Why C# compiler copies variable for locking?

Question

My original code is such like

Object mylock = new object();
void Test()
{
    lock(mylock)
    {
        
    }
}

Compiled into the following code

void Test
{
    object obj = mylock;  // add a temporary variable
    bool lockTaken = false;
    try
    {
        Monitor.Enter(obj, ref lockTaken);
    }
    finally
    {
        if (lockTaken)
        {
            Monitor.Exit(obj);
        }
    }
}

I want to know why add a temporary variable rather than use original variable directly as follows.

void Test
{
    bool lockTaken = false;
    try
    {
        Monitor.Enter(mylock, ref lockTaken);  // use original variable directly
    }
    finally
    {
        if (lockTaken)
        {
            Monitor.Exit(mylock);  // use original variable directly
        }
    }
}

Because otherwise mylock could be changed in another thread. And you really don't want to unlock the wrong object. — Jeremy Lakeman, Nov 30 '21 at 05:14

score 2 · Accepted Answer · answered Nov 30 '21 at 03:02

Several C# constructs (using, lock) get transformed into final code that way to prevent re-assingnment of the variable used. You really don't want finally section to act on something different than the value the original operation started with.

Sample of the code that would behave incorrectly if compiler simply uses current value of myLock in finally section:

 object myLock = new object();

 lock(myLock)
 {
    myLock = new object(); 
    ...
 }

Note that compiling such code also brings CS0728 warning because attempting to change lock variable will be ignored.

Why C# compiler copies variable for locking?

1 Answers1