c# - POST 方法中来自 JsonSerializer 的 NullReferenceException

Question

我试图在#C 中编写一个简单的 REST API，但在编写 POST 处理程序时，我很早就遇到了一个问题，其中包含一条相当无用的错误消息。我使用的是 .NET Core 3.0，该项目是使用 Visual Studio 中的 ASP.NET Core API 模板创建的。

这是我的代码的简化版本，它仍然会触发问题(这些类位于完整项目的单独文件中):

[Route("api/blub")]
[ApiController]
public class BlubController : ControllerBase
{
    [HttpGet]
    public IEnumerable<Blub> Get()
    {
        return new Blub[0];
    }

    [HttpPost]
    public ActionResult<Blub> PostBlub([FromBody] string[] paths)
    {
        return new Blub(paths);
    }
}

public class Blub
{
    public Blub(string[] paths)
    {
        this.Paths = paths;
        StartedAt = DateTime.Now;
    }

    public string[] Paths { get; }
    public DateTime StartedAt { get; }
}

我的 POST 请求的负载类似于以下内容:

{ "paths": [ "abc", "def" ] }

在对我的 api/blub 路由执行 POST 请求时出现以下异常:

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Text.Json.JsonSerializer.HandleStartObject(JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
   at System.Text.Json.JsonSerializer.ReadCore(JsonReaderState& readerState, Boolean isFinalBlock, ReadOnlySpan`1 buffer, JsonSerializerOptions options, ReadStack& readStack)
   at System.Text.Json.JsonSerializer.ReadAsync[TValue](Stream utf8Json, Type returnType, JsonSerializerOptions options, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonInputFormatter.ReadRequestBodyAsync(InputFormatterContext context, Encoding encoding)
   at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonInputFormatter.ReadRequestBodyAsync(InputFormatterContext context, Encoding encoding)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Binders.BodyModelBinder.BindModelAsync(ModelBindingContext bindingContext)
   at Microsoft.AspNetCore.Mvc.ModelBinding.ParameterBinder.BindModelAsync(ActionContext actionContext, IModelBinder modelBinder, IValueProvider valueProvider, ParameterDescriptor parameter, ModelMetadata metadata, Object value)
   at Microsoft.AspNetCore.Mvc.Controllers.ControllerBinderDelegateProvider.<>c__DisplayClass0_0.<<CreateBinderDelegate>g__Bind|0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

无论我的 POST 请求是否包含正确的主体，任何 POST 请求似乎都会触发此异常。 GET 请求可以正常工作。我试图登录到控制台并在我的函数中设置一个断点，但据我所知，甚至在我的函数被调用之前就抛出了异常。

为什么 JsonFormatter 会在此处抛出异常，我该如何解决？

Answer 1

类型映射有问题，要么将方法签名更改为:

[HttpPost]
public ActionResult<Blub> PostBlub([FromBody] MyRequest request)
{
    return new Blub(request.Paths);
}

public class MyRequest
{
    public string[] Paths { get; set; }
}

或将负载更改为:

 [ "abc", "def" ]