I'm developing an application for taking orders in C# and DevExpress, and I need a function that generates a unique order number. The order number must contain letters and digits and has a length of 20 ..
I've seen things like Guid.NewGuid() but I don't want it to be totally random, nor to be just an auto increment number ..

Can anyone help? even if it's a script in a different language, I need ideas desperately :)