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