How to add check box inside combobox in c#

You have to extend the ComboBox control by providing your own rendering strategy, and "manually" adding a CheckBox.

Theses open source project are ready to use :

http://www.codeproject.com/KB/combobox/CheckComboBox.aspx http://www.codeproject.com/KB/combobox/extending_combobox.aspx

It is a wrong usage of a ComboBox control, because the user has no possibility to see his choices. For multiple selection, I recommend you to consider this CheckedListBox control:

link to MSDN