Use
Update Panel.
Eg :
<asp:ScriptManager ID="ScriptManager"
runat="server" />
<asp:UpdatePanel ID="UpdatePanel1"
UpdateMode="Conditional"
runat="server">
<ContentTemplate>
<asp:dropdownlist id="DropDownList1" runat="server" class="form-control" autopostback="true" onselectedindexchanged="DropDownList1_SelectedIndexChanged" xmlns:asp="#unknown"></asp:dropdownlist>
<asp:requiredfieldvalidator id="RequiredFieldValidator1" runat="server" controltovalidate="DropDownList1" errormessage="Please Enter Project Name.." forecolor="#FF5050" xmlns:asp="#unknown"></asp:requiredfieldvalidator>
</ContentTemplate>
</asp:UpdatePanel>
Good luck.