ASP0004: Do not use action results with route handlers

Cause

A route handler delegate returns a value that implements xref:Microsoft.AspNetCore.Mvc.IActionResult.

Rule description

Route handler endpoints do not support executing MVC's IActionResult instances. Returning an IActionResult that doesn't implement IResult results in serializing the result instance rather than executing the result.

app.MapGet("/todos/{id}", (int id) => new JsonResult(new Todo { .. }));

How to fix violations

To fix a violation of this rule, make sure that endpoint's route handler returns an xref:Microsoft.AspNetCore.Http.IResult type by using the xref:Microsoft.AspNetCore.Http.Results extension methods.

app.MapGet("/todos/{id}", (int id) => Results.Json(new Todo { .. }));

When to suppress warnings

Do not suppress a warning from this rule. Returning an IActionResult that doesn't implement IResult results in serializing the result instance rather than executing the result.