C# 委托（和字符串）的不变性是如何在底层实现的？

Question

在 C# 中，为了以线程安全的方式调用委托，我们可以使用以下代码：

public static void TestInvokeDelegate1()
{
    CustomClass cc1 = new CustomClass("cc1");
    Action action = cc1.WriteName;

    Action action2 = action;
    if(action2 != null)
    {
        action2();
    }
}

或者，在 C# 6.0 或更高版本中，我们也可以使用：

public static void TestInvokeDelegate2()
{
    CustomClass cc1 = new CustomClass("cc1");
    Action action = cc1.WriteName;

    action?.Invoke();
}

众所周知，这些代码使用委托的不变性来实现线程安全。但是不变性是如何在 .net 编译器或运行时完成的呢？我使用Ildasm工具获取这两种方法的IL代码：

.method public hidebysig static void  TestInvokeDelegate1() cil managed
{
  //        32 (0x20)
  .maxstack  2
  .locals init (class [System.Runtime]System.Action V_0)
  IL_0000:  ldstr      "cc1"
  IL_0005:  newobj     instance void ess_cs.CustomClass::.ctor(string)
  IL_000a:  ldftn      instance void ess_cs.CustomClass::WriteName()
  IL_0010:  newobj     instance void [System.Runtime]System.Action::.ctor(object,
                                                                          native int)
  IL_0015:  stloc.0
  IL_0016:  ldloc.0
  IL_0017:  brfalse.s  IL_001f
  IL_0019:  ldloc.0
  IL_001a:  callvirt   instance void [System.Runtime]System.Action::Invoke()
  IL_001f:  ret
} // end of method Program::TestInvokeDelegate1

和，

.method public hidebysig static void  TestInvokeDelegate2() cil managed
{
  //        32 (0x20)
  .maxstack  8
  IL_0000:  ldstr      "cc1"
  IL_0005:  newobj     instance void ess_cs.CustomClass::.ctor(string)
  IL_000a:  ldftn      instance void ess_cs.CustomClass::WriteName()
  IL_0010:  newobj     instance void [System.Runtime]System.Action::.ctor(object,
                                                                          native int)
  IL_0015:  dup
  IL_0016:  brtrue.s   IL_001a
  IL_0018:  pop
  IL_0019:  ret
  IL_001a:  callvirt   instance void [System.Runtime]System.Action::Invoke()
  IL_001f:  ret
} // end of method Program::TestInvokeDelegate2

我不精通 IL，但是在这些 IL 代码中，我看到的只是复制对对象的引用。通过简单地复制引用，两个变量都指向同一个对象，不可变性在哪里？没有复制对象数据的代码，它是如何实现的？ （也是字符串的不变性）

---

附加文本：我编写的用于测试字符串和委托的不变性的代码：

（注意：我知道以下代码的 C# 级别的所有内容，它是我编写的，用于测试字符串或委托的不变性。似乎在分配指向它的变量时复制了该对象到另一个变量。但是在IL代码中，我找不到对象复制代码。我想知道的是在编译器或运行时（clr）级别的不变性的内部实现。后面发生了什么？）

public static void TestStringImmutable()
{
    string s1 = "abc";
    string s2 = s1;
    s1 = "ab";
    Console.WriteLine(s1); //"ab"
    Console.WriteLine(s2); //"abc"

    s1 = "abcde";
    Console.WriteLine(s1); //"abcde"
    Console.WriteLine(s2); //"abc"
}

和，

public static void TestDelegateImmutable()
{
    CustomClass cc1 = new CustomClass("cc1");
    CustomClass cc2 = new CustomClass("cc2");

    Action action = cc1.WriteName;
    action += cc2.WriteName;
    Action action2 = action;
    action -= cc2.WriteName;

    action(); //Output: "cc1"
    action2(); //Output: "cc1" and "cc2"

    action -= cc1.WriteName;
    Console.WriteLine(action == null); //true
    Console.WriteLine(action2 == null); //false
    Console.WriteLine(action2.GetInvocationList().Length); //2
}

---

CustomClass 类：

class CustomClass
{
    private string m_name = null;

    public CustomClass(string name)
    {
        m_name = name;
    }

    public void WriteName()
    {
        Console.WriteLine(m_name);
    }
}

Answer 1

看看我的解释：

public static void TestDelegateImmutable()
{
    CustomClass cc1 = new CustomClass("cc1");
    CustomClass cc2 = new CustomClass("cc2");

    Action action = cc1.WriteName; //Allocate action to an object refers to cc1.WriteName.
    action += cc2.WriteName; //Allocate action to an new object refers to cc1.WriteName and cc2.WriteName.
    Action action2 = action; //Allocate action2 to the object of action has been created above.
    action -= cc2.WriteName; //Allocate action to an new object refers to cc1.WriteName.

    action(); //Output: "cc1"
    action2(); //Output: "cc1" and "cc2"

    action -= cc1.WriteName; //Allocate action to null.
    Console.WriteLine(action == null); //true
    Console.WriteLine(action2 == null); //false
    Console.WriteLine(action2.GetInvocationList().Length); //2
}

希望对你有意义。