XmlSerializerOutputFormatter throws InvalidCastException on async method

[aguacongas]

Is this a Bug or Feature request?:

Bug, regression

Steps to reproduce or link to a repro project:

• Create an Asp.Net Core api project
• Update package to use 2.1.0-rc1-final

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Folder Include="wwwroot\" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.All" Version="2.1.0-rc1-final" />
  </ItemGroup>

  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="2.0.4" />
  </ItemGroup>

</Project>

• Add XmlSerializerOutputFormatter in MVC option

public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc(options =>
            {
                options.OutputFormatters.Add(new XmlSerializerOutputFormatter());
            });
        }

• Update a ValuesController method to use async

 public async Task<IEnumerable<string>> Get()
        {
            return await Task.FromResult(new string[] { "value1", "value2" });
        }

• run the project
• send a request using Accept: application/xml

GET /api/values HTTP/1.1
Host: localhost:61519
Accept: application/xml

Description of the problem:

An InvalidCastException is thrown

InvalidCastException: Unable to cast object of type 'System.String[]' to type 'System.Threading.Tasks.Task1[System.Collections.Generic.IEnumerable1[System.String]]'.

Version of Microsoft.AspNetCore.Mvc or Microsoft.AspNetCore.App or Microsoft.AspNetCore.All:

Microsoft.AspNetCore.All Version="2.1.0-rc1-final"

[mkArtakMSFT]

@aguacongas, thanks for contacting us.
@dougbu, can you please investigate this? Thanks!

[dougbu]

I'm able to reproduce this. Workaround is to use ActionResult<IEnumerable<string>> instead i.e.

[HttpGet]
public async Task <ActionResult<IEnumerable<string>>> Get()
{
    return await Task.FromResult(new string[] { "value1", "value2" });
}

[rynowak]

The issue is likely here:

Mvc/src/Microsoft.AspNetCore.Mvc.Core/Internal/ActionMethodExecutor.cs

Line 204 in 599d1a5

var actionResult = ConvertToActionResult(mapper, returnValue, executor.MethodReturnType);

This should be https://github.com/aspnet/Common/blob/dev/shared/Microsoft.Extensions.ObjectMethodExecutor.Sources/ObjectMethodExecutor.cs#L69 instead of MethodReturnType

[mkArtakMSFT]

Thanks for investigation, @dougbu and @rynowak.
@pranavkm, can you please work on a fix, so I can apply for approval? Thanks!