Locks held concurrently should be locked in a consistent sequence, otherwise the program can deadlock. This rule detects nested lock statements that lock variables in a different sequence in different parts of the program.

This problem can be avoided by ensuring that nested lock statements always lock variables in the same sequence.

The following example shows a program running two threads, which deadlocks because thread1 holds lock1 and is waiting to acquire lock2, whilst thread2 holds lock2 and is waiting to acquire lock1.

This problem is resolved by reordering the lock variables as shown below.
  • MSDN, C# Reference: lock Statement.
  • The CERT Oracle Coding Standard for Java: LCK07-J. Avoid deadlock by requesting and releasing locks in the same order.