Append level list to combo box

I have a problem with pyrevit
I have WPF form in xaml file it contains a combobox, this combo box I want to include all level in my model
I make a collector for levels and level name list
And append the list to combobox but it didn’t work
Any one can help?

Hi @Hasantahhan ,
if you want us to help you find the errors in your code, please show us!
It’s difficult to understand what’s wrong only from a description.