protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
{
CheckBox cb = (CheckBox)sender;

if (cb.Text == "全选")
{
foreach (GridViewRow gv in this.GridView1.Rows)
{
CheckBox cd = (CheckBox)gv.FindControl("CheckBox1");
cd.Checked = cb.Checked;
cb.Text = "反选";
}
}
else
{
cb.Text = "反选";
foreach (GridViewRow gv in this.GridView1.Rows)
{
CheckBox cd = (CheckBox)gv.FindControl("CheckBox1");
cd.Checked = cb.Checked;(提示的出错位置!)
cb.Text = "全选";
}

}
}

<asp:TemplateField HeaderText="全选">
 <HeaderTemplate>
     &nbsp;
    <asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="True" OnCheckedChanged="CheckBox1_CheckedChanged" />
   </HeaderTemplate>
 <ItemTemplate>
<asp:CheckBox ID="chkSelect" runat="server" AutoPostBack="True"/>
   </ItemTemplate>                                                                                            
</asp:TemplateField>