ASP.net 核心 API 返回 ActionResult<t> 不强制返回类型</t>

Question

Where an API is returning an ActionResult<T> there is no compile or run-time checks to ensure that the data type being returned is as declared.

For example:

public ActionResult<Dog> GetCat()
{
    var cat = new Cat("Moggy");
    return Ok(cat);
}

This compiles, runs and returns a Cat object.

The problem I have is that the OpenAPI defintion (Swagger/Swashbuckle) is being derived from the type declared on the method. This means the OpenAPI defintion can be out of sync with the code and there is no obvious way to find these instances.

In the example given, my API documentation would tell people that this API returns a Dog when really it returns a Cat.

Is there any way to enforce the return type at compile time, or failing that at run-time?

Answer 1

That is because your returned type OkObjectResult inherits the ActionResult . The returned child class does not impact the Parent class compilation.This is by design.

More details about the two class you could refer to the following source code:

1.The Ok method returns type of OkObjectResult :

// Summary:
//     A base class for an MVC controller without view support.
[Controller]
public abstract class ControllerBase
{       
      // Summary:
    //     Creates an Microsoft.AspNetCore.Mvc.OkObjectResult object that produces an Microsoft.AspNetCore.Http.StatusCodes.Status200OK
    //     response.
    //
    // Parameters:
    //   value:
    //     The content value to format in the entity body.
    //
    // Returns:
    //     The created Microsoft.AspNetCore.Mvc.OkObjectResult for the response.
    [NonAction]
    public virtual OkObjectResult Ok([ActionResultObjectValueAttribute] object value);

2.The OkObjectResult inherits ObjectResult :

    //
// Summary:
//     An Microsoft.AspNetCore.Mvc.ObjectResult that when executed performs content
//     negotiation, formats the entity body, and will produce a Microsoft.AspNetCore.Http.StatusCodes.Status200OK
//     response if negotiation and formatting succeed.
[DefaultStatusCode(200)]
public class OkObjectResult : ObjectResult
{
    //
    // Summary:
    //     Initializes a new instance of the Microsoft.AspNetCore.Mvc.OkObjectResult class.
    //
    // Parameters:
    //   value:
    //     The content to format into the entity body.
    public OkObjectResult(object value);
}

3.The ObjectResult inherits ActionResult :

public class ObjectResult : ActionResult, IStatusCodeActionResult, IActionResult    
{
   public ObjectResult(object value);
   //..
}

Is there any way to enforce the return type at compile time, or failing that at run-time?

ActionResult<T> enables you to return a type deriving from ActionResult or return a specific type.

To enforce compile error,you could just return cat which is a specific type.And you could see the error message disappear when you change Dog to Cat :

public ActionResult<Dog> GetCat()
{
    var cat = new Cat("Moggy");
    return cat;
}

Result: