Hi,
i got the solution for my problem,
just set the property of gridview
EnableCallBacks=False
aspx code
<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>
CS Code
protected void dxGVUser_SelectionChanged(object sender, EventArgs e)
{
txtUserName.Text = dxGVUser.GetSelectedFieldValues("UserName")[0].ToString();
}