Question

Can someone explain this behavior in Generics?

I have a generic function in C#

protected virtual void LoadFieldDataEditor <T> (ref T control, string strFieldName) where T : Control {   //T can be different types of controls inheriting from System.Web.UI.Control   if (control is TextBox)   {    //This line gives an error    //((TextBox)control).Text = 'test';     //This line works!     (control as TextBox).Text = 'Test';   } } 

On a side note, can I use switch case when I am doing a ‘Control is TextBox’ type of checking?

EDIT:

Forgot to add the error message Sorry!

Here you go:

Error   3   Cannot convert type 'T' to 'TextBox' 

EDIT:

While we are talking about generics, I have another question. (Wasn’t sure If I had to start a new post)

The method has been expanded to include another generic type

protected virtual void LoadFieldDataEditor <T1, T2> (T1 control, T2 objData, string strFieldName) where T1 : Control where T2 : BaseDataType {   //I will need to access field1.    //I don't know at compile time if this would be SomeType1 or   //SomeType2 but all of them inherit from BaseDataType.     //Is this possible using generics? }  public abstract class BaseDataType {}  public class SomeType1 : BaseDataType {    string field1;    string field2; } 

Answer 1

The rules for what a generic type can be converted to are quite tricky, and occasionally counterintuitive, as in this case. See section 6.2.6 of the C# spec for details. There are places where they could be laxer, and I think this is one of them. You can cast up to object and then down again, but that’s ugly.

In this case the better solution would be:

protected virtual void LoadFieldDataEditor <T> (ref T control,                                                 string strFieldName)      where T : Control {     TextBox textBox = control as TextBox;     if (textBox != null)     {         textBox.Text = 'test';     } } 

Aside from anything else, this only requires a single execution time check instead of two.

For the side note: no, you can’t use switch/case on types. (You could get the type name and switch on that, but it would be horrible.)