Mixins with C# 4.0

You can create mixin-like constructs in C# 4.0 without using dynamic, with extension methods on interfaces and the ConditionalWeakTable class to store state. Take a look here for the idea.

Here’s an example:

public interface MNamed { 
  // required members go here
}
public static class MNamedCode {
  // provided methods go here, as extension methods to MNamed

  // to maintain state:
  private class State { 
    // public fields or properties for the desired state
    public string Name;
  }
  private static readonly ConditionalWeakTable<MNamed, State>
    _stateTable = new ConditionalWeakTable<MNamed, State>();

  // to access the state:
  public static string GetName(this MNamed self) {
    return _stateTable.GetOrCreateValue(self).Name;
  }
  public static void SetName(this MNamed self, string value) {
    _stateTable.GetOrCreateValue(self).Name = value;
  }
}

Use it like this:

class Order : MNamed { // you can list other mixins here...
  ...
}

...

var o = new Order();
o.SetName("My awesome order");

...

var name = o.GetName();

The problem of using an attribute is that you can’t flow generic parameters from the class to the mixin. You can do this with marker interfaces.