Generic Type parameter of ActionResult doesn't enforce strict type.

Describe the bug

When you set a Type on the Generic Parameter of ActionResult and return any type other than that - it would still proceed. I mean, should this be expected? or not? or am I implementing web APIs incorrectly?

To Reproduce

1. dotnet new webapi
2. Create a controller with a method that ActionResult is used with a generic parameter and return a different type.

class Sample {}

[HttpGet("{id}")]
public ActionResult<Sample> GetCourt(int id)
{
    return Ok(1);
}

Exceptions (if any)

Further technical details

• ASP.NET Core version - 5.0 but this actually happens on different versions
• Include the output of dotnet --info - down below
• The IDE (VS / VS Code/ VS4Mac) you’re running on, and it’s version - VS19

dotnet info:

.NET SDK (reflecting any global.json):
 Version:   5.0.100
 Commit:    5044b93829

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.100\

Host (useful for support):
  Version: 5.0.0
  Commit:  cf258a14b7

.NET SDKs installed:
  5.0.100 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

[davidfowl]

We should try out best to improve the analyzer to fix this. If we can catch simple enough cases

[javiercn]

@javiercn thanks for contacting us.

I believe the that ActionResult<T> is only used for the purposes of being able to express the return type on the method signature.

Other action results convert implicitly to ActionResult<T> so I believe the behavior is by design and we don’t intend to enforce it, since we want to be able to let you return alternative object types in some cases, like when there is an error.

/cc: @pranavkm can confirm.