ASP.NET Core试图将对象强制转换为System.使用LINQ GroupBy时为Int64

Question

尝试执行以下ASP时。NET核心代码：

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)

为什么它似乎试图将一个对象序列化为int64？如何从web API终结点成功返回GroupBy的结果？

Answer 1

新的System.Text.Json序列化程序无法序列化IEnumerable<IGrouping<long, Foo>>。

解决方案

我建议换成老Json。Net序列化程序。Net Core项目。我已经测试过了，它在Json身上运行良好。网

有关切换到Json的信息，请参阅本文。网

这将是一件很容易的事情：

services.AddMvc()
.AddNewtonsoftJson();