public async Task Given_A_RemoteFileUrl_And_A_LocalFileFullPath_Should_Invoke_Download_Method_Once() { // Arrange var remoteFileUrl = new Uri("http://www.google.com"); const string localFileFullPath = @"c:\images\pic.png"; // Act await _sut.Download(remoteFileUrl, localFileFullPath); // Assert await _fileSystem.Received(1).Download(Arg.Is(remoteFileUrl), Arg.Is(localFileFullPath)); }