public void FileManager_WhenProvidingDirectory_ReturnsFileList() { var mockedFileSystem = new Mock<IFileSystem>(); var mgr = new FileManager(mockedFileSystem.Object); mockedFileSystem.Setup(x => x.GetFilesFromDirectory(It.IsAny<string>())) .Returns(new List<string> { "file1.zip", "file2.zip", "file3.zip" }); IList<string> files = mgr.EnlistFilesByDirectory("fooPath"); CollectionAssert.Contains(files,"file1.zip"); CollectionAssert.Contains(files, "file2.zip"); CollectionAssert.Contains(files, "file3.zip"); }