Restrict interface to specific classes or restrict a property/declare to specific class+interface

Question

I have an Interface IDataControl:

public interface IDataControl
{
    ...
}

Then, I have a class with a variable of List<IDataControl>.

List<IDataControl> items;

The problem is when I try to add elements of this collection in a "Controls" property of another form or another control:

panel.Controls.Add(items[i]);

The IDE says me that "The best overloaded method matching......" because "IDataControl" is not inherited from "Control".

I understand the error, and I kwnow that I can do some casting like:

panel.Controls.Add(   (Control)items[i]   );

But, I want to know if there is a "cleaner" way to do this, without requiring a casting, having also strict type verifying.

So, is it possible to indicate that "IDataControl" only can be implemented by Control derived objects?

public interface IDataControl where this : Control
{
    ...
}

Or, is it possible to declare a variable of a class and a interface?
Something like this:

Control:IDataControl item;
List<Control:IDataControl> items;

Thanks

any success with your problem? – Artiom Aug 20 '12 at 06:40 — Artiom, Aug 20 '12 at 06:40

Artiom · Answer 1 · 2012-08-17T12:48:46.773

As you already understood System.Windows.Forms.Control does know nothing about your interface.

public interface IDataControl where this : Control
{
    ...
}

You can't do that. It is Applied for generics type constraints Also interfaces can not be inherited from class. An interface has the following properties:

An interface is like an abstract base class: any non-abstract type that implements the interface must implement all its members.
An interface cannot be instantiated directly.
Interfaces can contain events, indexers, methods, and properties.
Interfaces contain no implementation of methods.
Classes and structs can implement more than one interface.
An interface itself can inherit from multiple interfaces.

What you need is perhaps next: When you create you controls, inherit them from your interface.

In this host control you need to create a method

private void AddDataControl(IDataControl dataControl)
{
   if(IDataControl is Control){
      //your logic
   }
   else{
      throw new ArgumentException();
   }
}

It's just general approach. Depends on your code.

Restrict interface to specific classes or restrict a property/declare to specific class+interface

1 Answers1