NSubstitute: Mock method is not returning expected

问题:

I'm new to NSubstitute, mocking, and unit testing in general.

I'm trying to use NSubstitute to remove some dependencies I have in my class under test, but methods in the mock objects are not behaving as I expect based on how I configured them. Here is an example I created in Visual Studio:

1. Interface and concrete class to be substituted. Notice, MyConcreteClass.MyMethod() returns false:

   public interface IMyInterface
   {
       bool MyMethod(string arg);
   }
   
   public class MyConcreteClass : IMyInterface
   {
       public bool MyMethod(string arg)
       {
           return false;
       }
   }
   

2. My class under test:

   public class MyTestedClass
   {
       private IMyInterface _concrete;
   
       public MyTestedClass()
       {
           _concrete = new MyConcreteClass();
       }
   
       public MyTestedClass(IMyInterface mock)
       {
           _concrete = mock;
       }
   
       public bool MyConcreteMethod(string arg)
       {
           return _concrete.MyMethod(arg);
       }
   }
   

3. My unit test class for MyTestedClass:

   [TestClass]
   public class UnitTest1
   {
       [TestMethod]
       public void Given_MyMethodIsUsingAMock_ShouldReturnTrue()
       {
           // Arrange
           var myMock = Substitute.For<IMyInterface>();
           myMock.MyMethod("blah").Returns(true);
           var myTestedObject = new MyTestedClass(myMock);
   
           // Act
           var result = myTestedObject.MyConcreteMethod("blah blah");
   
           // Assert
           Assert.AreEqual(result, true); // This assertion fails!
       }
   
       [TestMethod]
       public void Given_MyMethodIsNotMock_ShouldReturnFalse()
       {
           // Arrange
           var myTestedObject = new MyTestedClass();
   
           // Act
           var result = myTestedObject.MyConcreteMethod("blah blah");
   
           // Assert
           Assert.AreEqual(result, false); // This assertion passes.
       }
   }
   

4. Test results show that Given_MyMethodIsUsingAMock_ShouldReturnTrue() fails:

   MyUnitTests (2 tests) [0:00.190] Failed: 1 test failed
    MyUnitTests (2 tests) [0:00.190] Failed: 1 test failed
     UnitTest1 (2 tests) [0:00.190] Failed: 1 test failed
      Given_MyMethodIsNotMock_ShouldReturnFalse [0:00.000] Success
      Given_MyMethodIsUsingAMock_ShouldReturnTrue [0:00.189] Failed
   Assert.AreEqual failed. Expected:<False>. Actual:<True>. 
      at MyUnitTests.UnitTest1.Given_MyMethodIsUsingAMock_ShouldReturnTrue() in "c:\MyWorkspace\projects\NSubstituteMocking\MyUnitTests\UnitTest1.cs":line 23
   

It looks like I'm missing a trivial configuration, but it is eluding me.

回答1:

The MyMethod is arranged to return true when given "blah"

myMock.MyMethod("blah").Returns(true);

but is then provide it with "blah blah" when being acted upon.

var result = myTestedObject.MyConcreteMethod("blah blah");

As the expected/arranged parameter did not match, the mock did not behave as configured.

Provide the mock with what it expects to receive for it to behave as expected.

[TestMethod]
public void Given_Blah_MyConcreteMethod_ShouldReturnTrue() {
    // Arrange
    var myMock = Substitute.For<IMyInterface>();
    var arg = "blah";
    var expected = true;
    myMock.MyMethod(arg).Returns(expected);
    var myTestedObject = new MyTestedClass(myMock);

    // Act
    var actual = myTestedObject.MyConcreteMethod(arg);

    // Assert
    Assert.AreEqual(expected, actual); // This should pass
}

Note the use of variables to store the provided and expected values so that mistakes can be reduced when exercising the test.