Question

Example:

I’d like to have several specialized textboxes that derive from either TextBox or RichTextBox, which both derive from TextBoxBase:

class CommonFeatures<T> : T where T : TextBoxBase
{
  // lots of features common to the TextBox and RichTextBox cases, like
    protected override void OnTextChanged(TextChangedEventArgs e) 
    { 
        //using TextBoxBase properties/methods like SelectAll();  
    }
}

and then

class SpecializedTB : CommonFeatures<TextBox>
{
    // using properties/methods specific to TextBox
    protected override void OnTextChanged(TextChangedEventArgs e) 
    { 
        ... base.OnTextChanged(e); 
    }
}

and

class SpecializedRTB : CommonFeatures<RichTextBox>
{
    // using methods/properties specific to RichTextBox
}

Unfortunately

class CommonFeatures<T> : T where T : TextBoxBase

doesn’t compile (“Cannot derive from ‘T’ because it is a type parameter”).

Is there a good solution to this? Thanks.

Answer 1

C# generics don’t support inheritance from a parameter type.

Do you really need CommonFeatures to derive from TextBoxBase?

A simple workaround may be to use aggregation instead of inheritance. So that you would have something like this:

public class CommonFeatures<T> where T : TextBoxBase
{
    private T innerTextBox;

    protected CommonFeatures<T>(T inner)
    {
        innerTextBox = inner;
        innerTextBox.TextChanged += OnTextChanged;
    }

    public T InnerTextBox { get { return innerTextBox; } }

    protected virtual void OnTextChanged(object sender, TextChangedEventArgs e) 
    { 
        ... do your stuff            
    }
}

Like @oxilumin says, extension methods may also be a great alternative if you don’t really need CommonFeatures to be a TextBoxBase.