C# delegates, reference resolution time

问题:

I have a simple question about .net delegates. Say I have something like this:

    public void Invoke(Action<T> action)
    {
        Invoke(() => action(this.Value));
    }

    public void Invoke(Action action)
    {
        m_TaskQueue.Enqueue(action);
    }

The first function encloses a reference to this.Value. During runtime, when the first, method with generic parameter gets called, it will provide this.Value somehow to the second one, but how? These came into my mind:

• Call by value (struct) - the current value of this.Value gets passed, so if the m_TaskQueue executes it 5 minutes later, the value will not be in its recent state, it will be whatever it was when first referencing.
• Call by reference (reference type) - then the most recent state of Value will be referenced during execution of action but if I change this.Value to another reference before execution of action, it will still be pointing to the old reference
• Call by name (both) - where this.Value will be evaluated when the action gets called. I believe the actual implementation would be holding a reference to this then evaluate Value on that during actual execution of delegate since there is no call by name.

I assume it would be Call of name style but could not find any documentation so wondering if it is a well-defined behavior. This class is something like an Actor in Scala or Erlang so I need it to be thread safe. I do not want Invoke function to dereference Value immediately, that will be done in a safe thread for this object by m_TaskQueue.

回答1:

Let me answer your question by describing what code we actually generate for this. I'll rename your confusingly-named other Invoke method; it's not necessary to understanding what's going on here.

Suppose you said

class C<T>
{
  public T Value;
  public void Invoke(Action<T> action) 
  { 
      Frob(() => action(this.Value)); 
  } 
  public void Frob(Action action) 
  {  // whatever
  } 
}

The compiler generates code as though you had actually written:

class C<T>
{
  public T Value;

  private class CLOSURE
  {
     public Action<T> ACTION;
     public C<T> THIS;
     public void METHOD()
     {
       this.ACTION(this.THIS.Value);
     }
  }

  public void Invoke(Action<T> action) 
  { 
      CLOSURE closure = new CLOSURE();
      closure.THIS = this;
      closure.ACTION = action;
      Frob(new Action(closure.METHOD)); 
  } 
  public void Frob(Action action) 
  {  // whatever
  } 
}

Does that answer your question?

回答2:

The delegate stores a reference to the variable, not the value of it. If you want to keep the current value then (assuming it is a value type) you need to make a local copy of it:

public void Invoke(Action<T> action)
{
    var localValue = this.Value;
    Invoke(() => action(localValue));
}

If it is a mutable reference type you could make a local clone / deep copy.

回答3:

The real key is to remember that scope is lexical; it's something the compiler takes care of. So it captures variables, not their values. Whether those values are value types or reference types is another matter completely.

Maybe a slightly more extreme example of altering the behavior of the delegate will help:

var myVariable = "something";
Action a = () => Console.WriteLine(myVariable);
myVariable = "something else entirely"
a();

prints "something else entirely". In that light, it doesn't really matter how many times you wrap, save, or move the function; it still refers to the variable it enclosed. So, in short, what matters is the value of the enclosed variable when the delegate's actually executed.