Hi Wai Loon Ho,
If the textboxes, or any other inputs on the page, have required validators attached, on any postback the validators will be fired. Another option here would be to use separate validation groups for the dropdown, and the input controls. Another alternative would be to use client side code, and manually check the values of the control, if none of these standard approaches is suitable for you.
I hope these suggestions help.
Best wishes,
Yavor
the Telerik team