ECMA-334 C# Language Specification

15.12: The lock statement

The lock statement obtains the mutual-exclusion lock for a given object, executes a statement, and then releases the lock.

lock-statement
lock ( expression ) embedded-statement

The expression of a lock statement must denote a value of a reference-type. No implicit boxing conversion (13.1.5) is ever performed for the expression of a lock statement, and thus it is a compile-time error for the expression to denote a value of a value-type.

A lock statement of the form 
lock (x) ...
where x is an expression of a reference-type, is precisely equivalent to 
System.Threading.Monitor.Enter(x);  
try {  
   ...  
}  
finally {  
   System.Threading.Monitor.Exit(x);  
}
except that x is only evaluated once.

[Example: The System.Type object of a class can conveniently be used as the mutual-exclusion lock for static methods of the class. For example: 
class Cache  
{  
   public static void Add(object x) {  
      lock (typeof(Cache)) {  
         ...  
      }  
   }  
   public static void Remove(object x) {  
      lock (typeof(Cache)) {  
         ...  
      }  
   }  
}
end example]