浅谈.NET下的多线程和并行计算（十一）.NET异步编程模型基础 下
| (long)bytesRetrieved; } if (this.Offset != this.ContentLength) { if (this.Async) { this.WebClient.PostProgressChanged(this.AsyncOp, this.Progress); this.ReadStream.BeginRead(this.InnerBuffer, this.Offset, ((int) this.Length) - this.Offset, new AsyncCallback(WebClient.DownloadBitsReadCallback), this); } else { bytesRetrieved = this.ReadStream.Read(this.InnerBuffer, this.Offset, ((int)this.Length) - this.Offset); } return false; } } if (this.Async) { if (this.Progress.TotalBytesToReceive < 0L) { this.Progress.TotalBytesToReceive = this.Progress.BytesReceived; } this.WebClient.PostProgressChanged(this.AsyncOp, this.Progress); } if (this.ReadStream != null) { this.ReadStream.Close(); } if (this.WriteStream != null) { this.WriteStream.Close(); } else if (this.WriteStream == null) { byte[] dst = new byte[this.SgBuffers.Length]; if (this.SgBuffers.Length > 0) { BufferOffsetSize[] buffers = this.SgBuffers.GetBuffers(); int dstOffset = 0; for (int i = 0; i < buffers.Length; i++) { BufferOffsetSize size = buffers[i]; Buffer.BlockCopy(size.Buffer, 0, dst, dstOffset, size.Size); dstOffset += size.Size; } } this.InnerBuffer = dst; } return true; } WebClient的PostProgressChanged方法，在汇报进度的时候调用了AsyncOperation的Post方法：
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