BASeCamp Programming Blog

As I posted previously here , Sorting a Listview can be something of a pain in the butt.

In that article, I covered some basics on providing a class that would essentially give you sorting capabilities for free, without all the messy code that would normally be required. A lot of the code required for sorting is mostly boilerplate with a few modifications for sorting various types. As a result, the generic implementation works rather well.

However, as with any class, adding features never hurts. In this case, I got to thinking- why not have right-clicking the ColumnHeaders show a menu for sorting against that Column? Seems simple enough. I quickly learned that apparent simplicity often is misattributed.

I faced several issues. The first thought was that I could hook a Mouse event for Right-Clicking a column header. Unfortunately, I soon discovered two facts about the .NET ListView control. First, was that there was no event for right-clicking a header control. Second, no even was fired at all by the ListView control when you right-clicked a header.

This left me stymied. How the heck do I implement this feature? I discovered something of a “hack” however, in that when the ListView’s ContextMenuStrip property is set, that ContextMenu Strip will be shown regardless of the location the ListView is clicked. This at least gave me something to work with. Since a ContextMenuStrip’s “Opening” event can be easily hooked, we can use that as an entry point and perform needed calculations to determine if we are indeed on a columnheader.

Which brings me to the next problem, which is determining when a columnheader was in fact the item that was clicked. This requires determining the rectangle the Header control occupies, first. The Header Control is a child control of the ListView; as such, a platform Invoke using the EnumChildWindows() API was required, something like this:

 
private Rectangle _HeaderRect;
private delegate bool EnumWindowsCallBack(IntPtr hwnd,IntPtr lparam);
 [DllImport("user32.dll")] 
private static extern int EnumChildWindows(IntPtr hwndParent,EnumWindowCallBack callbackFunction,IntPtr lParam);
 [DllImport("user32.dll"] 
private static extern bool GetWindowRect(IntPtr hWnd,out RECT lpRect);

 [StructLayout(LayoutKind.Sequential)] 
private struct RECT
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;

}

private bool EnumWindowCallback(IntPtr hwnd, IntPtr lParam)
{
    RECT rct;
    if(!GetWindowRect(hwnd,out rct))
    {
    //first child of the listview should be the header control
    _HeaderRect=Rectangle.Empty; //likely the listview is not in Details mode, so there is no header control.
    }
    else
    {
        _HeaderRect = new Rectangle(rct.Left,rct.Top,rct.Right-rct.Left,rct.Bottom-rct.Top);
    }
    return false; //cancel enumeration.
}
private static ColumnHeader []  GetOrderedHeaders(ListView lvw)
{
    ColumnHeader []  returnarray = new ColumnHeader [lvw.Columns.Count] ;
    foreach(ColumnHeader loopheader in lvw.Columns)
    { 
        returnarray [loopheader.DisplayIndex]  = loopheader;

    }
    return returnarray;
}
 

Quite a bit of boilerplate to add in. Basically, the idea is that we will hook the contextMenu Opening event of the Listview, (and we add a context menu to hook if the listview in fact doesn’t have one) in our constructor; and then when we receive the event we need to determine if the click occured within the area of the header control of the listview, if so, we cancel the event (which stops the default context menu from appearing) and show our own menu for the columnheader, which we can acquire using a bit of math and the static “GetOrderedHeaders” function, which retrieves the array of columnheaders of a ListView in order of appearance Left to Right (since the user could rearrange the Columns).

So First, we need to add code to the GenericListViewSorter’s Constructor. We also have a few private variables that are added; in this case, we need a ContextMenuStrip variable called “_ghostStrip” which we will use if we need to create a context menu for the control, since we don’t want that to appear in the default case. Of course we create our own ContextMenuStrip which we will show in the event instead of the default when appropriate. so we add this beneath the existing code in the constructor:

 
    if(handleListView.ContextMenuStrip==null)
    {
        handleListView.ContextMenuStrip = new ContextMenuStrip();
        handleListView.ContextMenuStrip.Items.Add("GHOST"); //add a ghost item so we get the Opening Event
        _ghoststrip = handleListView.ContextMenuStrip;
}

