>We have a backup function built in our c/s application that creates a backup file of one database, using SQL Server 2008 R2.
>At this moment we do not check if another user is using the database and perhaps is doing updates, which could be a problem when doing a backup at the same time. It is possible to set the database to single user mode, but other users won't get notified and would be logged off. Is there a way to check if other users are logged on, and if no other user is logged on, then we could set the database to single user mode to prevent someone else to log on until the backup is finished.

This may not be the exact reply you were expecting but I am curious to know why you would like to backup like this. I have never had any use like that to do the backup. They are all done online at the precised time I have indicated from a .NET robot task, with or without anyone connected to the database, and usually they are several of them everytime a backup is done.