Is it good to test a condition then lock then re-test the condition [duplicate]

Question

Possible Duplicate:
Double-checked locking in .net

EDIT: lots of edits to clarify this question is not about singleton

I find myself writing code like this:

    if(resourceOnDiskNeedsUpdating)
    {
        lock(lockObject)
        {
            if(resourceOnDiskNeedsUpdating) // has a previous thread already done this?
                UpdateResourceOnDisk();
        }
    }
    return LoadResourceFromDisk();

UpdateResource() is a slow operation. Does this pattern make sense?
Are there better alternatives?

Answer 1

This called "double-checked locking".

You need a memory fence to make it correct.

See Wikipedia's article on double checked locking in .NET

Answer 2

An alternative I use would be to just use the 'volatile' keyword. http://msdn.microsoft.com/en-us/library/x13ttww7%28v=vs.71%29.aspx