JToken.WriteToAsync 不写入 JsonWriter

Question

我正在尝试创建一个以某种方式更改请求的中间件。我能够阅读它并更改内容，但我无法弄清楚如何正确设置流编写器以创建新主体。当我调用 normalized.WriteToAsync(jsonWriter) 时，MemoryStream 仍然为空，因此我收到 A non-empty request body is required. 异常。我在这里错过了什么？这是我目前所拥有的：

public async Task Invoke(HttpContext context)
{
    if (context.Request.ContentType == "application/json" && context.Request.ContentLength > 0)
    {
        using var scope = _logger.BeginScope("NormalizeJson");
        try
        {
            using var requestReader = new HttpRequestStreamReader(context.Request.Body, Encoding.UTF8);
            using var jsonReader = new JsonTextReader(requestReader);

            var json = await JToken.LoadAsync(jsonReader);
            var normalized = _normalize.Visit(json); // <-- Modify json and return JToken

            // Create new Body
            var memoryStream = new MemoryStream();
            var requestWriter = new StreamWriter(memoryStream);
            var jsonWriter = new JsonTextWriter(requestWriter);
            await normalized.WriteToAsync(jsonWriter); // <-- At this point the MemoryStream has still 0 length.

            var content = new StreamContent(memoryStream.Rewind()); // <-- Use helper extension to Seek.Begin = 0                 
            context.Request.Body = await content.ReadAsStreamAsync();
        }
        catch (Exception e)
        {
            _logger.Scope().Exceptions.Push(e);
        }
    }

    await _next(context);
}

LINQPad 等的演示:

async Task Main()
{
    var token = JToken.FromObject(new User { Name = "Bob" });

    var memoryStream = new MemoryStream();
    var requestWriter = new StreamWriter(memoryStream);
    var jsonWriter = new JsonTextWriter(requestWriter);
    await token.WriteToAsync(jsonWriter); 
    memoryStream.Length.Dump(); // <-- MemoryStream.Length = 0
}

public class User
{
    public string Name { get; set; }
}

Answer 1

您需要正确刷新并关闭 JsonTextWriter 和 StreamWriter 以完全填充 memoryStream，如下所示：

var memoryStream = new MemoryStream();
// StreamWriter implements IAsyncDisposable
// Leave the underlying stream open
await using (var requestWriter = new StreamWriter(memoryStream, leaveOpen: true)) 
{
    var jsonWriter = new JsonTextWriter(requestWriter); // But JsonTextWriter does not implement IAsyncDisposable, only IDisposable!
    try
    {
        await token.WriteToAsync(jsonWriter); 
    }
    finally
    {
        await jsonWriter.CloseAsync();
    }
}

演示 fiddle #1 here.

或者，由于您正在写入 MemoryStream，所以根本没有必要使用 async，您可以这样做：

var memoryStream = new MemoryStream();
using (var requestWriter = new StreamWriter(memoryStream, leaveOpen: true)) // Leave the underlying stream open
using (var jsonWriter = new JsonTextWriter(requestWriter))
{
    token.WriteTo(jsonWriter); 
}

演示 fiddle #2 here.

备注：

注意 for the StreamWriter. This syntax guarantees that the StreamWriter will be flushed and closed asynchronously, and can be used on any object that implements IAsyncDisposable的用法。（这仅在您写入文件流或其他非内存流时才真正重要。）
似乎 JsonTextWriter nor the base class JsonWriter 都没有实现 IAsyncDisposable，所以我不得不手动异步关闭 JSON writer，而不是通过 using 语句.外部 await using 应确保底层 StreamWriter 在发生异常时不会保持打开状态。

JToken.WriteToAsync 不写入 JsonWriter

JToken.WriteToAsync does not write to JsonWriter

c#

request

json.net

asp.net-web-api

asp.net-core-middleware