Implementing a C# interface with an F# type

Suppose you have the following C# interface ICalculator with 4 functions:

namespace Project.Domains
{
	public interface ICalculator
	{
		int Add(int x, int y);
		int Subtract(int x, int y);
		float Divide(float x, float y);
		int Multiply(int x, int y);
	}
}

We can implement this interface in an F# type as follows:

module CSharpExamples

open Project.Domains

type FSharpCalculator() =
    interface ICalculator with
        member this.Add(x, y) = x + y
        member this.Subtract(x, y) = x - y
        member this.Divide(x, y) = x / y
        member this.Multiply(x, y) = x * y

Interfaces must be implemented explicitly in F#. We declare each implementing member with the ‘member’ keyword. If any of the interface functions are missing then we get a compile time error similar to the following:

No implementation was given for ‘ICalculator.Multiply(x: int, y: int) : int’. Note that all interface members must be implemented and listed under an appropriate ‘interface’ declaration, e.g. ‘interface … with member …’.

Using the FSharpCalculator class is a bit strange at first though. At least I expected the following code to work just fine:

let calcImpl = new CSharpExamples.FSharpCalculator()
int res = calcImpl.Add(10, 20)

However, that results in an error:

The field, constructor or member ‘Add’ is not defined.

It turns out that we have to cast the implementation to the interface before we call its functions using the :> operator:

let calcImpl = new CSharpExamples.FSharpCalculator()
let calcAbs = calcImpl :> ICalculator
let sum = calcAbs.Add(10, 20);

Now it works as expected.

View all F# related articles here.