Cannot bind an event handler to the '<event name>' event because it is read-only
This error most often arises when you have tried to connect an event to a control that is inherited from a base class. If the control's member variable is Private, Visual Studio cannot connect the event to the method. Privately inherited controls cannot have additional events bound to them.
See Also
Tasks
How to: Inherit Forms Using the Inheritance Picker Dialog Box
How to: Create Event Handlers Using the Designer
Reference
Accessibility Levels (C# Reference)