asp.net点击表头增加排序标示

首先点击gridview的表头要有排序的功能才行，关于这个我的另一篇文章里有介绍
注意要把gridview表头的HtmlEncode的值改成false

 /**//// <summary>
    /// 为GridView.HeaderRow添加排序标识符
    /// </summary>
    /// <param name="headerRow">GridViewRow</param>
    private void changeHeaders(GridViewRow headerRow)
    {
        if (this.GridViewFileList.Rows.Count > 0) //有数据时才执行
        {
            for (int i = 0; i < headerRow.Cells.Count; i++)
            {
                if (headerRow.Cells[i] is DataControlFieldCell)
                {
                    DataControlField field = ((DataControlFieldCell)headerRow.Cells[i]).ContainingField;
                    if (field.HeaderText.IndexOf("↓") >= 0 || field.HeaderText.IndexOf("↑") >= 0)
                        field.HeaderText = field.HeaderText.Substring(0, field.HeaderText.Length - "<font color='#FF0000'>↓

</font>".Length); //把开始的排序方式去掉

                    if (this.HiddenFieldGridViewSortExpression.Value != "" && 

this.HiddenFieldGridViewSortExpression.Value.Trim().Equals(field.SortExpression.Trim()))
                    {
                            //保存了排序方式的隐藏控件
                        if (this.HiddenFieldGridViewSortDirection.Value.Equals("DESC")) //增加新的排序方式
                            field.HeaderText = field.HeaderText + "<font color='#FF0000'>↓</font>";
                        else
                            field.HeaderText = field.HeaderText + "<font color='#FF0000'>↑</font>";
                    }
                }
            }
        }
    }