What is Interface Duck Typing?

C# has a nominal type system, so the compatibility of types is done based on their names. In your example you have two classes with a Quack method, however there is no way to write a method which can take instances of these two classes and invoke their Quack method.

In C# 2, the solution would be to introduce an interface and have both classes implement it:

public interface IQuack
{
    void Quack();
}

public class Duck : IQuack { }
public class Human : IQuack { }

now you can create a method which take an IQuack instance and can call Human.Quack and Duck.Quack through it. In C#, methods are resolved ‘early’ at compile time, so you need to create a named type which supports the operations the method need so the compilation can succeed. Note there is still a runtime element to calling these methods, since the real implementation of IQuack.Quack needs to be resolved at runtime depending on the real type of the argument.

In a duck-typing system, no attempt is made to validate that a method exists before runtime. All that is required is that a given object supports the operation in that it has the right name and takes the required number of parameters (none in this case), hence the ‘if it quacks like a duck’ expression.

Duck typing in C# 2 can only be done using reflection, in this case you would accept an object argument and look for the required methods yourself:

public static void MakeQuack(object duck)
{
    MethodInfo quackMethod = duck.GetType().GetMethod("Quack", Type.EmptyTypes, null);
    if (quackMethod!=null)
    {
        quackMethod.Invoke(duck, new object[] { });
    }
    else
    {
        throw new ArgumentException("No Quack() method found on target");
    }
}

C#4 makes this much simpler with dynamic:

public static void MakeQuack(dynamic duck)
{
    duck.Quack();
}