also order it there, or grab it in stores within a few days.

From the product description:

Real Solutions for C# 4.0 Programmers

Need fast, robust, efficient code solutions for Microsoft C# 4.0? This book delivers exactly what you’re looking for. You’ll find more than 200 solutions, best-practice techniques, and tested code samples for everything from classes to exceptions, networking to XML, LINQ to Silverlight. Completely up-to-date, this book fully reflects major language enhancements introduced with the new C# 4.0 and .NET 4.0. When time is of the essence, turn here first: Get answers you can trust and code you can use, right now!

Beginning with the language essentials and moving on to solving common problems using the .NET Framework, C# 4.0 How-To addresses a wide range of general programming problems and algorithms. Along the way is clear, concise coverage of a broad spectrum of C# techniques that will help developers of all levels become more proficient with C# and the most popular .NET tools.

Fast, Reliable, and Easy to Use!

Write more elegant, efficient, and reusable code
Take advantage of real-world tips and best-practices advice
Create more effective classes, interfaces, and types
Master powerful data handling techniques using collections, serialization, databases, and XML
Implement more effective user interfaces with both WPF and WinForms
Construct Web-based and media-rich applications with ASP.NET and Silverlight
Make the most of delegates, events, and anonymous methods
Leverage advanced C# features ranging from reflection to asynchronous programming
Harness the power of regular expressions
Interact effectively with Windows and underlying hardware
Master the best reusable patterns for designing complex programs

I’ll be doing a book giveaway at some point as well, once I get my own shipment. Stay tuned!

Get it from Amazon

Get it from Barnes and Noble

How to position windows correctly at 120 DPI in WPF

Ben — Sun, 22 Nov 2009 07:13:52 +0000

If you want to place a window at a specific place in WPF, it will work pretty much as you expect—unless your DPI is 120 (the default is 96). Here’s a sample that shows how to put it where you want.

In this case, I want to put a window just under another control, aligned to its left side.

CalcWindow calc = new CalcWindow();

Point point = this.PointToScreen(

    new Point(0, this.ActualHeight));

PresentationSource source =

    PresentationSource.FromVisual(control);

double dpiX =

    96.0 * source.CompositionTarget.TransformToDevice.M11;

double dpiY =

    96.0 * source.CompositionTarget.TransformToDevice.M22;

calc.Left = point.X * 96.0 / dpiX;

calc.Top = point.Y * 96.0 / dpiY;

calc.Show();

The crux of this is getting the current DPI. You could use P/Invoke to call native methods to get this, but the transformation matrix contains the same information as well.

A WPF Numeric Entry Control

Ben — Mon, 16 Nov 2009 17:00:00 +0000

For my solution I wanted something that behaved very similarly to the WinForms NumericUpdown control.

Some of the specifications:

Allows user to set Value, MaxValue, MinValue, Increment, and LargeIncrement.
Text directly entered is limited to numbers
Pasted text is not intercepted, but when the control has lost focus it will be validated and reset to the previous value if necessary
Two buttons, for increment and decrement
Holding down the buttons with the mouse causes the number to increment continuously
Up and down increment and decrement by Interval
Page Up and Page Down increment and decrement by LargeInterval
This version only supports integers

Creating the control

To begin, create a new WPF project and add a new User Control called NumericEntryControl. This will create a pair of .cs and .xaml files.

In the XAML file, change the root element to be a .


    x:Class="NumericEntryDemo.NumericEntryControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
    mc:Ignorable="d"
    xmlns:my="clr-namespace:NumericEntryDemo" Width="200" Height="26">

Before we add the controls, let’s add some properties to our user control to hold the values the controls will use. These are dependency properties in order to take advantage of all the WPF goodness like data binding and animation. Let’s also add standard .Net property wrappers.

public partial class NumericEntryControl : UserControl

    public static readonly DependencyProperty ValueProperty =

        DependencyProperty.Register("Value",

            typeof(Int32), typeof(NumericEntryControl),

            new PropertyMetadata(0));

    public static readonly DependencyProperty MaxValueProperty =

        DependencyProperty.Register("MaxValue",

            typeof(Int32), typeof(NumericEntryControl),

            new PropertyMetadata(100));

    public static readonly DependencyProperty MinValueProperty =

        DependencyProperty.Register("MinValue",

            typeof(Int32), typeof(NumericEntryControl),

            new PropertyMetadata(0));