//create OUR context menu
_headerContextMenuStrip = new ContextMenuStrip();
//add a ghost item to make sure Opening will fire.
_HeaderContextMenuStrip.Items.Add("ghost");
handleListView.ContextMenuStrip.Opening += ContextMenuStrip_Opening;
handleListView.ContextMenuStripChanged += handleListView_ContextMenuStripChanged;


 

Of course we need to add the two referenced event handlers, too. The ContextMenuStripChanged being a rather simple implementation designed to keep changes in the contextmenu of the listview from causing us to balls up and stop showing ours (since we are now hooking a orphaned context menu not being shown by the listview).

 
void handleListView_ContextMenuStripChanged(object sender,EventArgs e)
{
    OurListView.ContextMenuStrip.Opening+=ContextMenuStrip_Opening;
}
 

Now the meat of the code is in the ContextMenuStrip_Opening() routine. This will need to determine wether its applicable to show the Column menu, or the already present menu (which it doesn’t show either if it happens to be the _ghoststrip). This is accomplished by use of the GetCursorPos() API routine paired with the already present GetWindowRect() implementation, which we update by calling EnumWindows.

 
void ContextMenuStrip_Opening(object sender,System.ComponentModel.CancelEventArgs e)
{
    //first, get screen coordinates of Cursor.
    POINTAPI gapi;
    GetCursorPos(out gapi);
    
    Point gotposition = new Point(gapi.X,gapi.Y);
    
    //acquire the HeaderRect of the control...
    EnumChildWindows(OurListView.Handle,new EnumWindowCallBack(EnumWindowCallback),IntPtr.Zero);
    //if the mouse position is within the retrieved rectangle, cancel the display of the normal menu and create and show ours.
    if(_HeaderRect.Contains(gotposition))
    {
        e.Cancel=true;
        int xoffset = gotposition.X - _HeaderRect.Left;
        ColumnHeader clickedheader = HeaderAtOffset(OurListView,xoffset);
        
        if(clickedheader != null)
        {
        //create the context menu as needed.
        _HeaderContextMenuStrip = new ContextMenuStrip();
        _HeaderContextMenuStrip.Tag = clickedheader;
        //two items, one for ascending order, one for descending order.
        ToolStripMenuItem AscendingHeaderItem = new ToolStripMenuItem(String.Format("Sort Column \"{0}\" Ascending",clickedheader.Text));
        ToolStripMenuItem DescendingHeaderItem = new ToolStripMenuItem(String.Format("Sort Column \"{1}\" Descending",clickedheader.Text));
        
        //if the current sort column is the header, check it off and disable it.
        
        if(CurrentSortColumn == clickedheader)
        {  
            if(OurListView.Sorting ==SortOrder.Ascending)
            {
                AscendingHeaderItem.Checked=true;
                AscendingHeaderItem.Enabled=false;
            }
            else if (OurListView.Sorting==SortOrder.Descending)
            {
              DescendingHeaderItem.Checked=true;
              DescendingHeaderItem.Enabled=false;
            }   
        
        }
        AscendingHeaderItem.Tag = ClickedHeader;
        DescendingHeaderItem.Tag = ClickedHeader;
        //set event handlers for the two items.
        AscendingHeaderItem.Click+= AscendingHeaderItem_Click;
        DescendingHeaderItem.Click+= DescendingHeaderItem_Click;
        //add them to the context menu strip.
        _HeaderContextMenuStrip.Items.Add(AscendingHeaderItem);
        _HeaderContextMenuStrip.Items.Add(DescendingHeaderItem);
        //display the menu.
        _HeaderContextMenuStrip.Show(gotposition);
        }
   } 
   else
   {
       //show the default menu, but only if it isn't the ghoststrip.
       if(OurListView.ContextMenuStrip == _ghoststrip)
           e.Cancel=true;
   
   }   

}


 

The events for the two buttons basically sort based on the columnheader in their tag, nothing particularly special there. the actual details can be seen in the source file itself, really.

It actually works quite well, I’m using it in a production application, and it’s working quite well.

