public override Stream GetRequestStream()
        {
            return InternalGetRequestStream().Result;
        }

        private Task<Stream> InternalGetRequestStream()
        {
            _requestStream = new RequestStream();

            return Task.FromResult((Stream)_requestStream);
        }

namespace System.Net
{
    // Cache the request stream into a MemoryStream.  This is the
    // default behavior of Desktop HttpWebRequest.AllowWriteStreamBuffering (true).
    // Unfortunately, this property is not exposed in .NET Core, so it can't be changed
    // This will result in inefficient memory usage when sending (POST'ing) large
    // amounts of data to the server such as from a file stream.
    internal sealed class RequestStream : Stream
    {
        private readonly MemoryStream _buffer = new MemoryStream();

        public RequestStream()
        {
        }

        public override void Flush()
        {
            // Nothing to do.
        }

        public override Task FlushAsync(CancellationToken cancellationToken)
        {
            // Nothing to do.
            return cancellationToken.IsCancellationRequested ?
                Task.FromCanceled(cancellationToken) :
                Task.CompletedTask;
        }

        public override long Length
        {
            get
            {
                throw new NotSupportedException();
            }
        }

        public override long Position
        {
            get
            {
                throw new NotSupportedException();
            }
            set
            {
                throw new NotSupportedException();
            }
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            throw new NotSupportedException();
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            ValidateBufferArguments(buffer, offset, count);
            _buffer.Write(buffer, offset, count);
        }

        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            ValidateBufferArguments(buffer, offset, count);
            return _buffer.WriteAsync(buffer, offset, count, cancellationToken);
        }

        public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback? asyncCallback, object? asyncState)
        {
            ValidateBufferArguments(buffer, offset, count);
            return _buffer.BeginWrite(buffer, offset, count, asyncCallback, asyncState);
        }

        public override void EndWrite(IAsyncResult asyncResult)
        {
            _buffer.EndWrite(asyncResult);
        }

        public ArraySegment<byte> GetBuffer()
        {
            ArraySegment<byte> bytes;

            bool success = _buffer.TryGetBuffer(out bytes);
            Debug.Assert(success); // Buffer should always be visible since default MemoryStream constructor was used.

            return bytes;
        }
    }
}

        public override WebResponse GetResponse()
        {
            try
            {
                _sendRequestCts = new CancellationTokenSource();
                return SendRequest(async: false).GetAwaiter().GetResult();
            }
            catch (Exception ex)
            {
                throw WebException.CreateCompatibleException(ex);
            }
        }

        private async Task<WebResponse> SendRequest(bool async)
        {
            var request = new HttpRequestMessage(new HttpMethod(_originVerb), _requestUri);

            bool disposeRequired = false;
            HttpClient? client = null;
            try
            {
                client = GetCachedOrCreateHttpClient(async, out disposeRequired);
                if (_requestStream != null)
                {
                	// 在这里使用到 RequestStream 数据
                    ArraySegment<byte> bytes = _requestStream.GetBuffer();
                    request.Content = new ByteArrayContent(bytes.Array!, bytes.Offset, bytes.Count);
                }

                // Copy the HttpWebRequest request headers from the WebHeaderCollection into HttpRequestMessage.Headers and
                // HttpRequestMessage.Content.Headers.
                foreach (string headerName in _webHeaderCollection)
                {
                    // The System.Net.Http APIs require HttpRequestMessage headers to be properly divided between the request headers
                    // collection and the request content headers collection for all well-known header names.  And custom headers
                    // are only allowed in the request headers collection and not in the request content headers collection.
                    // 拷贝 Head 逻辑
                }

                request.Headers.TransferEncodingChunked = SendChunked;

                _sendRequestTask = async ?
                    client.SendAsync(request, _allowReadStreamBuffering ? HttpCompletionOption.ResponseContentRead : HttpCompletionOption.ResponseHeadersRead, _sendRequestCts!.Token) :
                    Task.FromResult(client.Send(request, _allowReadStreamBuffering ? HttpCompletionOption.ResponseContentRead : HttpCompletionOption.ResponseHeadersRead, _sendRequestCts!.Token));

                HttpResponseMessage responseMessage = await _sendRequestTask.ConfigureAwait(false);

                HttpWebResponse response = new HttpWebResponse(responseMessage, _requestUri, _cookieContainer);

                return response;
            }
            finally
            {
                if (disposeRequired)
                {
                    client?.Dispose();
                }
            }
        }