dev exp aspxgridview server side events to get selected row details in to text box

<dx:ASPxGridView ID="dxGVUser" runat="server" AutoGenerateColumns="False"
                                                                                        ClientIDMode="AutoID" EnableCallBacks="False" KeyFieldName="UserId"
                                                                                        OnSelectionChanged="dxGVUser_SelectionChanged" Width="700px">
                                                                                        <Columns>
                                                                                            <dx:GridViewCommandColumn ShowInCustomizationForm="True" VisibleIndex="0">
                                                                                            </dx:GridViewCommandColumn>
                                                                                            <dx:GridViewDataTextColumn FieldName="UserId" Name="User Id"
                                                                                                ShowInCustomizationForm="True" VisibleIndex="2">
                                                                                            </dx:GridViewDataTextColumn>
                                                                                            <dx:GridViewDataTextColumn FieldName="UserName" Name="User Name"
                                                                                                ShowInCustomizationForm="True" VisibleIndex="3">
                                                                                            </dx:GridViewDataTextColumn>
                                                                                            <dx:GridViewDataTextColumn FieldName="FirstName" Name="First Name"
                                                                                                ShowInCustomizationForm="True" VisibleIndex="4">
                                                                                            </dx:GridViewDataTextColumn>
                                                                                            <dx:GridViewDataTextColumn Caption="Last Name" FieldName="LastName"
                                                                                                Name="Last Name" ShowInCustomizationForm="True" VisibleIndex="5">
                                                                                            </dx:GridViewDataTextColumn>
                                                                                            <dx:GridViewDataTextColumn Caption="Middle Name" FieldName="MiddleName"
                                                                                                Name="Middle Name" ShowInCustomizationForm="True" VisibleIndex="6">
                                                                                            </dx:GridViewDataTextColumn>
                                                                                        </Columns>
                                                                                        <SettingsBehavior AllowFocusedRow="True" AllowSelectByRowClick="True"
                                                                                            AllowSelectSingleRowOnly="True" ProcessSelectionChangedOnServer="True" />
                                                                                    </dx:ASPxGridView>

<asp:textbox id="txtUserName" runat="server" width="180px" xmlns:asp="#unknown"></asp:textbox>

protected void dxGVUser_SelectionChanged(object sender, EventArgs e)
       {


           txtUserName.Text = dxGVUser.GetSelectedFieldValues("UserName")[0].ToString();


       }

protected void select_empl(object sender, GridViewCommandEventArgs e)
    {
        int index = Convert.ToInt32(e.CommandArgument);
        GridViewRow row = GridView1.Rows[index];
        txtemployeecode.Text=row.Cells[1].Text;
        txtemployeename.Text=row.Cells[2].Text;

    }