Question

What is a best approach to make a function or set of statements thread safe in C#?

Answer 1

It depends on what you’re trying to accomplish.
If you want to make sure that in any given time only one thread would run a specific code use lock or Monitor:

public void Func(...)
{
   lock(syncObject)
   {
      // only one thread can enter this code
   }
}

On the other hand you want multiple threads to run the same code but do not want them to cause race conditions by changing the same point in memory don’t write to static/shared objects which can be reached by multiple at the same time.

BTW – If you want to create a static object that would be shared only within a single thread use the ThreadStatic attribute (http://msdn.microsoft.com/en-us/library/system.threadstaticattribute(VS.71).aspx).