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For you scenario, the best solution is to use the client-side OnRowSelect event of the RadGrid to send ajax request to the RadAjaxPanel with row index as argument. This will trigger the server-side OnAjaxRequest event of the RadAjaxPanel. Then in the handler of this event you can set the information in your text boxes depending on the selected row index.
I am attaching a simple project demonstrating this approach.
Additionally, I should mention that it is recommended to avoid using both RadAjaxManager and RadAjaxPanel at the same time. More information about that you can find in our help article: Ajaxifying Controls Wrapped in RadAjaxPanel and Added to RadAjaxManager Settings.
If you have any further questions, please feel free to contact us again.
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