I have an UserControl and would like to hide the resizing handles in design-time, just like the TextBoxControl, that has only two handles (left and right ones)... the height of the textbox is fixed, unless you say it is multiline. In that case, the textbox shows all nine resizing handles.
You need to implement a custom designer (ControlDesigner) for your control and then override the SelectionRules property.
Useful reference:
I think you would need to draw your own control and define the size in the class. I believe if you inherit a user control you'll always have the option to resize.
来源：https://stackoverflow.com/questions/2494865/how-can-i-hide-the-resizing-handles-of-a-control-in-windows-forms