c# - 委托(delegate)分配是否在 C# 中创建新副本？

Question

我读了一篇关于 C# 和性能注意事项的文章 ( here )

文中说委托(delegate)赋值会触发内存分配，例如:

every single local variable assignment like "Func fn = Fn" creates a new instance of the delegate class Func on the heap

我想知道这是否属实，如果属实 - 如何实现？我不熟悉引用赋值可以在 C# 中触发额外内存分配的任何方式。

Answer 1

这篇文章是对的。很容易测试:

static void Main(string[] args)
{
    Action<string[]> main = Main;
    Action<string[]> main2 = Main;
    Console.WriteLine(object.ReferenceEquals(main, main2)); // False
}

http://ideone.com/dgNxPn

如果您查看生成的 IL 代码 http://goo.gl/S47Wfy , 很清楚会发生什么:

    IL_0002: ldftn void Test::Main(string[])
    IL_0008: newobj instance void class [mscorlib]System.Action`1<string[]>::.ctor(object, native int)
    IL_000d: stloc.0
    IL_000e: ldnull

    IL_000f: ldftn void Test::Main(string[])
    IL_0015: newobj instance void class [mscorlib]System.Action`1<string[]>::.ctor(object, native int)
    IL_001a: stloc.1

所以有两个 newobj instance void class [mscorlib]System.Action1::.ctor(object, native int)`

请注意，您是对的，这是违反直觉的:

public class TestEvent
{
    public event Action Event;

    public TestEvent()
    {
        Action d1 = Print;
        Action d2 = Print;

        // The delegates are distinct
        Console.WriteLine("d1 and d2 are the same: {0}", object.ReferenceEquals(d1, d2));

        Event += d1;
        Event -= d2;

        // But the second one is able to remove the first one :-)
        // (an event when is empty is null)
        Console.WriteLine("d2 was enough to remove d1: {0}", Event == null);
    }

    public void Print()
    {
        Console.WriteLine("TestEvent");
    }
}

以 event 为例，您可以使用“等效但不相同”的委托(delegate)来删除另一个委托(delegate)，如示例所示。参见 https://ideone.com/xeJ6LO