NullReferenceException when api return null result.

[lovelangy]

To Reproduce

Steps to reproduce the behavior:

1. create an asp.net api project with version 2.2
2. add code

   [Route("api/[controller]")]
   [ApiController]
   public class ValuesController : ControllerBase
   {
     [Route("test")]
       public Task<string> Testapi()
       {           
               return null;
       }
     }

3. test the api with browser
4. it returns error

NullReferenceException: Object reference not set to an instance of an object.
lambda_method(Closure , object )
Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.GetAwaiter()
Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor+AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, object controller, object[] arguments)
System.Threading.Tasks.ValueTask<TResult>.get_Result()
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(HttpContext httpContext)

Expected behavior

NullReferenceException should be handled propertly and return some useful information.

[dpr-dev]

we can return some message, for example: "an action from MyGreatController returns zero"

[dpr-dev]

in netcore 3.0 i can't reproduce this. the action returns 404 and make redirect from 'localhost/url' to 'localhost'

[ApiController]
    [Route("api/[controller]")]
    public class SampleDataController : ControllerBase
    { 
        [HttpGet("[action]")]
        public Task<IEnumerable<WeatherForecast>> WeatherForecasts()
        {
            return null; 
        } 
    }

UDP: When I change Task<IEnumerable> to the Task I get an exception

[mkArtakMSFT]

Thanks for contacting us, @lovelangy.
What you see is actually by-design. What's happening is the method return type is Task<string> and when you return null, it's the same as:

Task<string> result = null;
return result;

When the ActionMethodExecutor is trying to evaluate the result of the action being called it tries to await for that Task<string> result, which is obviously null. Hence the NullReferenceException you observe.

You can instead return Task.FromResult<string>(null) and that would properly return.

As for the behavior in 3.0, we did add additional logic to treat null returns as 404 results.

//cc @pranavkm

[rynowak]

As for the behavior in 3.0, we did add additional logic to treat null returns as 404 results.

This is news to me, is there an issue tracking that?

[dpr-dev]

Thanks for contacting us, @lovelangy.
What you see is actually by-design. What's happening is the method return type is Task<string> and when you return null, it's the same as:

Task<string> result = null;
return result;

When the ActionMethodExecutor is trying to evaluate the result of the action being called it tries to await for that Task<string> result, which is obviously null. Hence the NullReferenceException you observe.

You can instead return Task.FromResult<string>(null) and that would properly return.

As for the behavior in 3.0, we did add additional logic to treat null returns as 404 results.

//cc @pranavkm

but why asp net core make redirect ?

[dpr-dev]

@pranavkm didn't we made a change to treat null results in API controllers as NotFound -s ?

the server returns 404, but the URL changes from "https: // localhost: 44379 / api / SampleData / test2" to "https: // localhost: 44379" and return 204

[rynowak]

I suspect that what's happening here is the action is throwing and your error handler is what's returning a 404.

[mkArtakMSFT]

So looks like I referred to something which we never did. So I'm removing my statement above regarding 3.0 to avoid spreading wrong information out.
I had a chat with @rynowak regarding this and what is most probably happening in your case is you're hitting the same exception as you do in 2.2, but... (and @rynowak just responded :) )