    public static readonly DependencyProperty IncrementProperty =

        DependencyProperty.Register("Increment",

            typeof(Int32), typeof(NumericEntryControl),

            new PropertyMetadata(1));

    public static readonly DependencyProperty LargeIncrementProperty =

        DependencyProperty.Register("LargeIncrement",

            typeof(Int32), typeof(NumericEntryControl),

            new PropertyMetadata(5));

    public Int32 Value

get

            return (Int32)GetValue(ValueProperty);

set

            SetValue(ValueProperty, value);

    public Int32 MaxValue

get

            return (Int32)GetValue(MaxValueProperty);

set

            SetValue(MaxValueProperty, value);

    public Int32 MinValue

get

            return (Int32)GetValue(MinValueProperty);

set

            SetValue(MinValueProperty, value);

    public Int32 Increment

get

            return (Int32)GetValue(IncrementProperty);

set

            SetValue(IncrementProperty, value);

    public Int32 LargeIncrement

get

            return (Int32)GetValue(LargeIncrementProperty);

set

            SetValue(LargeIncrementProperty, value);

Creating an incrementing TextBox

Add a TextBox inside the and bind its text to the value we created in our control:

<DockPanel d:LayoutOverrides="Width">

    <TextBox

        x:Name="_textbox"

        Margin="2,0"

        Text="{Binding Value,

            Mode=TwoWay,

            RelativeSource={RelativeSource FindAncestor,

                AncestorLevel=1,

                AncestorType={x:Type my:NumericEntryControl}},

            UpdateSourceTrigger=PropertyChanged}"

        HorizontalAlignment="Stretch"

        HorizontalContentAlignment="Right"

        VerticalContentAlignment="Center" />

DockPanel>

This will create a TextBox that sizes itself with its parent (a feature I wanted, but is not strictly necessary) and its text will be bound to the Value in our UserControl.

Handling text input

It used to be that you pointed with a mouse and entered text with a keyboard. However, it is common now to enter text with a stylus, gestures, or some future method not invented yet. Thankfully, WPF supports generic text input handling so you don’t have to concern yourself with the specific hardware.

public NumericEntryControl()

   InitializeComponent();

   _textbox.PreviewTextInput +=

        new TextCompositionEventHandler(_textbox_PreviewTextInput);

void _textbox_PreviewTextInput(object sender,

                   TextCompositionEventArgs e)

    if (!IsNumericInput(e.Text))

        e.Handled = true;

        return;

private bool IsNumericInput(string text)

    foreach (char c in text)

        if (!char.IsDigit(c))

            return false;

    return true;

This prevents anything except numbers from being entered, whether via character recognition or keyboard. It does not, however, prevent the user from pasting non-numeric text into the box. We’ll handle that later.

Validating Text input

It’s problematic to validate and correct user input as they are entering it. For example, if you set the MaxValue to 100, then every time you enter 1000, it jumps to 100, it can be jarring. It’s a similar situation with text pasted into the control. What the NumericUpDown control does is handle these sort of situations when the control loses focus.

To prepare for this, when the control gains focus, we need to save the last valid value so we have something to restore to.

When the control loses focus, we need to first verify that it is a number and if so, clip it to the bounds of our MinValue and MaxValue. If anything fails, set it back to the previous value.

public partial class NumericEntryControl : UserControl

    private int _previousValue = 0;

    public NumericEntryControl()

        InitializeComponent();

        _textbox.PreviewTextInput +=

            new TextCompositionEventHandler(

                _textbox_PreviewTextInput);

        _textbox.GotFocus +=

            new RoutedEventHandler(_textbox_GotFocus);

        _textbox.LostFocus +=

            new RoutedEventHandler(_textbox_LostFocus);

    void _textbox_GotFocus(object sender, RoutedEventArgs e)

        _previousValue = Value;

    void _textbox_LostFocus(object sender, RoutedEventArgs e)

        int newValue = 0;

        if (Int32.TryParse(_textbox.Text, out newValue))

            if (newValue > MaxValue)

                newValue = MaxValue;

            else if (newValue < MinValue)

                newValue = MinValue;

        else

            newValue = _previousValue;

        _textbox.Text = newValue.ToString();

Handle arrow keys

Just because WPF can handle text input from a variety of sources in a hardware-agnostic way doesn’t mean we should ignore the particular strengths of the keyboard. Specifically, we should handle the up and down arrows.

public NumericEntryControl()

...

