What should I do to make my StringCollection property editable during design-time?

Use this custom editor attribute for the property that is of the StringCollection type (this can be used for other Collection types too if you want to allow entry of strings into the collection during design-time).

Editor('System.Windows.Forms.Design.StringCollectionEditor, System.Design', 'System.Drawing.Design.UITypeEditor, System.Drawing'),
public YourCollection YourProp{get{...}set{...}}

StringCollectionEditor is not public so you have to use the above constructor override and specify the typename. This editor should allow you
to edit your collection without any problems.