public void TestExtensionExtraction(string fileName, string expectedExtension) { var environmentPathServiceMock = new Mock <IEnvironmentPathService>(); environmentPathServiceMock .Setup(m => m.GetExtension(It.IsAny <string>())) .Returns <string>(Path.GetExtension); var pathService = new PathService(environmentPathServiceMock.Object); var actualExtension = pathService.GetExtension(fileName); Assert.Equal(expectedExtension, actualExtension); }