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：
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：https://www.cnblogs.com/OpenCoder/p/12879728.html