C# ASP.NET Core Web API POST请求遇415 UnsupportedMediaType错误排查

场景说明 #

在HomeController中实现ApiTester方法，加载该页面时会向AuthController的ApiTest接口发起POST请求，首页跳转链接如下：

<a href="@Url.Action("ApiTester", "Home")">ApiTester Click Here</a>

HomeController代码 #

using Microsoft.AspNetCore.Mvc;
using Newtonsoft.Json;
using System.Diagnostics;
using System.Net.Http.Headers;
using System.Text;
using TestWebApplication.Models;

namespace TestWebApplication.Controllers
{
    public class HomeController : Controller
    {

        public IActionResult Index()
        {
            return View();
        }

        [HttpGet]
        public async Task<IActionResult> ApiTester()
        {
            var client = new HttpClient();
            var values = new Dictionary<string, string> { { "Name", "Alice" } };
            var content = new FormUrlEncodedContent(values);
            content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");

            var response = await client.PostAsync("https://localhost:7284/api/Auth/ApiTest", content);

            if (response.IsSuccessStatusCode)
            {
                var result = await response.Content.ReadAsStringAsync();
                ViewBag.ApiResponse = result;
            }
            else
            {
                ViewBag.ApiResponse = "Error: " + response.StatusCode;
            }

            return View();
        }
    }
}

AuthController代码 #

using Microsoft.AspNetCore.Mvc;
using Newtonsoft.Json.Linq;

namespace TestWebApplication.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class AuthController : ControllerBase
    {
        [HttpGet("GetTest")]
        public IActionResult GetTest()
        {
            return Ok(new { message = "Get Test Succesfull!" });
        }

        [HttpPost("ApiTest")]
        public IActionResult ApiTest([FromBody] Dictionary<string, string> values)
        {
            string name = values["name"];
            return Ok(new { message = "200: Post Test Succesfull!" });
        }
    }
}

Program.cs代码 #

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddControllersWithViews();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Home/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

app.UseAuthorization();

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.Run();

问题 #

当前请求返回Error: UnsupportedMediaType（状态码415），但移除ApiTest方法的[FromBody]参数后请求可正常执行，请问该错误的原因是什么？

错误原因 #

1. 请求格式与接口接收规则不匹配

   • 客户端发送的是application/x-www-form-urlencoded格式的表单数据，对应Content-Type头也是该类型。
   • 但服务端的ApiTest方法用[FromBody]标记参数，这个特性专门用于接收JSON/XML格式的请求体（Content-Type通常为application/json），无法处理表单格式的数据。

2. ASP.NET Core参数绑定机制限制

   • 当使用[FromBody]时，ASP.NET Core会调用JSON格式化器去解析请求体，但你的请求是表单格式，没有对应的格式化器能处理，因此返回415错误。
   • 移除[FromBody]后，ASP.NET Core会自动从表单数据中绑定参数，所以请求能正常执行。

内容的提问来源于stack exchange，提问作者Waller