public async Task MockFile_AppendAllLinesAsync_ShouldPersistNewLinesToNewFile() { // Arrange string path = XFS.Path(@"c:\something\demo.txt"); var fileSystem = new MockFileSystem(new Dictionary <string, MockFileData> { { XFS.Path(@"c:\something\"), new MockDirectoryData() } }); var file = new MockFile(fileSystem); // Act await file.AppendAllLinesAsync(path, new[] { "line 1", "line 2", "line 3" }); // Assert Assert.AreEqual( "line 1" + Environment.NewLine + "line 2" + Environment.NewLine + "line 3" + Environment.NewLine, file.ReadAllText(path)); }