Gridview with paging enabled and checkBox, save items to database

I have a Gridview with paging enabled and checkBox, with Columns UserName, etc. On ButtonClick event I want to save the UserName to Database for selected checkbox. I am unable to get the UserName for the selected checkbox and stuck. Help appriciated.

    protected void BtnSaveAuthorization_Click(object sender, EventArgs e)
    { 
     }    

      protected void GVRegisterNewCarton_PageChanging(object sender, GridViewPageEventArgs e)
    {

        string pageId = string.Format("Page{0}", GVRegisterNewCarton.PageIndex);
        bool[] selectedCheckboxes = new bool[GVRegisterNewCarton.PageSize];
        for (int i = 0; i < GVRegisterNewCarton.Rows.Count; i++)
        {
            TableCell cell = GVRegisterNewCarton.Rows[i].Cells[0]; 
            selectedCheckboxes[i] = (cell.FindControl("chkSelectAdd") as CheckBox).Checked; 
        } 
        ViewState[pageId] = selectedCheckboxes;
        GVRegisterNewCarton.PageIndex = e.NewPageIndex;
        BindData();


        //Bind the gridview again 



    }

    protected void GVRegisterNewCarton_PreRender(object sender, EventArgs e) 

    { 
        string pageId = string.Format("Page{0}", GVRegisterNewCarton.PageIndex); 
        bool[] selectedCheckboxes = ViewState[pageId] as bool[]; 
        if (selectedCheckboxes != null) 
        { 
            for (int i = 0; i < GVRegisterNewCarton.Rows.Count; i++)
            {
                TableCell cell = GVRegisterNewCarton.Rows[i].Cells[0];
                (cell.FindControl("chkSelectAdd") as CheckBox).Checked = selectedCheckboxes[i];
            }
        } 
    }