.WhenCalled(invocation => invocation.ReturnValue = new TestResult(){IsTrue = true, Message = "BBB"})

[TestClass]
public class ChrisTest
{
    private IRuleService ruleService;

   [TestMethod]
    public void ShouldNotChangeReturnedTestResult()
    {
        ruleService = MockRepository.GenerateMock<IRuleService>();
        var testResult = new TestResult();
        testResult.IsTrue = true;
        testResult.Message = "AAA";
        ruleService.Stub(a => a.GetTestResult()).Return(testResult)
            .WhenCalled(invocation => invocation.ReturnValue = new TestResult(){IsTrue = true, Message = "BBB"});

        var testClass = new TestClass(ruleService);
        testClass.KillTheString();
        Assert.IsTrue(testClass.StringIsThere());
        
    }

    public interface IRuleService
    {
        TestResult GetTestResult();
    }

    public class TestResult
    {
        public bool IsTrue { get; set; }
        public string Message { get; set; }
    }

    private class TestClass
    {
        private readonly IRuleService ruleService;

        public TestClass(IRuleService ruleService)
        {
            this.ruleService = ruleService;
        }

        public void KillTheString()
        {
            var result = ruleService.GetTestResult();
            result.Message = string.Empty;
        }

        public bool StringIsThere()
        {
            var result = ruleService.GetTestResult();
            return !string.IsNullOrEmpty(result.Message);
        }
    }
}