如何使用 System.Text.Json API 将流反序列化为对象 [英] How to deserialize stream to object using System.Text.Json APIs

问题描述

我收到来自 web api 调用的响应作为流，需要将其反序列化为模型.

I'm receiving a response from a web api call as a stream and need to deserialize it to a model.

这是一个通用方法，所以我不能说代码的哪些部分会使用它以及响应负载是什么.

This is a generic method, so I can't say which parts of code will use this and what's the response payload.

方法如下:

public async Task<T> InvokeAsync<T>(string method)
{
    Stream response = await this.httpClientWrapper.InvokeAsync(method);
    var serializer = new JsonSerializer();
    using var streamReader = new StreamReader(response);
    using var reader = new JsonTextReader(streamReader);
    return serializer.Deserialize<T>(reader);
}

我正在尝试删除 Newtonsoft 并使用 System.Text.Json API.

I'm trying to remove Newtonsoft and use System.Text.Json API.

我发现了这个移植指南 在 Github 的 corefx 存储库中，其中部分 从流/字符串中读取 状态:

I found this porting guide in corefx repo in Github, where section Reading from a Stream/String states:

我们目前(从 .NET Core 3.0 预览版 2 开始)没有一个方便的直接从流中读取 JSON 的 API(同步或异步).用于同步读取(尤其是小有效载荷)，您可以读取 JSON 有效载荷直到流结束成一个字节数组并将其传递给阅读器

We currently (as of .NET Core 3.0 preview 2) do not have a convenient API to read JSON from a stream directly (either synchronously or asynchronously). For synchronous reading (especially of small payloads), you could read the JSON payload till the end of the stream into a byte array and pass that into the reader

所以按照这个建议，我想出了以下几点:

So following this advise I come up with the following:

public async Task<T> InvokeAsync<T>(string method)
{
    Stream response = await this.httpClientWrapper.InvokeAsync(method);
    var length = response.Length;
    var buffer = ArrayPool<byte>.Shared.Rent((int)length);
    var memory = new Memory<byte>(buffer);
    await response.WriteAsync(memory);
    var result = JsonSerializer.Deserialize<T>(memory.Span);
    ArrayPool<byte>.Shared.Return(buffer);
    return result;
}

所以我的问题是 - 我是否正确理解了建议，这是要走的路?

So my question is - did I understand the advise correctly and this is the way to go ?

这个实现可能在很多方面都可以改进，但最让我烦恼的是从池中租用字节数组，例如Stream.Length 是一个 long，我将它转换为 int，这会导致 OverflowException.

This implementation probably can be improved on many aspects, but what bothers me most is renting the byte array from the pool e.g. Stream.Length is a long and I convert it to int which can cause OverflowException.

我试图查看系统.IO.Pipelines 并使用 JSON API 的 ReadOnlySequence 重载，但它变得非常复杂.

I tried to look into System.IO.Pipelines and use ReadOnlySequence<byte> overloads of JSON API, but it gets very complicated.

推荐答案

我认为文档需要更新，因为 .NET Core 3 有一个 直接从方法流.使用它很简单，假设流是用 UTF8 编码的:

I believe that documentation needs to be updated because .NET Core 3 has a method to read from a stream directly. Using it is straight-forward, assuming the stream is encoded in UTF8:

private static readonly JsonSerializerOptions Options = new JsonSerializerOptions();

private static async Task<T> Deserialize<T>(HttpResponseMessage response)
{
    var contentStream = await response.Content.ReadAsStreamAsync();
    var result = await JsonSerializer.DeserializeAsync<T>(contentStream, Options);
    return result;
}

需要注意的一点是，默认情况下 HttpClient 将在返回之前缓冲内存中的响应内容，除非您在调用 SendAsync 时将 HttpCompletionOption 设置为 ResponseHeadersRead:

One thing to watch out for is that by default HttpClient will buffer the response content in-memory before returning unless you set the HttpCompletionOption to ResponseHeadersRead when invoking SendAsync:

var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, token);

这篇关于如何使用 System.Text.Json API 将流反序列化为对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！