Gridview中加入求和和求平均值

前台代码：

<asp:GridView ID="GridView1" ShowFooter="true" runat="server" AutoGenerateColumns ="false"  
            style="border:solid 1px black" onrowdatabound="GridView1_RowDataBound">
        <Columns>
        <asp:BoundField DataField="编号" HeaderText ="编号" />
        <asp:BoundField DataField ="姓名" HeaderText ="姓名" />
        <asp:BoundField DataField ="邮政编码" HeaderText ="邮政编码" />
        <asp:BoundField DataField ="家庭住址" HeaderText ="家庭住址" />
       <asp:TemplateField HeaderText ="出生年月">
       <ItemTemplate>
       <%#Eval("出生年月","{0:yyyy-MM-dd}")%>
       </ItemTemplate>
       </asp:TemplateField>
        <asp:BoundField DataField ="起薪" HeaderText ="起薪" />
        </Columns>
        </asp:GridView>

后台代码：

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

    }
    public void databind()
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["Conn"].ToString());
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = "select * from T_Message";
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        this.GridView1.DataSource = ds.Tables[0];
        this.GridView1.DataBind();
    }
    double sum = 0;
    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        foreach (TableCell item in e.Row.Cells)
        {
            item.Attributes.Add("style", "border-color:black");
        }
      
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            sum = sum + Convert.ToInt32(e.Row.Cells[5].Text);
        }
        if (e.Row.RowType == DataControlRowType.Footer)
        {
            e.Row.Cells[4].Text = "每月支出薪水";
            e.Row.Cells[2].Text = "平均薪水";
            e.Row.Cells[3].Text = (sum / GridView1.Rows.Count).ToString();
            e.Row.Cells[5].Text = sum.ToString();
        }
    }