Polymorphism Through Extension Methods?

Question

I have a class library which contain some base classes and others that are derived from them. In this class library, I'm taking advantage of polymorphism to do what I want it to do. Now in a consuming application, I want to change the behavior of some code based on the runtime type of the child classes. So assume the following:

public class Base { }
public class Child1 : Base { }
public class Child2 : Base { }

Now in the consuming application I want do something as follows (note that all of the following classes are in the consuming application and cannot be referenced in the class library):

public interface IMyInterface1 { }
public interface IMyInterface2 { }
public static class Extensions
{
    public static void DoSomething(this Base myObj, Object dependency)
    {

    }

    public static void DoSomething(this Child1 myObj, Object dependency)
    {
        IMyInterface1 myInterface = dependency as IMyInterface1;
        if (myInterface != null)
        {
            //Do some Child1 specific logic here
        }
    }

    public static void DoSomething(this Child2 myObj, Object dependency)
    {
        IMyInterface2 myInterface = dependency as IMyInterface2;
        if (myInterface != null)
        {
            //Do some Child2 specific logic here
        }
    }
}

UPDATE:

This does not work. It always calls the extension method of the base class. Is there some other way that will allow me to do this and avoid having to explicitly check for the runtime type? The reasons is because more classes that are derived from the Base could be added and corresponding extension methods could come from some other external assembly.

Thanks in advance.

Answer 1

If you can not use the keyword "dynamic" (older version of .NET), you can use reflection to achieve the same thing.

In place of :

Base base1 = new Child1();
Extensions.DoSomething(base1 as dynamic);

you can write :

Base base1 = new Child1();

MethodInfo method = typeof(Extensions).GetMethod("DoSomething", new System.Type[] { base1.GetType() });
if (method) {
    method.Invoke(new object[] { base1 });
}

Answer 2

Below is the minimal example showing how to mimic polymorphism with extension methods.

void Main()
{
    var elements = new Base[]{
        new Base(){  Name = "Base instance"},
        new D1(){    Name = "D1 instance"},
        new D2(){    Name = "D2 instance"},
        new D3(){    Name = "D3 instance"}

    };

    foreach(Base x in elements){
        x.Process();
    }
}

public class Base{
    public string Name;
}
public class D1 : Base {}
public class D2 : Base {}
public class D3 : Base {}


public static class Exts{

    public static void Process(this Base obj){
        if(obj.GetType() == typeof(Base)) Process<Base>(obj); //prevent infinite recursion for Base instances
        else Process((dynamic) obj);
    }

    private static void Process<T>(this T obj) where T: Base
    {
        Console.WriteLine("Base/Default: {0}", obj.Name);
    }

    public static void Process(this D1 obj){
        Console.WriteLine("D1: {0}", obj.Name);
    }

    public static void Process(this D2 obj){
        Console.WriteLine("D2: {0}", obj.Name);
    }
}

Outputs:

    Base/Default: Base instance
    D1: D1 instance
    D2: D2 instance
    Base/Default: D3 instance

Answer 3

As @SLaks has already stated you cannot call the method as an extension method (even with a dynamic type) ... you can however call the static method with a dynamic type

So, although this will fail

Base base1 = new Child1();
(base1 as dynamic).DoSomething();

This will work

Base base1 = new Child1();
Extensions.DoSomething(base1 as dynamic);

Answer 4

No, that won't work.

Extension methods are statically dispatched, using the same mechanism as overload resolution.

If you have a variable of compile-time type Base, the compiler will always call the base extension method, regardless of the runtime type.

Instead, you can make the base extension method check the runtime type and call the appropriate other extension method.

Answer 5

I was looking for the same thing just now.

You could add one more method to your extension class like this:

public static void DoSomething(this Base myObj, Object dependency)
{       
    if(myObj.IsSubclassOf(Base))
    {
      // A derived class, call appropriate extension method.  
      DoSomething(myObj as dynamic, dependency);
    }
    else
    {
        // The object is Base class so handle it.
    }
} 

You don't need the if/else check if the base class is abstract (or never used in the wild):

public static void DoSomething(this Base myObj, Object dependency)
{ 
    DoSomething(myObj as dynamic, dependency);
}

[Edit] Actually this won't work in your case as you don't implement support for all derived objects (so could still get infinite recursion). I guess you could pass something to check for recursion but the given answer is the simplest. I'll leave this here as it might spark more ideas.