I’ve seen some for web using javascript, but I am looking for a Windows Form control that is a ComboBox that when opened displays a list of checkboxes and labels, allowing to select one or more, with a “Select All” option.
Would like a non-commercial solution.
I hate to suggest a commercial solution, however, I am using the DevExpress Winforms suite and it has exactly what you are looking for. Plus it really let’s you really customize it (as you see in the example below).