Only one user at a time

You can use the lock syntax to provide thread syncronisation

http://msdn.microsoft.com/en-us/library/c5kehkcz.aspx[^]

That's to do with threading, rather than 'user' actions though.

For your comment on Gregory.Gadow's answer, if you wanted an ID to be unique you'd certainly leave that to an underlying database operation (an Identity field) to allocate.

Are you talking about lock keyword which .net framework provide.

e.g.

object objLock = new object();
lock(objLock)
{
    //Do the required stuff here.
}