MVVM combobox binding issue

I have a project with a number of comboboxes, all binding correctly. Since installing SL5 I started experiencing all sorts of binding issues as well as the debugger being crippled. The problem is that when you switch from SL4 to SL5 and then back to SL4, not all the references are reset. Long story sort, I had to reinstall vs2010 to fix those issues. Now I am testing the dev environment and although I can get a textbox to bind from the VM, I cannot get a combobox to bind.

What I would like to know is what is the absolute minimum requirements for a combobox to bind. My ViewModel's property SET is getting its data.I have the comboboxes ItemSource set and the DisplayMemberPath set.I have tried using the ItemTemplate syntax but nothing works. The RaisePropertyChanged is being fired. Could this be a UI threading issue? The only difference between this combobox and the others is that there is a call to the database that collects values that are then used in a second call to the database. Those results are then passed to the property in the VM which raises the RaisePropertychanged event, but the binding doesnt render开发者_开发百科. I have mocked an entity in the class constructor and the binding works, but when I load the entities from the database it doesnt work.

I found the issue. This might be helpful to others. I was instantiating a ViewModels ObservableCollecion in the Completed event of the method and then looping through the collection from the database and addng it to the collection. Binding was not happening. I moved the instantiation of the ObservableCollection to the ViewModels constructor instead and the Binding started working. Still a bit puzzled as to why the newing up of the ObservableCollection has to be in the ViewModels constructor and not in the Completed event of the method fetching the data.