    _textbox.PreviewKeyDown +=

        new KeyEventHandler(_textbox_PreviewKeyDown);

void _textbox_PreviewKeyDown(object sender, KeyEventArgs e)

    switch (e.Key)

        case Key.Up:

            IncrementValue();

            break;

        case Key.Down:

            DecrementValue();

            break;

        case Key.PageUp:

            Value = Math.Min(Value + LargeIncrement, MaxValue);

            break;

        case Key.PageDown:

            Value = Math.Max(Value - LargeIncrement, MinValue);

            break;

        default:

            //do nothing

            break;

private void IncrementValue()

    Value = Math.Min(Value + Increment, MaxValue);

private void DecrementValue()

    Value = Math.Max(Value - Increment, MinValue);

IncrementValue() and DecrementValue() are pulled out as their own method because they’re used in the button-handling code as well (see below).

The code so far is a perfectly usable textbox that accepts only numbers and can be incremented using the keyboard. Typically, however, we also need to support the mouse, and for that we need buttons (unless you want to do something exotic like programs like Photoshop and Lightroom do, where text boxes have support for incrementing gestures—that’s another article).

A button you can hold down

Adding buttons to increment once per click is pretty easy, but we really want to be able to hold down the buttons and have the TextBox increment. Let’s start by adding the XAML for the buttons:

<UserControl

    x:Class="NumericEntryDemo.NumericEntryControl"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

    mc:Ignorable="d"

    xmlns:my="clr-namespace:NumericEntryDemo"

    Width="200" Height="26"

    <DockPanel d:LayoutOverrides="Width">

        <Button x:Name="buttonDecrement"

                DockPanel.Dock="Left"

                Content="-"

                Width="{Binding ActualHeight,

                    ElementName=buttonDecrement, Mode=Default}"

                Height="{Binding ActualHeight,

                    ElementName=_textbox, Mode=Default}"/>

        <Button x:Name="buttonIncrement"

                DockPanel.Dock="Right"

                Content="+"

                Width="{Binding ActualHeight,

                    ElementName=buttonDecrement, Mode=Default}"

                Height="{Binding ActualHeight,

                    ElementName=_textbox, Mode=Default}"/>

        <TextBox

            x:Name="_textbox"

            Margin="2,0"

            Text="{Binding Value,

                Mode=TwoWay,

                RelativeSource={RelativeSource FindAncestor,

                    AncestorLevel=1,

                    AncestorType={x:Type my:NumericEntryControl}},

                UpdateSourceTrigger=PropertyChanged}"

            HorizontalAlignment="Stretch"

            HorizontalContentAlignment="Right"

            VerticalContentAlignment="Center" />

    DockPanel>

UserControl>

Note that the button Width property is bound to its own ActualHeight property, and the Height property is bound to the TextBox’s ActualHeight. This has the effect of keeping the buttons square, the same height as the TextBox. It’s an effect I wanted, but you can easily dispose of it. With these buttons, our control finally takes shape:

How fast should we increment?

Before writing the code to do the incrementing as we hold the button down, it’s worth considering how fast the incrementing should occur. Ideally, we would want it to increment at about the same rate as if we were holding down the up key on the keyboard. The keyboard repeat rate is an operating system value that we can retrieve.

There are actually two values:

private static int _delayRate =

    System.Windows.SystemParameters.KeyboardDelay;

private static int _repeatSpeed =

    Math.Max(1, System.Windows.SystemParameters.KeyboardSpeed);

The delay rate is how long we should wait before starting the repetition, and is given in multiples of 250ms. Roughly speaking, humans can determine and control actions with lengths of time of about 200ms, so 250ms is a good value to start with. Any shorter and the repetition might start when it was not intended (say, if they just click the button instead of holding it down).

The keyboard speed is the number of times per second we should repeat—sort of. The value can be 0, so because of the way I use it below I want to sure it’s at least 1.

To allow us to hold the button down, we need to override the default mouse handling of a button which is to disable the standard LeftMouseButtonDown/Up messages. Instead, we need to handle the PreviewMouseLeftButtonDown message and its corresponding Up message.

When we handle the down message, we need to set a timer for the keyboard delay value. When we handle the timer’s tick, we need to increment (or decrement) and change the timer’s interval to the repeat speed. This repeat speed is calculated merely by dividing 1000ms (1s) by the rate per second. There may be better ways, but this gets pretty close to the rate experienced by the keyboard on my computer. Finally, when the mouse button is released we need to stop the timer. We also do a final increment, which will cover the case where the user clicks instead of holds.

We also need to capture the mouse in case the user moves it off the button—otherwise the timer will just keep incrementing forever.

