使用三元运算符了解 C# 编译错误 [英] Understanding C# compilation error with ternary operator

问题描述

我已从 Wrox Professional ASP.NET 4.0 MVC 4 一书第 179 页(了解 Web 应用程序中的安全向量"一章)中复制了以下代码，并对其进行了少量修改 protected 并在我抽象的应用程序范围 Controller

I have copied the following code from Wrox Professional ASP.NET 4.0 MVC 4 book, page 179 (Chapter "Understanding the Security Vectors in a Web Application") with the little modification of making it protected and storing as utility method in my abstract application-wide Controller

protected ActionResult RedirectToLocal(string returnUrl)
{
    if (Url.IsLocalUrl(returnUrl))
    {
        return Redirect(returnUrl);
    }
    else
    {
        return RedirectToAction("Index", "Home");
    }
}

上面的代码旨在保护 MVC 应用程序免受开放重定向攻击，这不是问题的主题.

The code above is aimed at securing the MVC application from open redirection attacks, which are not subject of the question.

代码显然格式良好，可以编译并且我相信它可以工作.

The code is obviously well-formed, compiles and I trust it works.

问题出现在将上面的代码巧妙地"改成下面一行代码时

The problem arises when "smartly" changing the code above into the following one-liner

return (Url.IsLocalUrl(returnUrl)) ? Redirect(returnUrl) : RedirectToAction("Index", "Home");

上面的单行代码应该和扩展代码完全一样(不，ReSharper 没有建议我替换，这是我的主动).

The one-liner above is supposed to do exactly the same as the extended code (no, ReSharper didn't suggest me to replace, it was my initiative).

编译错误如下:System.Web.Mvc.RedirectResult 和 System.Web.Mvc.RedirectToRouteResult 之间没有隐式转换.

ReSharper 然后来帮忙并建议以下修改

ReSharper then comes to help and suggests the following modification

return (Url.IsLocalUrl(returnUrl)) ? (ActionResult) Redirect(returnUrl) : RedirectToAction("Index", "Home");

问题是为什么我必须强制转换 Redirect 方法"?

由于 Redirect 和 RedirectToAction 都返回 ActionResult 的子类(通过 F12 验证)，并且该子类是函数的返回值，它应该是自动兼容的.或者至少，以我对 C# 的了解，要么两个代码都能编译，要么都不能.

The question is "why do I have to cast Redirect method"?

Since both Redirect and RedirectToAction return a subclass of ActionResult (verified via F12) and that subclass is the return value of the function, it should be automatically compatible. Or at least, in my knowledge of C#, either both codes compile or they both don't.

更笼统地说，这个问题可以重新表述如下:

To be more general, the question could be reformulated as follows:

假设我有 A、B 和 C 类

Suppose I have class A, B and C

abstract class A {}
class B: A{}
class C: A{}

假设以下函数有效

private A Function(){
    if (condition) return new B();
    else return new C();
}

为什么下面的单行代码不能编译?

Why the following one-liner doesn't compile?

private A Function(){
    return (condition) ? new B(): new C();
}

推荐答案

它无法编译，因为编译器根据两个返回值之一决定单行的返回类型.由于类型 B 不派生自类型 C，或者相反，这两种类型并不相互排斥，并且 if(作为一个整体)的返回类型不能以这种方式派生.

It doesn't compile because the compiler decides what the return-type of the one-liner is based upon one of the two return-values. Since type B doesn't derive of type C, or the other way around, the two types are not mutually exclusive, and the return type of the if (as a whole) cannot be derived that way.

如果您想以单行方式执行此操作，则必须将返回值强制转换为您要返回的类型，即类型 A.

If you would want to do it in a one-liner, you have to cast the return-values to the type that you want to return, which is type A.

private A Function(){
    return (condition) ? ((A)new B()): ((A)new C());
}

更新:我没有看到你已经在你的问题中做了演员，我很抱歉.

UPDATE: I didn't see that you already do the casting in your question, my apologies.

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