In this demo, I will use the sample I developed in a previous demo - Cascading Dropdownlists in GridView. I modify only the web user control named "wucCity.ascx" by replacing its content with an AutoComplete extender strategy from the ToolKit to set and retrieve the city based on the first letters typed by the user. Everything else on the webform remains the same and the demo still functions as required.

I also added a TextBoxWatermarkExtender to the AutoComplete TextBox to improve the user experience when editing the content of the TextBox.