I have a project with a number of different classes querying and modifying data in a common set of tables. I've set up a .dbml file which provides us with a DataContext class. My question is whether a single instance of the DataContext should be used by all objects, or whether multiple instances are safe to use. I'm also wondering about thread safety in the case of a single DataContext, and whether access to it's methods should be synchronized.
I use an new instance of DataContext for every transaction.
Reusing old instances can be troublesome, and will bloat the content of the DataContext, since any item that has been loaded at some time, will have to be tracked - your app will get slower and slower, bloating up memory.
If you need an item longer than for a transaction, you can detach it from the DataContext by cloning the item, and can reattach it later to a new and fresh DataContext using Attach().
I even can clone an item, send it over the network with WCF, get it back in some later call, attach it to a new DataContext and save the changes (of course I need a timestamp column for this).