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.NET 4: Monitor.Enter replaced by Monitor.Enter

Mon, August 31, 2009, 06:27 PM under dotNET
The most common way to protect resources from concurrent access from multiple threads in .NET is by using the C# lock (or SyncLock in VB) statement. 
lock(obj)
{
 // only 1 thread executes this at a time
}
Under the covers it generates code that is equivalent to:
    Monitor.Enter(obj);
    try
    {
 // only 1 thread executes this at a time
    }
    finally
    {
        Monitor.Exit(obj);
    }
If you compile the above under Visual Studio 2010 and look at the code generation in your favorite disassembler (e.g. Reflector before it gets a chance to be updated for .NET 4 and hide the detail) you'll find different code generation similar to this:
    bool taken = false;
    try
    {
        Monitor.Enter(obj, ref taken);
        // only 1 thread executes this at a time
    }
    finally
    {
        if (taken)
        {
            Monitor.Exit(obj);
        }
    }
The most important thing to note is the introduction of a new overload for Monitor.Enter (and there is one for TryEnter too) which are public so you can even use them directly.

Instead of explaining here what problem the new APIs are trying to address (asynchronous exceptions occurring just before the try block and after the call to Monitor.Enter), I defer you to Joe's old blog post on orphaned locks.

Don't be too surprised if the original Monitor.Enter overloads (without the boolean parameter) become obsolete.
Comments [2] |#Permalink
Tuesday, 01 September 2009 07:23:48 (Pacific Daylight Time, UTC-07:00)
They do have the obsolete attribute on them now.
justinmchase
Wednesday, 09 September 2009 18:59:53 (Pacific Daylight Time, UTC-07:00)
justinmchase: That's what I get for sitting on the draft blog post for way too long… Moreover, this has already been covered in more detail on this blog post:
http://blogs.msdn.com/ericlippert/archive/2009/03/06/locks-and-exceptions-do-not-mix.aspx
Daniel Moth
Comments are closed.