IsAssignableFrom

program.cs

class Program {

    static void Main(string[] args) {

        var type = typeof(Foo);
        var isIFoo = typeof(IFoo).IsAssignableFrom(type);
    }
}

public interface IFoo {

    void Do();
}

public class Foo : IFoo {

    public void Do() {
        Console.WriteLine("Done!");
    }
}