ASP.NET Core如何将任何类型转换为ActionResult <T> 控制器动作的返回类型？

Question

I have simple action in WebApi controller in ASP.NET Core 2.2 that looks like this:

[HttpGet("test123")]
public ActionResult<string> Test123()
{
    return new OkResult();
}

This compiles fine but I am wondering how is it possible that OkResult object is converted to ActionResult<string> ?

These classes have different inheritance chain: OkResult -> StatusCodeResult -> ActionResult while ActionResult<TValue> only implements IConvertToActionResult In other words, ActionResult<string> is not base type for OkResult class.

If I do that manually and change code to:

[HttpGet("test123")]
public ActionResult<string> Test123()
{
    var a = new OkResult();
    var b = a as ActionResult<string>;  // Error CS0039

    return b;
}

the code wont compile with the conversion error:

Error CS0039: Cannot convert type 'Microsoft.AspNetCore.Mvc.OkResult' to 'Microsoft.AspNetCore.Mvc.ActionResult' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

How is it possible that first code works while the second doesn't? How is return type converted from objects that don't have common base type?

Answer 1

The following two implicit operators from ActionResult<TValue>

/// <summary>
/// Implictly converts the specified <paramref name="value"/> to an <see cref="ActionResult{TValue}"/>.
/// </summary>
/// <param name="value">The value to convert.</param>
public static implicit operator ActionResult<TValue>(TValue value)
{
    return new ActionResult<TValue>(value);
}

/// <summary>
/// Implictly converts the specified <paramref name="result"/> to an <see cref="ActionResult{TValue}"/>.
/// </summary>
/// <param name="result">The <see cref="ActionResult"/>.</param>
public static implicit operator ActionResult<TValue>(ActionResult result)
{
    return new ActionResult<TValue>(result);
}

Source

Is what allows for the use of multiple return types in the action.

[HttpGet("test123")]
public ActionResult<string> Test123() {
    if(someCondition) return "String value"; //<--String
    return Ok(); //<-- OkResult
}

When the string is returned the ActionResult<TValue>(TValue value) operator is invoked, returning a valid ActionResult<TValue> and vice versa of the other operator.

Answer 2

Your first example takes advantage of an implicit user-defined type conversion operator, which looks like this :

public static implicit operator ActionResult<TValue>(ActionResult result)
{
    return new ActionResult<TValue>(result);
}

Your second example, using as , is unable to use the implicit conversion operator as, according to the docs , it:

...performs only reference conversions, nullable conversions, and boxing conversions. The as operator can't perform other conversions, such as user-defined conversions, which should instead be performed by using cast expressions.