Onfocus of Amount textbox i have to make alert as "select Product" if Product dropdown is not selected with any value

<script type="text/javascript">

        function check() {            
            if ((document.getElementById("DropDownValue").selectedIndex)== 0) {               
alert("Please select the Name!");               
return false;
            }}
    script>

<asp:DropDownList ID="DropDownValue" runat="server">
        <asp:ListItem Text="Manish" Value="Manish">asp:ListItem>
        <asp:ListItem Text="Gaurav" Value="Gaurav">asp:ListItem>
        <asp:ListItem Text="Ravi" Value="Ravi">asp:ListItem>
    asp:DropDownList>

       <asp:TextBox onfocus="Javascript:check();" ID="TextBoxAlert"
runat="server">asp:TextBox>

i think you will get some idea from this solution.