ASP.NET Core : Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead（转载）

问：

---

ASP.NET core server, AllowSynchronousIO is set to false

        new WebHostBuilder()
        .UseKestrel(options =>
        {
            options.AllowSynchronousIO = false;
        })

In the action, it outputs a JsonResult

    public async Task<IActionResult> SanityCheck()
    {
        Dictionary<string, string> dic = await GetDic();

        return this.Json(dic);
    }

And it ends with an exception

System.InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.

Can't I return a JsonResult with AllowSynchronousIO=false ?

答：

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You might have the following problem: https://github.com/aspnet/AspNetCore/issues/8302

And you can find more info here: https://github.com/aspnet/AspNetCore/issues/7644

A workaround until the issue is being solved is to allow Synchronous IO. Put this in Startup.cs for either Kestrel or IIS:

public void ConfigureServices(IServiceCollection services)
{
    // If using Kestrel:
    services.Configure<KestrelServerOptions>(options =>
    {
        options.AllowSynchronousIO = true;
    });

    // If using IIS:
    services.Configure<IISServerOptions>(options =>
    {
        options.AllowSynchronousIO = true;
    });
}

从下面的链接可以知道，微软从ASP.NET Core 3.0开始，在默认情况下，禁用了HttpReqeuest.Body和HttpResponse.Body的同步读写方法，需要设置AllowSynchronousIO为true，才能在HttpReqeuest.Body和HttpResponse.Body上使用同步读写方法，否则就会抛出诸如本文所述的异常信息。

[Announcement] AllowSynchronousIO disabled in all servers

原文链接