Validate at least one CheckBox checked inside ListView in ASP.Net

This Way:

HTML:

<form id="form1" runat="server">
    <div>
        <asp:ListView ID="ListView1" runat="server">
            <LayoutTemplate>
                <table border="0" cellpadding="1">
                    <tr style="background-color: #E5E5FE">
                        <th align="left">
                            CustomerID
                        </th>
                        <th align="left">
                            Name
                        </th>
                        <th align="left">
                            City
                        </th>
                    </tr>
                    <tr runat="server" id="itemPlaceholder" />
                </table>
            </LayoutTemplate>
            <ItemTemplate>
                <tr>
                    <td>
                        <asp:CheckBox ID="CheckBox1" Text='<%# Eval("CustomerID") %>' runat="server" />
                    </td>
                    <td>
                        <asp:Label ID="lblName" runat="server" Text='<%# Eval("Name") %>'></asp:Label>
                    </td>
                    <td>
                        <asp:Label ID="lblCity" runat="server" Text='<%# Eval("City") %>'></asp:Label>
                    </td>
                </tr>
            </ItemTemplate>
        </asp:ListView>
        <asp:Button ID="Button1" Text="Search" OnClick="Save" runat="server" />
    </div>
    </form>

C#:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            this.BindList();
        }
    }

    private void BindList()
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[3]{  
                            new DataColumn("CustomerId", typeof(int)),  
                            new DataColumn("Name", typeof(string)),
                            new DataColumn("City", typeof(string))});
        dt.Rows.Add(1, "Jake", "Madrid");
        dt.Rows.Add(2, "John", "London");
        dt.Rows.Add(3, "Micheal", "Lisbon");
        dt.Rows.Add(3, "Azim", "Mumbai");
        this.ListView1.DataSource = dt;
        this.ListView1.DataBind();
    }

    protected void Save(object sender, EventArgs e)
    {
        int count = 0;
        Button btn = sender as Button;
        foreach (ListViewItem item in this.ListView1.Items)
        {
            if ((item.FindControl("CheckBox1") as CheckBox).Checked == true)
            {
                count++;
            }
        }
        if (count == 0)
        {
            string message = "Please check at least one CheckBox";
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            sb.Append("<script type = 'text/javascript'>");
            sb.Append("window.onload=function(){");
            sb.Append("alert('");
            sb.Append(message);
            sb.Append("')};");
            sb.Append("</script>");
            ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());

        }
    }

Output

Thank You.