C# – Virtual Extension Methods


I have a class that gets used in a client application and in a server application.
In the server application, I add some functionality to the class trough extension methods. Works great. Now I want a bit more:

My class (B) inherits from another class (A).

I'd like to attach a virtual function to A (let's say Execute() ), and then implement that function in B. But only in the server. The Execute() method would need to do stuff that is only possible to do on the server, using types that only the server knows about.

There are many types that inherit from A just like B does, and I'd like to implement Execute() for each of them.

I was hoping I could add a virtual extension method to A, but that idea doesn't seem to fly. I'm looking for the most elegant way to solve this problem, with or without extension methods.

Best Solution

No, there aren't such things as virtual extension methods. You could use overloading, but that doesn't support polymorphism. It sounds like you might want to look at something like dependency injection (etc) to have different code (dependencies) added in different environments - and use it in regular virtual methods:

class B {
     public B(ISomeUtility util) {
         // store util
     public override void Execute() {
         if(util != null) util.Foo();

Then use a DI framework to provide a server-specific ISomeUtility implementation to B at runtime. You can do the same thing with a central static registry (IOC, but no DI):

    override void Execute() {
        ISomeUtility util = Registry.Get<ISomeUtility>();
        if(util != null) util.Foo();

(where you'd need to write Registry etc; plus on the server, register the ISomeUtility implementation)