public async Task <bool> IsFileOnDevice(string filename, CancellationToken cancellationToken)
 {
     if (FilesOnDevice.Any() == false)
     {
         await GetFilesAndCrcsAsync(DefaultTimeout, 0, cancellationToken)
         .ConfigureAwait(false);
     }
     return(FilesOnDevice.ContainsKey(filename));
 }