Thank you for writing.
In order to make the text area of the RadDropDownList read-only you should just set the DropDownStyle
property to DropDownList.
This way the user will not be able to edit the text and he/she should chose from one of the predefined options.
If you do not want to change the drop down style for some reason, you can use the Validating event and cancel the change and perhaps display a message to the user.
I hope this helps. Let me know if you have additional questions.
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