Implementing class

namespace MockingCSharp
{
	public class ImplementingMockableClass
	{
		IInterface mockedClass;
		public ImplementingMockableClass (IInterface mockable)
		{
			mockedClass = mockable;
		}
		
		public string OutputString()
		{
			return "This is the string: - " + mockedClass.GetString();
		}
	}
}

Interface

namespace MockingCSharp
{
	public interface IInterface 
	{
		string GetString();
	}
	
	public class ClassWithInterface : IInterface
	{
		public string GetString()
		{
			return "ClassWithInterface - Mockable";	
		}
	}
}

mocking_CLR_interfaces.rb

require 'mocking_csharp.dll'
include MockingCSharp

$: << 'C:/IronRuby/lib/IronRuby/gems/1.8/gems/caricature-0.7.2/lib'
require 'caricature'
include Caricature

i = ImplementingMockableClass.new(ClassWithInterface.new)
puts i.OutputString()

isolation = Isolation.for(IInterface)
isolation.when_receiving(:get_string) do |exp|
    exp.return('MOCKED!!')
end

i = ImplementingMockableClass.new(isolation)
puts i.OutputString()

Output

This is the string: - ClassWithInterface - Mockable
This is the string: - MOCKED!!