问题描述

让我们想象一下简单的委托调用:

Lets imagine simple delegate calls:

void Main()
{
    Func<int, int, string> tfunc = null;
    tfunc += Add; // bind first method
    tfunc += Sub; // bind second method 

    Console.WriteLine(tfunc(2, 2));
}

private string Add(int a, int b)
{
    return "Add: " + (a + b).ToString();
}

private string Sub(int a, int b)
{
    return "Sub: " + (a - b).ToString();
}

这个程序的结果是:

Sub: 0

那么，为什么没有调用 Add 方法呢?我期待调用方法 Add，然后 然后 方法 Sub.

So, why Add method was not called? I'm expecting to call Method Add, and then method Sub.

推荐答案

Add被正确链接调用，看看结果

Add was correctly chained and called, take a look at the result of

void Main()
{
    Func<int, int, string> tfunc = null;
    tfunc += Add; // bind first method
    tfunc += Sub; // bind second method 

    Console.WriteLine(tfunc(2, 2));
}

private string Add(int a, int b)
{
    Console.WriteLine("Inside Add");
    return "Add: " + (a + b).ToString();
}

private string Sub(int a, int b)
{
    Console.WriteLine("Inside Sub");
    return "Sub: " + (a - b).ToString();
}

它是:

Inside Add
Inside Sub
Sub: 0

没有被链接，因为没有办法访问它，是 Add 方法的结果.返回值的委托，在链接的情况下，返回最后调用的方法的值，即添加到委托的最后一个方法.

What is not chained, because there is no way to access it, is the result of the Add method. Delegates that return a value, in case of chaining, return the value of the last method invoked, that is the last method that was added to the delegate.

这在 C# 4.0 语言规范 的第 15.4 部分中指定

This is specified in part 15.4 of the C# 4.0 language specification

调用一个委托实例，其调用列表包含多个条目通过调用调用列表，同步，按顺序....如果委托调用包含输出参数或返回值，它们的最终值将来自于列表中的最后一位代表.

Invocation of a delegate instance whose invocation list contains multiple entries proceeds by invoking each of the methods in the invocation list, synchronously, in order. ... If the delegate invocation includes output parameters or a return value, their final value will come from the invocation of the last delegate in the list.

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