When designing interfaces member method's signatures are defined by their type’s signatures rather having their signatures inferred from the methods parameters. There are roughly 3 ways to define a methods signature and these lead to different compiled forms and different ways of calling these methods within F#. This below interface illustrates this:

        type TestInterace = 
            interface
                abstract DoSomethingCurryStyle : string -> string -> unit  
                        // C# signature "FastFunc<string, object> DoSomethingCurryStyle(string param1);"
                abstract DoSomethingTupleStyle : (string * string) -> unit 
                        // C# signature "void DoSomethingTupleStyle(Tuple<string, string> tupple);"
                abstract DoSomethingCSharpStyle : string * string -> unit 
                        // C# signature "void DoSomethingCSharpStyle(string param1, string param2);"
            end

The important thing to note is the only difference between tuple style and the C# is that the C# style has no parenthesis. The different way of calling these methods are illustrated below:

        let TestTestInterface (thing : TestInterface ) =
                thing.DoSomethingCurryStyle "hello" "hello";
                let param = "hello", "hello" in
                thing.DoSomethingTupleStyle param;
                thing.DoSomethingCSharpStyle ("hello", "hello");
                ()

An interface can be implemented by a class as follows:

  type TestInterface = interface
     abstract DoSomethingElse : string * string -> unit
  end


  type TestClass() = class
    interface TestInterface with
         member x.DoSomethingElse(s,t) = printf "Hi %s and %s" s t
    end
  end

As of F# 1.9.2 much of this syntax optional (just to encourage you that you won't always have to write all those 'end' tokens...). You can then write just:

  type TestInterface = 
      abstract DoSomethingElse : string * string -> unit


  type TestClass() = 
     interface TestInterface with
         member x.DoSomethingElse(s,t) = printf "Hi %s and %s" s t