GridView复选框的问题

前台：
    <asp:GridView ID="GridView1" runat="server">
    <Columns>
    <asp:TemplateField HeaderText="选择" >
    <ItemTemplate>
         <asp:CheckBox ID="CheckBox1" runat="server"/>
    </ItemTemplate>
    </asp:TemplateField>
    </Columns>
    </asp:GridView>
        <br />
        <asp:Button ID="Button1" runat="server" onclick="Button1_Click1" Text="提交" />
        <asp:Label ID="Label1" runat="server"></asp:Label>

后台：
    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection sqlconstr = new SqlConnection();
        sqlconstr.ConnectionString = "server = localhost;database = xuanke; Trusted_Connection=SSPI";
        string safeSql = "Select * FROM kecheng";
        SqlCommand cmd = new SqlCommand(safeSql, sqlconstr);
        sqlconstr.Open();
        SqlDataReader myReader = cmd.ExecuteReader();
        this.GridView1.DataSource = myReader;
        GridView1.DataBind();
        sqlconstr.Close();
    }

    protected void Button1_Click1(object sender, EventArgs e)
    {
        CheckBox cb = (CheckBox)GridView1.Rows[1].Cells[0].FindControl("CheckBox1");
        if (cb.Checked)
            Label1.Text = "123";
        
    }
为什么点击checkbox没反应