Question

Bellow is simplified version of the code I have:

public interface IControl<T>
{
    T Value { get; }
}

public class BoolControl : IControl<bool>
{
    public bool Value
    {
        get { return true; }
    }
}

public class StringControl : IControl<string>
{
    public string Value
    {
        get { return ""; }
    }
}
public class ControlFactory
{
    public IControl GetControl(string controlType)
    {
        switch (controlType)
        {
            case "Bool":
                return new BoolControl();
            case "String":
                return new StringControl();
        }
        return null;
    }
}

The problem is in GetControl method of ControlFactory class. Because it returns IControl and I have only IControl<T> that is a generic interface. I cannot provide T because in Bool case it’s going to bool and in String case it’s going to be string.

Any idea what I need to do to make it work?

Answer 1

Just derive IControl<T> from IControl.

public interface IControl<T> : IControl
{
    T Value { get; }
}

UPDATE

If I missunterstood you, and you don’t want a non-generic interface, you will have to make the method GetControl() generic, too.

public IControl<T> GetControl<T>()
{
    if (typeof(T) == typeof(Boolean))
    {
        return new BoolControl(); // Will not compile.
    }
    else if (typeof(T) == typeof(String))
    {
        return new StringControl(); // Will not compile.
    }
    else
    {
        return null;
    }
}

Now you have the problem that the new controls cannot be implicitly casted to IControl<T> and you would have to make this explicit.

public IControl<T> GetControl<T>()
{
    if (typeof(T) == typeof(Boolean))
    {
        return new (IControl<T>)BoolControl();
    }
    else if (typeof(T) == typeof(String))
    {
        return (IControl<T>)new StringControl();
    }
    else
    {
        return null;
    }
}

UPDATE

Changed the cast from as IControl<T> to (IControl<T>). This is prefered because it will cause an exception if there is a bug while as IControl<T> silently returns null.