Why doesn't C# allow a null value to be locked?

Question

C# doesn't allow locking on a null value. I suppose I could check whether the value is null or not before I lock it, but because I haven't locked it another thread could come along and make the value null! How can I avoid this race condition?

Answer 1

You cannot lock on a null value because the CLR has no place to attach the SyncBlock to, which is what allows the CLR to synchronize access to arbitrary objects via Monitor.Enter/Exit (which is what lock uses internally)

Answer 2

Lock on a value that is never null, e.g.

Object _lockOnMe = new Object(); Object _iMightBeNull; public void DoSomeKungFu() {     if (_iMightBeNull == null) {         lock (_lockOnMe) {             if (_iMightBeNull == null) {                 _iMightBeNull = ...  whatever ...;             }         }     } } 

Also be careful to avoid this interesting race condition with double-checked locking: Memory Model Guarantees in Double-checked Locking