How can I write a conditional lock in C#?

Action doThatThing = someMethod;

if (condition)
{
  lock(thatThing)
  {
     doThatThing();
  }
}
else
{
  doThatThing();
}

private static object lockHolder = new object();

if (ActionIsValid()) {
  lock(lockHolder) {
    if (ActionIsValid()) {
       DoSomething();
    }
  }
}

if (someCondition) {
            lockObj.EnterReadLock();
            try { Foo(); }
            finally { lockObj.ExitReadLock(); }
        } else {
            lockObj.EnterWriteLock();
            try { Foo(); }
            finally { lockObj.ExitWriteLock(); }
        }

bool locked = false;
if (condition) {
    Monitor.Enter(lockObject);
    locked = true;
}
try {
    // possibly critical section
}
finally {
    if (locked) Monitor.Exit(lockObject);
}

private Monkey GetScaryMonkey(int numberOfHeads){
    Monkey ape = null;
    lock(this) {
        ape = new Monkey();
        ape.AddHeads(numberOfHeads);
    }
    return ape;
}

private Monkey GetScaryMonkey(int numberOfHeads){
    if ( numberOfHeads > 1 ) {
         lock(this) {
            return CreateNewMonkey( numberOfHeads );
        }
    }
    return CreateNewMonkey( numberOfHeads );
}