<ItemTemplate>
                        <asp:label id="DataSourceLabel"    wrap="true"  Text= '<%# Eval("DataSource") %>' runat="server" />
                        <asp:Button ID="FullButton" Visible="false" Text=" full " CssClass= "GridFullButton"
                                    CommandArgument="<%# ((GridViewRow) Container).RowIndex %>"
                                    onclick="FullButton_Click" runat="server" CommandName='Full' />                       
                    </itemtemplate>

        protected void MyGridView_RowCommand(object sender, GridViewCommandEventArgs e)
        {
            if (e.CommandName == "Full")
            {  
                //получаю номер строки
                int index = Convert.ToInt32(e.CommandArgument);
 
                       ((TextBox)this.DataSourceTextPanel.FindControl("DataSourceTextBox")).Text =
                  ---->    (string)System.Web.UI.DataBinder.Eval(row.DataItem, "DataSource");
            }
       }

DataSourceFullButton.CommandArgument = System.Web.UI.DataBinder.Eval(e.Row.DataItem, "SessionID").ToString();

int SessionID = Convert.ToInt32(e.CommandArgument);  

        
        String SelectCommand = String.Concat("SELECT DataSource FROM SessionSheet WHERE SessionID=",SessionID.ToString());
        this.FullViewSqlDataSource.SelectCommand = SelectCommand;
 
        DataView dv = (DataView)FullViewSqlDataSource.Select(System.Web.UI.DataSourceSelectArguments.Empty);
        String FullDataSourceString = (String)dv.Table.Rows[0][0];
 
        DataSourceTextBox = (TextBox)this.DataSourceTextPanel.FindControl("DataSourceTextBox");
        DataSourceTextBox.Text = FullDataSource;