Daily Archives: August 2, 2006

Factory Design Pattern to the Rescue: Practical Example

Design patterns really are quite useful. I have a situation in the code I’m working on where I was obviously repeating a lot of the same patterns and code (functions that were 90% the same–the only thing different was the specific class being instantianted): perfect candidate for factory techniques.

Let’s say we have the following set of classes representing a data access layer meant to abstract some database information from the client code. We have a BaseDBObject class that defines all of the common. We derive from that for each table we want to access.

class BaseDBObject
{
    protected BaseDBObject(Database database) {...}
    public void SetProperty(string name, object value) {...}
    //...more common functionality

};

Derived from this base are lots of classes that implement table-specific database objects. To control object creation, constructors are declared protected and static member functions are used. To wit:

class MyTableObject : BaseDBObject
{
    protected MyTableObject(Database database) : base(database) { }
    public static void Create(Database database, int param1, string param2)
    {
        string query = "INSERTO INTO MyTable (param1, param2) VALUES (@PARAM1, @PARAM2)";
        SqlCommand cmd = new SqlCommand(query, database.GetConnection());
        //paramterize query
        try {
            //exeute query
            //error check
            MyTableObject mto = new MyTableObject();
            //set object properties to match what's inserted
            return mto;
        }
        catch (SqlException ex)
        {
            //handle exception
        }
        finally
        {
            //close connection
        }
    }
    //...
    public static IList<MyTableObject> LookupById(Database database, int id)
    {
        string query = "SELECT * FROM MyTable WHERE ID = @ID";
        SqlCommand cmd = new SqlCommand(query, database.GetConnection());
        //parameterize query
        try
        {
            //execute query
            SqlDataReader reader = cmd.ExecuteReader(...);
            List<MyTableObject> list = new List<MyTableObject>();
            while (reader.Read())
            {
                MyTableObject mto = new MyTableObject();
                //set properties in mto
                list.Add(mto);
               
            }
            return mto;
        }
        catch (SqlException ex)
        {
            //handle exceptions
        }
        finally
        {
            //close connections
        }
    }
};

There are two functions here that must be created for every single table object derivation. That can be a LOT of code, and most of it is doing the same thing. There are a number of simple ways to handle some of the repetition:

  1. There will be multiple LookupByXXXXX functions. They can all specify the query they will use and pass it to a common function that executes and returns a list the class’s objects.
  2. Paramterizing queries can be accomplished by a function that takes a query string, a list of parameters (say, in a struct that describes each parameter), and produces a paramterized SqlCommand, ready for execution.
  3. Other helper functions that do the actual execution and checking of errors.

In the end, however, you are still left with two things that can’t be relegated to helper functions: MyTableObject mto = new MyTableObject(); and List<MyTableObject> list = new List<MyTableObject>(); One possible solution is to use reflection to dynamically generate the required objects. From a performance and understandability perspective, I don’t think this is a first choice.

Which leaves a factory method. My first attempt involved using templates to simplify this (you will see why shortly). Something like this:

class DatabaseObjectFactory<T> where T : BaseDBObject, new()
{
    public T Create(Database database) { return new T(database); } 
    public IList<T> CreateList() { return new List<T>(); }
};

This way, I could simply define a function in the base class BaseDBObject, which I could call like this:

Lookup(database, query, arguments, new DatabaseObjectFactory<MyTableObject>());

and that would automagically return a list of the correct objects. The problem with this approach, however, lies in the Create function. .Net can’t pass arguments to a constructor of T. It can only return new T() with no parameters. Nor can you access properties of BaseDBObject through T after creation. Back to the drawing board…

Now I had to face the problem of creating a duplicate inheritance hierarchy of object factories. This is what I had hoped to avoid by using generics. I designed an interface like this:

interface IDatabaseObjectFactory
{
    BaseDBObject Create(Database database);
    IList<BaseDBObject> CreateList();
};

And in each table object derivation I declare a private class and static member like this:

private class MyTableObject : IDatabaseObjectFactory
{
    public BaseDBObject Create(Database database) { return new MyTableObject(database); }
    public IList<BaseDBObject> CreateList() { return new List<MyTableObject>(); }
};
private static IDatabaseObjectFactory s_factory = new MyTableObjectFactory();

Now, I can have a Lookup function in BaseDBObject that accepts an IDatabaseObjectFactory parameter. At the expense of creating a small, simple factory class for each table object that needs it, I can remove roughly 50 lines of code from each of those classes. Smaller code = fewer bugs and easier maintenance.

The base lookup function would look something like this:

protected Lookup(Database database, string query, ICollection<QueryArgument>, IDatabaseObjectFactory factory)
{
    //paramterize query
    //execute query
    //ignoring error-handling for sake of brevity
    SqlDataReader reader = cmd.ExecuteReader(...);
    IList<BaseDBObject> list = factory.CreateList();
    while (reader.Read())
    {
        BaseDBObject obj = factory.Create(database);
        obj.Fill(reader);    //another common function that
                             // automatically fills in all properties
                             //of object from SqlDataReader
        list.Add(obj);
    }
    return list;
}

But what about theMyTableObject.Create()? It’s possible to do something like this, but in a different way. In order to handle inserting rows in a table that uses identity fields (that you don’t know until after creation), I created a utility function that inserted the data using database, query string, and QueryArgument objects. Then, instead of creating the object directly, I do a Lookup based on values that I know are unique to the row I just inserted. This ensures I get the most complete object (at the expense of an extra database trip).


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