Implementing a C# interface with an F# object expression

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 with an F# type like we saw in this post. Another option is to use an object expression. It is useful if we don’t want to create a specific F# type for it. The syntax is very similar to the F# type solution. The implementation is enclosed within curly braces:

open Project.Domains

let calcImplObjExp = { new 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 }
        
let mult = calcImplObjExp.Multiply(4, 5)

Be careful with the indentation and putting the “with” keyword to the right place though. The following won’t work:

let calcImplObjExp = { new 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 }

View all F# related articles here.