How can I write a conditional lock in C#?

I think that question cries "race condition!". What if the condition turns from true to false shortly after the check, but before a thread enters the critical section of code? Or while a thread is in the process of executing it?

I'm no threading expert, but it sounds like you might be looking for something like this (double-checked locking). The idea is to check the condition both before and after acquiring the lock.

private static object lockHolder = new object();

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

Action doThatThing = someMethod;

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