Is it possible to implement mixins in C#?

Question

I\'ve heard that it\'s possible with extension methods, but I can\'t quite figure it out myself. I\'d like to see a specific example if possible.

Thanks!

Answer 1

You could also augment the extension method approach to incorporate state, in a pattern not unlike WPF's attached properties.

Here is an example with minimum boilerplate. Note that no modification are required on the target classes, including adding interfaces, unless you need to deal with the target class polymorphically - in which case you end up with something very close to actual Multiple Inheritance.

// Mixin class: mixin infrastructure and mixin component definitions
public static class Mixin
{ 
    // =====================================
    // ComponentFoo: Sample mixin component
    // =====================================

    //  ComponentFooState: ComponentFoo contents
    class ComponentFooState
    {
        public ComponentFooState() {
            // initialize as you like
            this.Name = "default name";
        }

        public string Name { get; set; }
    }

    // ComponentFoo methods

    // if you like, replace T with some interface 
    // implemented by your target class(es)

    public static void 
    SetName(this T obj, string name) {
        var state = GetState(component_foo_states, obj);

        // do something with "obj" and "state"
        // for example: 

        state.Name = name + " the " + obj.GetType();


    }
    public static string
    GetName(this T obj) {
        var state = GetState(component_foo_states, obj);

        return state.Name; 
    }

    // =====================================
    // boilerplate
    // =====================================

    //  instances of ComponentFoo's state container class,
    //  indexed by target object
    static readonly Dictionary
    component_foo_states = new Dictionary();

    // get a target class object's associated state
    // note lazy instantiation
    static TState
    GetState(Dictionary dict, object obj) 
    where TState : new() {
        TState ret;
        if(!dict.TryGet(obj, out ret))
            dict[obj] = ret = new TState();

        return ret;
    }

}

Usage:

var some_obj = new SomeClass();
some_obj.SetName("Johny");
Console.WriteLine(some_obj.GetName()); // "Johny the SomeClass"

Note that it also works with null instances, since extension methods naturally do.

You might also consider using a WeakDictionary implementation to avoid memory leaks caused by the collection's holding on to target class references as keys.