HttpResponseMessage 的处理是调用请求流的处理

Question

我有一个获取 Stream 参数并将其传递给服务器的方法

public async Task<string> Execute(Stream archive)
    {
        archive.Seek(0, SeekOrigin.Begin);
        using var content = new MultipartFormDataContent();
        content.Add(new StreamContent(archive), "file1", "file1");
        var result = "";
        using (var response = await _client.PostAsync(_uri, content))
        {
            if (response.IsSuccessStatusCode)
            {
                var stringResult = await response.Content.ReadAsStringAsync();
                result = stringResult;
            }
        }
        // here archive is already disposed
        return result;
    }

现在我实现这个方法的重试策略。如果调用此方法的外部代码得到 "" 作为结果，则它会尝试再次调用此方法。但档案是为那一刻准备的。我看到存档流在处理响应后立即处理。为什么？这个方法后需要外面的stream参数怎么办？

Answer 1

StreamContent 将处理 Stream，如 it's source. And that will be disposed by the MultipartContent 中所述。这将在 PostAsync 中处理...整个链。

一个解决方案是 subclass Stream 并删除 dispose 方法，就像提议的 here 一样。但您必须确保自己处理原始流！编辑：更新。 Stream是一个抽象class，所以如果你知道具体的流类型会更容易，例如

public sealed class NoDisposeMemoryStream : MemoryStream
{
    protected override void Dispose(bool disposing) { }
}

否则您将需要编写自己的完整 Stream 包装器，见底部。

另一种解决方案是在最里面的 using 块中实现重试机制，在每次失败时重置 archive 寻源。这可能更安全。

    public sealed class NoDisposeStream : Stream
    {
        private Stream _original;
        public NoDisposeStream(Stream original) => _original = original
            ?? throw new ArgumentNullException(nameof(original));
        public override bool CanRead => _original.CanRead;
        public override bool CanSeek => _original.CanSeek;
        public override bool CanWrite => _original.CanWrite;
        public override long Length => _original.Length;
        public override long Position { get => _original.Position; set { _original.Position = value; } }
        public override void Flush() => _original.Flush();
        public override int Read(byte[] buffer, int offset, int count) => _original.Read(buffer, offset, count);
        public override long Seek(long offset, SeekOrigin origin) => _original.Seek(offset, origin);
        public override void SetLength(long value) => _original.SetLength(value);
        public override void Write(byte[] buffer, int offset, int count) => _original.Write(buffer, offset, count);
        protected override void Dispose(bool disposing) { }
    }

Answer 2

发生这种情况是因为 HttpClient PostAsync 处理了您传递的内容。 https://github.com/microsoft/referencesource/blob/master/System/net/System/Net/Http/HttpClient.cs

相关代码：

public Task<HttpResponseMessage> PostAsync(Uri requestUri, HttpContent content,
                CancellationToken cancellationToken)
{
    HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, requestUri);
    request.Content = content;
    return SendAsync(request, cancellationToken);
}

然后 SendAsync 调用 DisposeRequestContent，其实现如下：

private void DisposeRequestContent(HttpRequestMessage request)
{
   Contract.Requires(request != null);
    
   // When a request completes, HttpClient disposes the request content so the user  doesn't have to. This also
   // ensures that a HttpContent object is only sent once using HttpClient (similar to   HttpRequestMessages
   // that can also be sent only once).
   HttpContent content = request.Content;
   if (content != null)
   {
     content.Dispose();
   }
}

评论说原因

HttpResponseMessage 的处理是调用请求流的处理

Disposing of HttpResponseMessage is calling disposing of request's stream

c#

httpclient

.net-core

httpresponsemessage