如何在 ASP .NET Core 最小 API 中使用 415（不支持的媒体类型）上的 JSON 正文进行响应？

问：

如果调用没有标头的 ASP .NET Core 最小 API 终结点，则会收到状态代码为 415 的空正文响应。我需要处理这个错误并用一些错误JSON进行响应。我无法让终结点使用空字符串以外的任何内容进行响应。此外，异常处理程序不会截获任何内容。Content-Type

下面是一个示例应用和测试，用于解释该问题。

应用程序接口

using System.Runtime.CompilerServices;
using Microsoft.AspNetCore.Mvc;

[assembly: InternalsVisibleTo("TestProject1")]

var builder = WebApplication.CreateBuilder(args);

builder.Services.Configure<RouteHandlerOptions>(o => o.ThrowOnBadRequest = true);

var app = builder.Build();

app.MapPost("/hi", ([FromBody] Request request) => "Hello World!");
app.MapPost("/throwerror", ([FromBody] Request request) => true ? throw new Exception("Ouch") : "");

app.UseExceptionHandler(handlerApp => handlerApp.Run(async context =>
{
    context.Response.StatusCode = 400;
    await context.Response.WriteAsync("Threw Error");
}));

app.Run();

record Request(int Id);

测试

using System.Text;
using Microsoft.AspNetCore.Mvc.Testing;
namespace TestProject1;

[TestClass]
public class UnitTest1
{
    //This test doesn't pass. I think it should hit the exception handler and set the body to "Threw Error"
    //What I need to achieve is the ability to give a meaningful error message in the body if the consumer
    //doesn't specify a Content-Type header 
    [TestMethod]
    public async Task TestHiRespondsWithTextFromExceptionHandlerOnNoContentType()
    {
        var factory = new WebApplicationFactory<Program>().WithWebHostBuilder(builder => { });
        var client = factory.CreateClient();
        var response = await client.PostAsync(new Uri("http://localhost/hi"), 
            //No content type
            new StringContent( "{ \"Id\": \"123\" }", Encoding.UTF8));
        var body = await response.Content.ReadAsStringAsync();
        //this would prove the exception handler is handling this
        Assert.AreEqual("Threw Error", body);
    }
    
    [TestMethod]
    public async Task TestSuccessRespondsWithHelloWorld()
    {
        var factory = new WebApplicationFactory<Program>().WithWebHostBuilder(builder => { });
        var client = factory.CreateClient();
        var response = await client.PostAsync(new Uri("http://localhost/hi"), 
            new StringContent( "{ \"Id\": \"123\" }", Encoding.UTF8,"application/json"));
        var body = await response.Content.ReadAsStringAsync();
        Assert.AreEqual("Hello World!", body);
    }
    
    [TestMethod]
    public async Task TestThrowErrorRespondsWithTextFromExceptionHandler()
    {
        var factory = new WebApplicationFactory<Program>().WithWebHostBuilder(builder => { });
        var client = factory.CreateClient();
        var response = await client.PostAsync(new Uri("http://localhost/throwerror"), 
            new StringContent( "{ \"Id\": \"123\" }", Encoding.UTF8,"application/json"));
        var body = await response.Content.ReadAsStringAsync();
        //this proves the exception handler is handling this
        Assert.AreEqual("Threw Error", body);
    }
}

请注意，第一个测试失败，因为它从未命中异常处理程序。就好像有一些中间件在任何其他中间件之前执行，并且它永远不会抛出异常。它只是在给我做任何事情的机会之前用 415 回应。如何挂接到这个中间件以使用正文进行响应？Unsupported Media Type

底部的两个测试通过。