Support for route handler filters in minimal APIs issue

[axzxs2001]

.net version is 7.0.100-preview.3.22179.4

The following code will work：

string SayHello(string name) => $"Hello, {name}!.";
app.MapGet("/hello/{name}", SayHello)
    .AddFilter(async (RouteHandlerInvocationContext context, RouteHandlerFilterDelegate next) =>
    {
        return await next(context);
    });

But the following code doesn't work：

app.MapGet("/hello/{name}", (string name) => $"Hello, {name}!.")
    .AddFilter(async (RouteHandlerInvocationContext context, RouteHandlerFilterDelegate next) =>
    {
        return await next(context);
    });

System.NullReferenceException:“Object reference not set to an instance of an object.”

[martincostello]

I've just come across this too, but with .AddFilter<T>(). You can repro using the unit tests of the aspnetcore-41188 branch of this repo.

For example, the filter is added here:

// Get all Todo items
builder.MapGet("/api/items", async (
    ClaimsPrincipal user,
    ITodoService service,
    CancellationToken cancellationToken) =>
    {
        return Results.Ok(await service.GetListAsync(user.GetUserId(), cancellationToken));
    })
    .AddFilter<RateLimitFilter>()
    .ProducesProblem(StatusCodes.Status429TooManyRequests)
    .RequireAuthorization();

The NullReferenceException is thrown when next is awaited here:

var rateLimit = GetRateLimitPolicy(userId, operation);

try
{
    return await rateLimit.ExecuteAsync(async () => await next(context));
}
catch (RateLimitRejectedException ex)
{
    return Results.Extensions.RateLimited(ex.RetryAfter);
}

 Can_Manage_Todo_Items_With_Api
   Source: ApiTests.cs line 46
   Duration: 14 ms

  Message: 
System.Net.Http.HttpRequestException : Response status code does not indicate success: 500 (Internal Server Error).

  Stack Trace: 
HttpResponseMessage.EnsureSuccessStatusCode()
HttpClientJsonExtensions.GetFromJsonAsyncCore[T](Task`1 taskResponse, JsonSerializerOptions options, CancellationToken cancellationToken)
ApiTests.Can_Manage_Todo_Items_With_Api() line 52
--- End of stack trace from previous location ---

  Standard Output: 
[2022-04-15 14:04:00Z] dbug: AspNet.Security.OAuth.GitHub.GitHubAuthenticationHandler[1]
      HandleChallenge with Location: https://localhost:443/sign-in-github?code=3dc3e33a-c5d8-4413-9f7b-cb546fcab89b&state=CfDJ8E9WBL1MnW1Nn2fpw_o_MnEBnqeKTHVhuSSQawRf_Ipcj9vIb8GU5SbWI9heULZrYC-ccdmtJwWVo4v9JjSS-Sg1f62oqJu4KB7ZQ4SwQNJOLP0lmLLkgeYeaaS6m1sZcFcdXfrRwUZ_Q6jtMtNhnXlUVXI7hBMnZfb2oxKpnw9_Vl5KsMIYsgUGzdLqaL6RpH_ZattA2dJsd_GLT3PU4yI; and Set-Cookie: .AspNetCore.Correlation.FLc18UATvq8WEyiZUwuuOYK1E8Zz8J1WJLHJvXdBRdg=N; expires=Fri, 15 Apr 2022 14:19:00 GMT; path=/sign-in-github; secure; samesite=none; httponly.
[2022-04-15 14:04:00Z] info: AspNet.Security.OAuth.GitHub.GitHubAuthenticationHandler[12]
      AuthenticationScheme: GitHub was challenged.
[2022-04-15 14:04:00Z] fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method81(Closure, RouteHandlerInvocationContext)
   at TodoApp.RateLimitFilter.<>c__DisplayClass7_0.<<InvokeAsync>b__0>d.MoveNext() in C:\Coding\martincostello\polly-rate-limiting\src\TodoApp\RateLimitFilter.cs:line 31
--- End of stack trace from previous location ---
   at Polly.RateLimit.AsyncRateLimitEngine.ImplementationAsync[TResult](IRateLimiter rateLimiter, Func`3 retryAfterFactory, Func`3 action, Context context, CancellationToken cancellationToken, Boolean continueOnCapturedContext)
   at Polly.AsyncPolicy.ExecuteAsync[TResult](Func`3 action, Context context, CancellationToken cancellationToken, Boolean continueOnCapturedContext)
   at TodoApp.RateLimitFilter.InvokeAsync(RouteHandlerInvocationContext context, RouteHandlerFilterDelegate next) in C:\Coding\martincostello\polly-rate-limiting\src\TodoApp\RateLimitFilter.cs:line 31
   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.Authorization.Policy.AuthorizationMiddlewareResultHandler.HandleAsync(RequestDelegate next, HttpContext context, AuthorizationPolicy policy, PolicyAuthorizationResult authorizeResult)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

[BrennanConroy]

I've just come across this too, but with .AddFilter<T>().

Right, because the issue is with the generated code and how we handle inline delegates vs. methods. So the workaround until preivew4 is to move your handler code into a method.