Route handler filters don't handle all return types

[BrennanConroy]

Filters currently wrap the user delegate in a ValueTask<object?> before handing off the function to the normal route handling logic. This causes issues with any non object returning methods like void, Task, Task<T>, etc. returning methods.

Void returning method with no-op filter:

void Method() { }
app.MapGet("/method", Method).AddFilter((c, n) => n(c));

Output:

ArgumentException: Expression of type 'System.Void' cannot be used for parameter of type 'System.Object' of method 'System.Threading.Tasks.ValueTask`1[System.Object] WrapObjectAsValueTask(System.Object)' (Parameter 'arg0')

Task returning method with no-op filter:

Task Method() => Task.CompletedTask;
app.MapGet("/method", Method).AddFilter((c, n) => n(c));

Output:

{"result":{},"id":54,"exception":null,"status":5,"isCanceled":false,"isCompleted":true,"isCompletedSuccessfully":true,"creationOptions":0,"asyncState":null,"isFaulted":false}

[lohithgn]

@BrennanConroy @danroth27
I cant event compile the code snippet provided in Preview 3 blog post:

app.MapGet("/hello/{name}", (string name) => $"Hello, {name}!")
    .AddFilter((context, next) =>
    {
        var name = (string) context.Parameters[0];
        if (name == "Bob")
        {
            return Result.Problem("No Bob's allowed");
        }
        return next(context);
    });

It throws up error: cannot convert type IResult to ValueTask<Object?>

I found @captainsafia example here: https://github.com/captainsafia/MinimalFiltersExample/blob/main/Program.cs

Code compiles but i have a run time exception for next delegate:

Stack trace:

System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method2(Closure, RouteHandlerInvocationContext)
   at Program.<>c__DisplayClass0_0.<<<Main>$>b__3>d.MoveNext() in C:\w\aspnet\WebApplication1\Program.cs:line 13
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Http.RequestDelegateFactory.ExecuteObjectReturn(Object obj, HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

How do we get this to work ?

[BrennanConroy]

The null ref is issue #41188, you can workaround it by using a method instead of an inline delegate.