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Before we extend the control, let us determine the new properties we would like to have in the control.

• AllowMultipleColumnSorting: When set to true, this should enable sorting on more than one column.
• SortAscImageUrl and SortDescImageUrl: These properties allow the page developer to optionally select an image for indicating the sort direction in the column header.  When these properties are left blank, the control inserts a glyph symbol to indicate the sort direction.  We would like to enable the sort direction indicator for default (single-column) sort behavior, too.

The following code block shows how we define the new properties.

Listing 2

        /// <summary>
        /// Enable/Disable MultiColumn Sorting.
        /// </summary>
        [
        Description("Whether Sorting On more than one column is enabled"),
        Category("Behavior"),
        DefaultValue("false"),
        ]
        public bool AllowMultiColumnSorting
        {
            get
            {
                object o = ViewState["EnableMultiColumnSorting"];
                return (o != null ? (bool)o : false);
            }
            set
            {
                AllowSorting = true; //Set default Sort property to true
                ViewState["EnableMultiColumnSorting"] = value;
            }
        }


        /// <summary>
        /// Get or Set Image location to be used to display Ascending Sort order.
        /// </summary>
        [
        Description("Image to display for Ascending Sort"),
        Category("Misc"),
        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;
            }
        }
        /// <summary>
        /// Get or Set Image location to be used to display Ascending Sort order.
        /// </summary>
        [
        Description("Image to display for Descending Sort"),
        Category("Misc"),
        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;
            }
        }

The design-time attributes for the properties are worth noting.  The Category, Description, and DefaultValue attributes allocate the properties to the corresponding classification in the IDE's Property Browser. The Editor attribute enables a URL Editor for easily choosing the image location.