当使用 LINQ GroupBy 时，ASP.NET 内核尝试将 object 转换为 System.Int64

Question

When trying to execute the following ASP.NET Core code:

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));
}

I get:

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)

Why does it seem to be trying to serialize an object to an int64? How can I successfully return the result of a GroupBy from a web API endpoint?

Answer 1

New System.Text.Json serializer cannot serialize IEnumerable<IGrouping<long, Foo>> .

Solution

I would recommend switching to old Json.Net serializer in your ASP.Net Core project. I've tested it and it works fine with Json.Net.

See this article for information about switching to Json.Net.

It will be something as easy as that:

services.AddMvc()
    .AddNewtonsoftJson();