In this article, I will show you how to create a multiselect in an ASP.NET 4.0 page. I will keep this article short and sweet so you can just use the code in your applications. Lets see how I implement this.

Now, place one textbox in your design view and assign the popup extender for that textbox. Important note here is you must be place your textbox control within the "Updatepanel". Afterwards, you create one panel and within this panel you put one checkboxlist. You assign this panel to the "PopupControlID" property of the Popup Extender.

Now see this code:

Finally we must do only one modification in code behind, because each time clicking the checkboxlist that selected value will added into the textbox control. So in selected index changed event you will place this code.

Finish. Hopefully this article can help you.