public async Task AppendAllLinesAsync() { if (File.Exists(_filePath)) { File.Delete(_filePath); } File.WriteAllLines(_filePath, _lineContent); await FileHelper.AppendAllLinesAsync(_filePath, _appendLineContent); CollectionAssert.AreEqual(_lineContent.Union(_appendLineContent).ToArray(), File.ReadAllLines(_filePath)); File.Delete(_filePath); }