How to declare a variable/param of type "Control implementing interface ISomething"?

Question

I wish to declare a variable in such a way as it can be assigned only values which derive from Control and also implement the ISomething interface.

I intend to add the ISomething interface to derivatives of controls.

I would like to derive SpecialTextBox and SpecialDatePicker From TextBox and DatePicker and implement the ISomething interface on each.

I would like to be able to assign each of these controls to a variable whose type is "Control which also implements ISomething" so that from there they could either have their ISomething methods invoked or could be added to a control collection on a form.

So.... How do I declare a variable of Type "Control which also implements ISomething"?

Ideally the answer to be in VB.Net but I would be interested in a C# method also.

Answer 1

One way to do this is with generics - i.e. a generic method:

void Foo<T>(T control) where T : Control, ISomething
{
    // use "control"
    // (you have access to all the Control and ISomething members)
}

Now you can call Foo only with other variables that are a Control that implements ISomething - you don't need to specify the generic, though:

Foo(someName);

is all you need. If you've given it something that isn't both a Control and ISomething, the compiler will tell you.

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Update: I don't "do" VB, but reflector tells me that the above translates as:

Private Sub Foo(Of T As { Control, ISomething })(ByVal control As T)

End Sub