Here’s the code:

private DispatcherTimer _timer =

    new DispatcherTimer();

private static int _delayRate =

    System.Windows.SystemParameters.KeyboardDelay;

private static int _repeatSpeed =

    Math.Max(1, System.Windows.SystemParameters.KeyboardSpeed);

private bool _isIncrementing = false;

public NumericEntryControl()

...

    buttonIncrement.PreviewMouseLeftButtonDown +=

        new MouseButtonEventHandler(

            buttonIncrement_PreviewMouseLeftButtonDown);

    buttonIncrement.PreviewMouseLeftButtonUp +=

        new MouseButtonEventHandler(

            buttonIncrement_PreviewMouseLeftButtonUp);

    buttonDecrement.PreviewMouseLeftButtonDown +=

        new MouseButtonEventHandler(

            buttonDecrement_PreviewMouseLeftButtonDown);

    buttonDecrement.PreviewMouseLeftButtonUp +=

        new MouseButtonEventHandler(

            buttonDecrement_PreviewMouseLeftButtonUp);

    _timer.Tick += new EventHandler(_timer_Tick);

void buttonIncrement_PreviewMouseLeftButtonDown(

    object sender, MouseButtonEventArgs e)

    buttonIncrement.CaptureMouse();

    _timer.Interval =

        TimeSpan.FromMilliseconds(_delayRate * 250);

    _timer.Start();

    _isIncrementing = true;

void buttonIncrement_PreviewMouseLeftButtonUp(

    object sender, MouseButtonEventArgs e)

    _timer.Stop();

    buttonIncrement.ReleaseMouseCapture();

    IncrementValue();

void buttonDecrement_PreviewMouseLeftButtonDown(

    object sender, MouseButtonEventArgs e)

    buttonDecrement.CaptureMouse();

    _timer.Interval =

        TimeSpan.FromMilliseconds(_delayRate * 250);

    _timer.Start();

    _isIncrementing = false;

void buttonDecrement_PreviewMouseLeftButtonUp(

    object sender, MouseButtonEventArgs e)

    _timer.Stop();

    buttonDecrement.ReleaseMouseCapture();

    DecrementValue();

void _timer_Tick(object sender, EventArgs e)

    if (_isIncrementing)

        IncrementValue();

    else

        DecrementValue();

    _timer.Interval =

        TimeSpan.FromMilliseconds(1000.0 / _repeatSpeed);

And voila! A NumericEntryControl that’s basic and easy-to-use for both keyboard and mouse.

Further improvements

This isn’t the last word in numeric entry controls, by any means. There are many ways to accomplish it, and this is one that worked well for me. There are a number of further enhancements you could do (and perhaps should do):

More validation
Ensure that MaxValue >= MinValue
Set focus to the TextBox when the control gains focus (maybe)
Define strokes to increment and decrement with a stylus
The user can change the keyboard repeat rate through Control Panel. This program could be modified to listen for updates to this value.

There’s also another, really cool feature that I will add to this in a future post.

Download full source.

Found this article helpful? Want a resource of hundreds of similar, how-to tips? I’ve written a book that covers topics like this and more in C# 4. Check out C# 4.0 How-To!

Review: The Code Book – the most entertaining book on cryptography you’ll ever read

Ben — Wed, 04 Nov 2009 03:30:22 +0000

I recently wandered into a thrift store and as is usual in these stores I headed to the book section.(I live by the maxim that you can never own too many books.) The only thing that really caught my eye was a hardcover edition of The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography by Simon Singh. A quick perusal indicated it would be worth the shelf space.

However, I severely underestimated how much I would enjoy this book. I’m about half way through and find it so interesting and compelling a read that I feel I have to write about it.

The thing that really strikes me about this book is how entertaining and readable it is. I am not a big crypto or security guru, so I came to this book largely ignorant of of the topics and history.

The book is a wonderful blend of how encryption works with the history that surrounded its development. For such a seemingly-complex topic, Singh seems to have hit the perfect blend of history and instruction. Starting with the rudimentary Caesarian ciphers (basically, letter substitution) he builds up the techniques in the millenia-old one-upsmanship game between those who encipher and those who attempt to decipher. He starts with frequency analysis and history progresses he builds upon the previous techniques in explaining ever-more sophisticated attacks.

