why should we avoid lock(this)?

Answer 1

Because this is not encapsulated by the class and thus it is hard to reason about who locks on this. I.e. in order to find out what part of the code is locking on this you need to go through a lot. If, on the other hand, you restrict locking to a private member, it is easy to reason about where locking takes place.

Answer 2

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.