Philosophical Geek » WPF

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.

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.

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