How to keep focus on the TextBox after TextChanged event is fired in ASP.Net

This way

HTML

 <asp:TextBox ID="Inv_Qty" runat="server"  ForeColor="blue" AutoPostBack="true" OnTextChanged = "CheckQTY_TextChange"></asp:TextBox>
    <script type = "text/javascript">
        function SetFocus() {
            window.onload = function () {
                setTimeout(function () {
                    alert('Invalid Input,Not allow 0 or negative number. Please check again!!');
                    document.getElementById('<%=Inv_Qty.ClientID %>').focus(); ;
                }, 100)
            }
        };
    </script>

Code

    protected void CheckQTY_TextChange(object sender, EventArgs e)
    {
        int value = 0;
        if (!int.TryParse (this.Inv_Qty.Text.Trim(), out value) || value < 0)
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "Message", "SetFocus();", true);
        }
    }