Joe,

>I need to display some data on the MainWindow which is an mmBaseWindow. I cannot call RegisterPrimaryBizObj. So I create the bizobj and set it into the datacontext and it works ok. Is there any reason or way to use RegisterPrimaryBizObj on the MainWindow?

By main window are you referring to the MDI Main Form? The benefits of registering a business object with a form are so you can use the form level methods such as Save() . These methods allow UI action to be taken like confiming a user wants to delete, etc. There is nothing wrong with using the business object the way you state.
Tim