this.GrdViewMain.DataSource = Membership.GetAllUsers();
  this.GrdViewMain.DataBind();

<asp:GridView ID="GrdViewMain" runat="server" AutoGenerateColumns="true" DataKeyNames="UserName"
      OnRowCommand="GrdViewMain_RowCommand" OnRowDataBound="GrdViewMain_RowDataBound"
      SkinID="MainGridView">
      <Columns>
        <asp:ButtonField ButtonType="Image" CommandName="DeleteRowByGrid" HeaderText="C"
          ImageUrl="~/Immagini/cancella.gif">
          <HeaderStyle ForeColor="Red" />
        </asp:ButtonField>
        <asp:ButtonField ButtonType="Image" CommandName="UpdateRowByGrid" HeaderText="A"
          ImageUrl="~/Immagini/modifica.gif">
          <HeaderStyle ForeColor="Blue" />
        </asp:ButtonField>
      </Columns>
    </asp:GridView>

protected void GrdViewMain_RowCommand(object sender, GridViewCommandEventArgs e)
  {
    if (e.CommandName.Equals("UpdateRowByGrid"))
    { .. }
  }