Surrounding this technical (yet still easy to understand) instruction is a fairly thorough narration of the history where these ciphers played a crucial role, from ancient Egypt, Julius Caesar, Mary Queen of Scots’ plot against Queen Elizabeth I, to legends of buried treasure protected by unbreakable ciphers, to the formidable German Enigma machine–

And let me be clear—he will teach you exactly how Enigma worked! It’s not that complicated, once you understand the principles that came before it.

Similarly, he explains how first the Polish and then the British cracked the Enigma as it evolved from the 1930’s to the end of the war, the mistakes the Germans made, and the things that, for example, the German Navy did to increase the security of it so that it became essentially unbreakable without external help (i.e., finding a codebook).

He discusses the Navajo code talkers (there is more to it than simply speaking an obscure language!), public key security, and finally quantum computing.

If you enjoy computer security topics, history, or just like to geek out, this is a great book to read.

Coincidentally, Simon Singh is also at the center of an unfortunate controversy in the UK surrounding some (I believe justified) comments he made about the British Chiropractic Association’s practices of encouraging procedures that could be considered unsafe. Rather than defend the practices with facts, they have decided to sue him for libel (the UK is infamous for having draconian libel laws that strongly favor accusers). You can learn more at his website.

You can also read a thorough explanation of the events at the Bad Astronomy blog.

Bing LEGO mosaic

Ben — Sat, 24 Oct 2009 20:04:44 +0000

This is a simple project I’ve been working on during the weekends lately. It’s the Bing logo done in LEGO plates. I’ll be moving it into my office soon.

It took maybe 6 hours of work and is about 3.4ft (1m) wide.

Feel free to leave comments at flickr, or check it out at my other LEGO posting sites (there are a couple of other shots of it):

C# 4.0 How-To now available for pre-sale!

Ben — Fri, 16 Oct 2009 03:51:47 +0000

For the last year, aside from starting a great job with Bing, I’ve also been working on a book about C# 4.0 and the upcoming .Net framework. The news: it is finally available for presale! This book is not your typical C# reference. It’s designed to be an easy guide to how to accomplish specific tasks, using a problem/solution approach. Some examples:

How to use P/LINQ (new in .Net 4!)
Override Equals and implement IEquatable correctly
Enforce coding contracts (new in .Net 4!)
Convert numbers to strings in arbitrary bases
Various ways of rounding, including “snapping” to specific intervals.
Dynamic discovery of WCF services
Make your Silverlight 3 application run out-of-the-browser
Speed up array access
Easily split work among multiple processors
Localize WinForms, WPF, ASP.Net, and Silverlight apps

…and hundreds of other topics, covering everything from the basics of C# to WPF, ASP.Net, interaction with the operating system, common application patterns and more. I cover all the new stuff that’s in both the C# language and the .Net 4 framework classes, as well as existing functionality.

Each topic begins with a brief description of when/where/why you would need to use the technique, followed by a brief explanation and source code.

I often just want a reference I can quickly dive into to remind me of how something is done. This book is my attempt to put in writing what I find valuable, both when I was learning C# and now when I just need to locate a sample quickly.

C# 4.0 How-To at Amazon
C# 4.0 How-To at Barnes and Noble

Over the next few months I’ll talk more about what’s in the book, and hopefully get back into blogging more programming topics.

Lots, and lots of tiles

Ben — Wed, 07 Oct 2009 15:22:13 +0000

Over in my flickr account, I’ve posted a picture of part of the destruction of the LEGO shuttle. I just wanted to show the amount of tiles used in the project—it was a lot, but I didn’t count. The shuttle is sadly no more, but new projects are coming…

Lots, and lots of tiles

Ben — Wed, 07 Oct 2009 15:19:57 +0000

BrickCon 2009 this weekend—get your LEGO fix in Seattle!

Ben — Fri, 02 Oct 2009 12:00:48 +0000

If you’re in Seattle this weekend, BrickCon 2009 is going on, and I’ll be participating for the first time, displaying my Space Shuttle model.

The convention has more space than ever this year: 35,000 square feet.

The public hours are Saturday and Sunday, 11am-3pm. It’s only $7 per person (or max of $20 per family) and you don’t want to miss the amazing models that will be there.

More Photos

Ben — Sat, 15 Aug 2009 20:52:19 +0000

I finally logged back into my flickr account and uploaded some recent photos from an evening trip to Mercer Island. It was around sunset, the light was beautiful, and I had about two hours to myself to just shoot what I could.

This hand was part of many holding a chain along some docks. The sunset was truly beautiful, but it was difficult to find a decent vantage point., given the people at the beach and the many trees. For more photos, check out my flickr stream.