Some obvious enhancements, of course, include making it possible to customize the shown menu, to present other options; perhaps a delegate or event that can be hooked that is given the Strip and the clicked column, and any number of other parameters? This would essentially give the equivalent of a ColumnHeaderRightClicked type event, too.

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Anybody who has used windows is probably familiar with the ListView control. It is used in Windows Explorer; it is even used for the desktop. Heck, the ListView control even has implementations on Linux and Mac, and in the latter case it was there first.

The ListView itself can display in several modes. Normally, it shows things as Icons. But it can also be set to show Small Icons, a List, in some Operating Systems, there is a ‘Tile’ option, or even options like Large,Medium, and other sizes of Icons. My Personal favourite is the details mode.

HA! Bet you didn't expect me to use an image from Windows 95! Expect the unexpected, chaps.

Because I mostly see and use Listviews in Details mode, I also force people who use my software to deal with Details mode. Mostly because the reason I am displaying a ListView is to show some data in a somewhat tabular format and not just give them a few icons to drag around with minimal actual information, but I digress. Anyway, I think a good question at this point might be to look at what different parts this particular ListView has. First, the gray “buttons” at the top, which serve to title each column, are referred to affectionately as ColumnHeaders. Under each ColumnHeader there is data for a given “subitem” of each item. For example, the “Size” entry here is a Subitem for each drive. An interesting feature of columnheaders that is nearly universal is that you can click on one, and it will sort by that column.

Another interesting thing, is that in many programming environments, the ListView control doesn’t actually provide this feature for you, and you have to code it yourself. It is rather frustrating. In particular, Visual Basic 6 allows you to sort, but you can’t really customize what you sort by; it always treats it as text. In one of my VB6 applications, BCSearch (which is available for download from my Downloads page) I managed to use a Custom control, available from VBAccelerator.com, which exposes additional functionality of the ListView Control on top of that provided in either of the MS provided libraries for use within Visual Basic. One of these features is that it has better support for sorting. I still had to add my own “arrow” to show the sort direction, though.

BCSearch showing results sorted by Size

The VBAccelerator control exposes a number of events and properties for controlling sorting, which I use to properly sort the various subitems, so that various entries like date or size aren’t sorted as text.

Curiously, the .NET Windows Forms ListView control, while having more functionality, still leaves a lot of effort to the programmer for what ideally ought to be a free feature supported by the OS. In fact it IS a free feature supported by the OS. Thankfully, the .NET control does in fact provide a feature for customizing sort functionality, And all you need is a class to implement IComparer. the IComparer will be used to compare the listitems as the Listview sorts. But if you have, say, Date and Time fields and size fields or other fields that can’t just be sorted as text, you are going to need to implement your own special comparer for each. This amounts to quite a bit of glue code; on top of that, you will need to handle the ColumnClick events on the ColumnHeader, change the sort mode, and sort it, and so forth.

To combat this bloating code, I wrote a relatively small class designed to encapsulate sorting. The idea being that you create a instance of this class for each listview, pass in the ListView to it’s constructor, and the class handles all the details. It worked quite well. There was a minor issue that amounted to a gigantic pain in the ass but at the same time made the result a lot better.

As you can see, it it relatively small (overall). the API code at the top might be a bit confusing, but it is a result of what can only be described as an oversight on Microsoft’s part; see, originally, I was changing the sort arrow header by simply changing the columnheader image. This worked, sorta of, but there was no way to remove the image and it had this weird effect where it would basically move the text and make it aligned sorta weird. Turns out that the way the ListView would “normally” show sort order icons was a built in feature of the Listview since Common Controls 6 (XP). After some SDK digging I was able to use the Platform Invoke feature of C# to call all the appropriate API functions and “force” the Listview to show the sort order in the header appropriately.

The class also exposes a custom delegate which can be implemented and passed in to the constructor, which will allow for “custom” sorts. This is useful if columns contain data like dates, or numbers that you don’t want to be sorted using the normal “text” comparison.

All in all, It’s a class I’ve added to my “toolbox”, alongside my INIFile class for accessing INI Files. did I write about that one? I forget.

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