排序一个函数在c# gridview

我正在构建一个小的web应用程序来跟踪赢/输/平均。数据保存在XML文件中(因为只有几行)。我想尝试并给页面的访问者，按每一列可能排序的能力(玩，赢，平均)。我可以让它对play和Won进行排序，但不能对Average进行排序，因为Average是一个计算值。

我如何得到它排序GetWinAverage函数的结果?

现在的样子:

<asp:GridView ID="Results" runat="server" AutoGenerateColumns="false" AllowSorting="true" OnSorting="gvResults_Sorting" EnableViewState="true">
        <Columns>
            <asp:TemplateField HeaderText="Played" SortExpression="Played">
                <ItemTemplate>
                    <asp:Label ID="lblPlayed" runat="server" Text='<%# Eval("Played") %>' />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        <Columns>
            <asp:TemplateField HeaderText="Won" SortExpression="Won">
                <ItemTemplate>
                    <asp:Label ID="lblWon" runat="server" Text='<%# Eval("Won") %>' />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        <Columns>
            <asp:TemplateField HeaderText="Average" SortExpression="Average">
                <ItemTemplate>
                    <asp:Label ID="lblAverage" runat="server" Text='<%# GetWinAverage(Eval("Won"), Eval("Played")) %>' />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>

gettwinaverage看起来像这样:

public string GetWinAverage(object gamesWon, object gamesPlayed)
    {
        string strReturn = "";
        double iWon = Convert.ToDouble(gamesWon);
        double iPlayed = Convert.ToDouble(gamesPlayed);
        if (iPlayed < 0)
        {
            return "0%";
        }
        else
        {
            double dReturn = (iWon / iPlayed) * 100;
            strReturn = string.Format("{0:N2}%", dReturn);
            return strReturn;
        }
    }

排序是这样的:

private const string ASCENDING = " ASC";
    private const string DESCENDING = " DESC";
    public SortDirection GridViewSortDirection
    {
        get
        {
            if (ViewState["sortDirection"] == null)
                ViewState["sortDirection"] = SortDirection.Ascending;
            return (SortDirection)ViewState["sortDirection"];
        }
        set { ViewState["sortDirection"] = value; }
    }
    protected void gvResults_Sorting(object sender, GridViewSortEventArgs e)
    {
        string sortExpression = e.SortExpression;
        if (GridViewSortDirection == SortDirection.Ascending)
        {
            GridViewSortDirection = SortDirection.Descending;
            SortGridView(sortExpression, DESCENDING);
        }
        else
        {
            GridViewSortDirection = SortDirection.Ascending;
            SortGridView(sortExpression, ASCENDING);
        } 
    }
    private void SortGridView(string sortExpression, string direction)
    {
        //  You can cache the DataTable for improving performance
        DataTable dt = dtPlayers;
        DataView dv = new DataView(dt);
        dv.Sort = sortExpression + direction;
        gvResults.DataSource = dv;
        gvResults.DataBind();
    }