NSubstitute: Mock method is not returning expected result

Question

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.

Answer 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.