带有CancellationToken的SHA256哈希

Question

对于需要在后台运行的大型文件，我需要一个SHA256哈希函数。我想有可能取消执行（我认为带有CancellationToken）。使用库功能时，我没有找到根据要求取消该过程的方法。

 private byte[] SHA256_Hash(string fileName) {
        byte[] result = null;
        using (SHA256 sha256 = SHA256.Create()) {
            int bufferSize = 10 * 1024 * 1024; // 10MB
            using (var stream = new BufferedStream(File.OpenRead(fileName), bufferSize)) {
                result = sha256.ComputeHash(stream);
            }
        }
        return result;
    }

Answer 1

例如，您可以尝试取消流阅读（stream.ReadAsync）

private static async Task<byte[]> SHA256_HashAsync(string fileName,
                                                   CancellationToken token) { 
  using (SHA256 sha256 = SHA256.Create()) {
    // we are going to read large file block after block
    // each read can be cancelled
    var buffer = new byte[8 * 1024]; // <- typical 8k bytes block

    using (var stream = new BufferedStream(File.OpenRead(fileName), buffer.Length)) {
      int read = 0;

      // read next block...
      while ((read = await stream.ReadAsync(buffer, 0, buffer.Length, token)) != 0)
        sha256.TransformBlock(buffer, 0, read, buffer, 0); //... add it ti SHA

      // final (may be incomplete) block
      sha256.TransformFinalBlock(buffer, 0, read); 

      return sha256.Hash;
    }
  }
}

可能的用法

  using (CancellationTokenSource src = new CancellationTokenSource(5000)) {
    try {
      byte[] hash = await SHA256_HashAsync(@"c:\MyLongFile.dat", src.Token);

      // relevant code here
    }
    catch (TaskCanceledException) {
      // 5 seconds is not enough to compute SHA256 hash
    }
  }