基于编译时类型的运行时类型的调用方法 [英] Calling method based on run-time type insead of compile-time type

问题描述

在应用程序中，我需要.NET根据其运行时类型而不是其编译时类型来调用方法.

In an application, I need .NET to call a method based on its run-time type instead of its compile-time type.

简化示例:

    class A { }

    class B : A { }

    static void Main(string[] args)
    {
        A b = new B();
        Print(b);
    }

    static void Print(A a)
    {
        Console.WriteLine("Called from A");
    }

    static void Print(B b)
    {
        Console.WriteLine("Called from B");
    }

上面的代码实际上会打印Called from A，但是我需要它是Called from B.

The above code will actually print Called from A, but I need it to be Called from B.

这可以按预期工作:

static void Print(A a)
{
    var b = a as B;
    if (b != null)
       return Print(b);
    else
       Console.WriteLine("Called from A");
}

但是出于可维护性的考虑，这是不可取的.

But for maintainability's sake, it is not desirable.

我相信这个问题与此类似:

I believe this question is similar to this one: Why isn't this method chosen based on the runtime-type of its object?, but for .NET instead of Java.

推荐答案

如果您使用的是.NET 4或更高版本，最简单的方法是使用

The simplest approach if you're using .NET 4 or higher is to use dynamic typing:

dynamic b = new B();
Print(b);

几乎所有使用类型为dynamic的值的表达式都将被动态调用，"mini-C#编译器"将在执行时应用与编译时相同的规则，但是使用这些动态值的实际执行时间类型. (尽管其类型在编译时是静态已知的表达式仍会被视为具有这些类型-它不会使有关重载解析的所有内容变为动态.)

Almost all expressions using a value of type dynamic will be invoked dynamically, with a "mini-C# compiler" applying the same rules at execution time as it would have done at compile-time, but using the actual execution-time type of those dynamic values. (Expressions whose types are known statically at compile-time will still be regarded as having those types though - it doesn't make everything about overload resolution into dynamic.)

如果您不使用.NET 4，则难度会有所增加-您可以使用反射，也可以对选项进行硬编码，但都不有趣.

If you're not using .NET 4, it's somewhat harder - you could either use reflection, or hard-code the options, neither of which is fun.

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