How to lock & unlock the Richtextbox control in VB.NET?

Thr is only way to lock & unlock the richtextbox by property window. but i hav to lock & unlock it during runtime.

These are the properties for TextBox

Enabled: Default value is True. To disable, set the property to False.
Multiline: Setting this property to True makes the TextBox multiline which allows to accept multiple lines of text. Default value is False.
PasswordChar: Used to set the password character. The text displayed in the TextBox will be the character set by the user. Say, if you enter *, the text that is entered in the TextBox is displayed as *.
ReadOnly: Makes this TextBox readonly. It doesn't allow to enter any text.
Visible: Default value is True. To hide it set the property to False.