Guidelines of when to use locking

Question

I would like to know if there are any guidelineswhich a developer should follow as to when (and where) to place locks.

For instance: I understand that code such as this should be locked, to avoid the possibility of another thread changing the value of SomeHeapValue unexpectedly.

class Foo
{
  public SomeHeapObject myObject;
  public void DoSummat(object inputValue_)
  {
    myObject.SomeHeapValue = inputValue_;
  }

}

My question is, however, how deep does one go with the locking? For instance, if we have this code:

class Foo
{
  public SomeHeapObject myObject;
  public void DoSummat(object inputValue_)
  {
    myObject.SomeHeapValue = GetSomeHeapValue();
  }

}

Should we lock in the DoSummat(...) method, or should we lock in the GetSomeHeapValue() method?

Are there any guidelines that you all keep in mind when strcturing multi-threaded code?

Answer 1

The best guide for locking and threading I found, is this page (this is the text I consult when working with locking and threading):

http://www.albahari.com/threading/

You want the paragraph "Locking and Thread Safety", but read the rest as well, it is very well written.

Answer 2

• Lock as little as possible, but as much as needed.

• Avoid locks when possible - in .NET 4.0 there are alternatives that are not causing a context switch.

• Try not to lock multiple times. Structure your API accordingly. For example a queue. DeQueue - make an alternative DeQueue(int amount) that can dequeue many items with one lock.