Warning AD0001 : Analyzer 'Microsoft.AspNetCore.Analyzers.RouteHandlers.RouteHandlerAnalyzer' NullReferenceException due to dynamic in method signature

[peeveen]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Warning is shown when building a webapp that uses a dynamic parameter in a minimal API endpoint.

AD0001: Analyzer 'Microsoft.AspNetCore.Analyzers.RouteHandlers.RouteHandlerAnalyzer' threw an exception of type 'System.NullReferenceException' with message 'Object reference not set to an instance of an object.'.

Expected Behavior

No warning.

Steps To Reproduce

A minimal API webapp, with the following type of endpoint:

endpointRouteBuilder.MapPost("/blah",
    async (
        ... various params ...
        [FromBody] dynamic someOtherThing
    ) => {
        ... do stuff ...
    }
);

The [FromBody] is not really important. Same thing if I remove it.
It seems to be the dynamic parameter that causes it.
I know that using a dynamic here is kinda strange, but I have configured the services (via services.ConfigureHttpJsonOptions) with a custom JSON converter than can convert data to a dynamic.
Weirdly, after seeing this warning, repeating the operation (e.g. dotnet build) completes without the warning. It seems to only occur once immediately after a code change.

Exceptions (if any)

System.NullReferenceException

.NET Version

9.0.202

Anything else?

No response

[captainsafia]

Closing as a dupe of #56831.