Cannot implicitly convert type 'Microsoft.AspNetCore.Mvc.NotFoundResult' to 'System.Threading.Tasks.Task'

Question

Consider a controller action like this:

[HttpGet]
[Route("blog/{slug}")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public async Task<Blog> Get(string slug)
{
    return await Task.Run(() =>
    {
        var model = Data.GetBlog(slug);
        if (model == null)
        {
            return NotFound();
        }
        return Ok(model);
    });
}

I am getting an error for each of the returns:

Cannot implicitly convert type 'Microsoft.AspNetCore.Mvc.NotFoundResult' to 'System.Threading.Tasks.Task'
Cannot implicitly convert type 'Microsoft.AspNetCore.Mvc.OkObjectResult' to 'System.Threading.Tasks.Task'
Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type

If I comment out either one of the response types (so only have the NotFound() or only have the Ok()), it compiles fine.

I thought both inherit from same base class and so both could be used within the same action.

Where am I going wrong?

p.s. The Task.Run is me being lazy until I write the async Data methods.

Answer 1

The Task.Run code is returning IActionResult derived results that you try to return as a Blog type. Hence the error

This is what the ActionResult<T> is for.

Consider the following refactor.

[HttpGet]
[Route("blog/{slug}")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public async Task<ActionResult<Blog>> Get(string slug) {
    Blog model = await Task.Run(() => Data.GetBlog(slug));
    if (model == null) {
        return NotFound();
    }
    return model;    
}

Reference Controller action return types in ASP.NET Core Web API