Question

Possible Duplicate:
Why is lock(this) {…} bad?

In C# to make a critical region thread safe we can use lock() statement. The lock statement takes an object. What is wrong if we pass this to the lock statement?

Answer 1

From http://msdn.microsoft.com/en-us/library/c5kehkcz.aspx:

In general, avoid locking on a public
type, or instances beyond your code’s
control. The common constructs lock
(this), lock (typeof (MyType)), and
lock ("myLock") violate this
guideline:

• lock (this) is a problem if the
  instance can be accessed publicly.
• lock (typeof (MyType)) is a problem if
  MyType is publicly accessible.
• lock(“myLock”) is a problem because
  any other code in the process using
  the same string, will share the same
  lock.

Best practice is to define a
private object to lock on, or a
private static object variable to
protect data common to all instances.