Why doesn’t ActionResult<T> work in .NET Azure Functions isolated worker model?

Question

I recently upgraded a .NET Function App from the in-process worker model to the isolated worker model.
After the upgrade, I noticed that none of the HTTP-triggered APIs were returning any data.

On further investigation I found that the return types were declared as ActionResult<T>.
When I changed the return types to IActionResult, the APIs started working again.

Example

I created a simple example app using the isolated model to illustrate the issue.

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Worker;

namespace FunctionApp1;

public class Function1
{
    [Function("Function1")]
    public IActionResult Run([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequest req)
    {
        return new OkObjectResult("Hello World"); //Works fine
    }

    [Function("Function2")]
    public ActionResult<string> Run2([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequest req)
    {
        return "Hello World"; //Returning Nothing
    }

    [Function("Function4")]
    public ActionResult<string> Run4([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequest req)
    {
        return new OkObjectResult("Hello World"); //This is how the api's were returning data before, Returning nothing after update
    }

    [Function("Function3")]
    public async Task<ActionResult<MyReturnType>> Run3([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequest req)
    {
        return await Task.FromResult<MyReturnType>(new MyReturnType("Jack", "BlacK")); // Returning Nothing
    }

    public record MyReturnType(string Name, string Surname);
}

Questions

Is ActionResult<T> not supported in the isolated worker model?
Why can the runtime unwrap ActionResult<T> in the in-process model but not in the isolated worker model?
Am I missing some configuration, or is the only supported pattern to return HttpResponseData/IActionResult?

Your function works within the Function host, when operating in the in-process model, and it can depend on the MVC pipeline for features like content negotiation and automatic wrapping. ActionResult<T> functions there because the MVC layer is capable of converting the T to JSON, selecting a content type, and other tasks. Your code executes in a distinct process without the ASP.NET Core MVC pipeline when using the isolated worker model. The host has no idea what to do with ActionResult<T> — Akshay Kumar
– Akshay Kumar, Commented Sep 18 at 8:56

D A · Accepted Answer · 2025-09-18 10:48:24Z

For an Azure Function App to work with HTTP triggers there are two different approaches that can be used: ASP.NET Core integration model and built-in model.

Built-in HTTP model - the HTTP request message is translated into HttpRequestData object and the requested return type is HttpResponseData. Read more here.
ASP.NET Core integration - HTTP request and response objects using types from ASP.NET Core including HttpRequest, HttpResponse, and IActionResult.

Looking at your code (understanding that you did a migration to isolated model), you are using ASP.NET Core integration. This requires to :

Replace in your host builder ConfigureFunctionsWorkerDefaults with ConfigureFunctionsWebApplication
Add a reference in your project to the Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore package, version 1.0.0 or later.

Update your project to use these specific package versions:2

Microsoft.Azure.Functions.Worker.Sdk, version 1.11.0. or later Microsoft.Azure.Functions.Worker, version 1.16.0 or later.

Azure Functions aren't designed to work with MVC-style ActionResult<T> return types out of the box. Azure Functions use function bindings and return types that are specific to the Functions runtime. ActionResult<T> is part of ASP.NET Core MVC, the Functions runtime doesn't interpret ActionResult<T> the same way. As a result, the runtime doesn't know how to serialize or return the value properly, leading to an empty response. Your only solution is to use IActionResult as a return type or try to use a custom serializer like they describe it here.

Collectives™ on Stack Overflow

Why doesn’t ActionResult<T> work in .NET Azure Functions isolated worker model?

Example

Questions

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Example

Questions

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related