GridView多列排序及排序方向图标

网上找到的一个控件，有一点小问题，排序点击几次图标会消失，再点击一次才会出现。来源不可考证，如果作者看到请联系我，我再添加来源信息。

public class WebGridView : GridView
    {
        /**/
        /// 
        /// 是否启用或者禁止多列排序
        /// 
        [
        Description("是否启用多列排序功能"),
        Category("排序"),
        DefaultValue("false"),
        ]
        public bool AllowMultiColumnSorting
        {
            get
            {
                object o = ViewState["EnableMultiColumnSorting"];
                return (o != null ? (bool)o : false);
            }
            set
            {
                AllowSorting = true;
                ViewState["EnableMultiColumnSorting"] = value;
            }
        }
        /**/
        /// 
        /// 升序时显示图标
        /// 
        [
        Description("升序时显示图标"),
        Category("排序"),
        Editor("System.Web.UI.Design.UrlEditor", typeof(System.Drawing.Design.UITypeEditor)),
        DefaultValue(""),

        ]
        public string SortAscImageUrl
        {
            get
            {
                object o = ViewState["SortImageAsc"];
                return (o != null ? o.ToString() : "");
            }
            set
            {
                ViewState["SortImageAsc"] = value;
            }
        }
        /**/
        /// 
        /// 降序时显示图标
        /// 
        [
         Description("降序时显示图标"),
         Category("排序"),
         Editor("System.Web.UI.Design.UrlEditor", typeof(System.Drawing.Design.UITypeEditor)),
         DefaultValue(""),
         ]
        public string SortDescImageUrl
        {
            get
            {
                object o = ViewState["SortImageDesc"];
                return (o != null ? o.ToString() : "");
            }
            set
            {
                ViewState["SortImageDesc"] = value;
            }
        }
        protected override void OnSorting(GridViewSortEventArgs e)
        {
            if (AllowMultiColumnSorting)
            {
                e.SortExpression = GetSortExpression(e);
            }

            base.OnSorting(e);
        }
        protected override void OnRowCreated(GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.Header)
            {
                if (SortExpression != String.Empty)
                {
                    DisplaySortOrderImages(SortExpression, e.Row);
                    this.CreateRow(0, 0, DataControlRowType.EmptyDataRow, DataControlRowState.Normal);
                }
            }
            base.OnRowCreated(e);
        }
        /**/
        /// 
        ///  获取排序表达式
        /// 
        protected string GetSortExpression(GridViewSortEventArgs e)
        {
            string[] sortColumns = null;
            string sortAttribute = SortExpression;

            if (sortAttribute != String.Empty)
            {
                sortColumns = sortAttribute.Split(",".ToCharArray());
            }
            if (sortAttribute.IndexOf(e.SortExpression) > 0 || sortAttribute.StartsWith(e.SortExpression))
            {
                sortAttribute = ModifySortExpression(sortColumns, e.SortExpression);
            }
            else
            {
                sortAttribute += String.Concat(",", e.SortExpression, " ASC ");
            }
            return sortAttribute.TrimStart(",".ToCharArray()).TrimEnd(",".ToCharArray());

        }
        /**/
        /// 
        ///  修改排序顺序
        /// 
        protected string ModifySortExpression(string[] sortColumns, string sortExpression)
        {
            string ascSortExpression = String.Concat(sortExpression, " ASC ");
            string descSortExpression = String.Concat(sortExpression, " DESC ");

            for (int i = 0; i < sortColumns.Length; i++)
            {

                if (ascSortExpression.Equals(sortColumns[i]))
                {
                    sortColumns[i] = descSortExpression;
                }

                else if (descSortExpression.Equals(sortColumns[i]))
                {
                    Array.Clear(sortColumns, i, 1);
                }
            }

            return String.Join(",", sortColumns).Replace(",,", ",").TrimStart(",".ToCharArray());

        }
        /**/
        /// 
        ///  获取当前的表达式对所选列进行排序
        /// 
        protected void SearchSortExpression(string[] sortColumns, string sortColumn, out string sortOrder, out int sortOrderNo)
        {
            sortOrder = "";
            sortOrderNo = -1;
            for (int i = 0; i < sortColumns.Length; i++)
            {
                if (sortColumns[i].StartsWith(sortColumn))
                {
                    sortOrderNo = i + 1;
                    if (AllowMultiColumnSorting)
                    {
                        sortOrder = sortColumns[i].Substring(sortColumn.Length).Trim();
                    }
                    else
                    {
                        sortOrder = ((SortDirection == SortDirection.Ascending) ? "ASC" : "DESC");
                    }
                }
            }
        }
        /**/
        /// 
        ///  绘制升序降序的图片
        /// 
        protected void DisplaySortOrderImages(string sortExpression, GridViewRow dgItem)
        {
            string[] sortColumns = sortExpression.Split(",".ToCharArray());

            for (int i = 0; i < dgItem.Cells.Count; i++)
            {
                if (dgItem.Cells[i].Controls.Count > 0 && dgItem.Cells[i].Controls[0] is LinkButton)
                {
                    string sortOrder;
                    int sortOrderNo;
                    string column = ((LinkButton)dgItem.Cells[i].Controls[0]).CommandArgument;
                    SearchSortExpression(sortColumns, column, out sortOrder, out sortOrderNo);
                    if (sortOrderNo > 0)
                    {
                        string sortImgLoc = (sortOrder.Equals("ASC") ? SortAscImageUrl : SortDescImageUrl);

                        if (sortImgLoc != String.Empty)
                        {
                            Image imgSortDirection = new Image();
                            imgSortDirection.ImageUrl = sortImgLoc;
                            dgItem.Cells[i].Controls.Add(imgSortDirection);

                        }
                        else
                        {

                            if (AllowMultiColumnSorting)
                            {
                                Literal litSortSeq = new Literal();
                                litSortSeq.Text = sortOrderNo.ToString();
                                dgItem.Cells[i].Controls.Add(litSortSeq);

                            }
                        }
                    }
                }
            }

        }
    }