// Arrange
var stub = MockRepository.GenerateStub<IDemo>();
stub.Stub(x => x.StringArgString(Arg<string>.Is.Equal("input"))).Return("output");

// Act
var firstCallResult = stub.StringArgString("input");
var secondCallResult = stub.StringArgString("input");

// Assert
Assert.Equal("output", firstCallResult);
Assert.Equal(firstCallResult, secondCallResult);