當使用 LINQ GroupBy 時，ASP.NET 內核嘗試將 object 轉換為 System.Int64

Question

嘗試執行以下 ASP.NET 核心代碼時：

public class Foo
{
    public long A { get; set; }
    public string B { get; set; }
}

[HttpGet]
[Route("Foos")]
public ActionResult<IEnumerable<IGrouping<long, Foo>>> Foos()
{
    var foos = new List<Foo> { new Foo { A = 1, B = "Foo" }, new Foo { A = 2, B = "Bar" }, new Foo { A = 2, B = "Baz" } };
    return new ActionResult<IEnumerable<IGrouping<long, Foo>>>(foos.GroupBy(f => f.A));
}

我得到：

System.InvalidCastException: Unable to cast object of type 'Foo' to type 'System.Int64'.
   at System.Text.Json.JsonPropertyInfoNotNullable`4.OnWriteEnumerable(WriteStackFrame& current, Utf8JsonWriter writer)
   at System.Text.Json.JsonPropertyInfo.WriteEnumerable(WriteStack& state, Utf8JsonWriter writer)
   at System.Text.Json.JsonSerializer.HandleEnumerable(JsonClassInfo elementClassInfo, JsonSerializerOptions options, Utf8JsonWriter writer, WriteStack& state)
   at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 originalWriterDepth, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteAsyncCore(Stream utf8Json, Object value, Type inputType, JsonSerializerOptions options, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
   at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|29_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters()
--- End of stack trace from previous location where exception was thrown ---
   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.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

為什么它似乎試圖將 object 序列化為 int64？ 如何從 web API 端點成功返回 GroupBy 的結果？

Answer 1

新System.Text.Json序列化程序無法序列化IEnumerable<IGrouping<long, Foo>> 。

解決方案

我建議在您的 ASP.Net Core 項目中切換到舊的 Json.Net 序列化程序。 我已經對其進行了測試，它可以與 Json.Net 一起正常工作。

有關切換到 Json.Net 的信息，請參閱本文。

事情就這么簡單：

services.AddMvc()
    .AddNewtonsoftJson();