What is AutoPostBack feature in ASP.NET?

In case it is required for a server side control to postback when any of its event is triggered, then the AutoPostBack property of this control is set to true.

What is AutoPostback?

AutoPostBack automatically posts the page back to the server when state of the control is changed.

Can a DropDownList fire validation controls?

Yes, a DropDownList control can fire validation control when the CausesValidation property of that validation control is set to true and the AutoPostBack property is set to true.

What is the use of AutoPostBack ?

AutoPostBack is a feature available that is available on few controls.
The main purpose of this feature is that, if there is any change done on the client side, then it should be handled on the server side.

Usage:

<asp:TextBox AutoPostBack="TRUE|FALSE" runat="server"/>
Example:

<form runat="server">
<asp:TextBox id="txtBox1" runat="server" AutoPostBack="TRUE" />
</form>