C# TreeView with multiple selection

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The .NET TreeView control has no built-in multiple selection. Let there be. This article about C# depicts a tree view control with multiple selection, derived from the base .NET TreeView control. It supports CTRL and SHIFT combinations.

How to use it

This control is a C# control. Once compiled, it becomes a managed .NET component and behaves much like good ol' ActiveX components for VB developers. To use it in your application, you have at least two options. The first is to simply reference the TreeViewMS project given in the source project, by clicking right on your current Windows Form application and choosing Add Reference, then browse to TreeViewMS.csproj. The screen capture below shows the steps:

Adding a reference to the new TreeView control to your code

You may also add the tree view control once for all in your Toolbox window, then simply drag&drop it onto your Form. In order to do this, show up the .NET Studio Toolbox window, then right click and choose Customize toolbox, then choose the .NET Framework components tab, and browse to the compiled control: TreeViewMS.dll, as in the capture below:

Adding the new tree view control to the Visual Studio .NET toolbox

Once you have added this tree view control in a form, you may start to use it like the base .NET TreeView control. The addition lies in a new exposed property, SelectedNodes, which returns the collection of selected TreeNode items. If we take the demo app, which adds selected items in the list view on the right, then code goes like this:

private TreeViewMS.TreeViewMS treeViewMS1;
private System.Windows.Forms.ListView listView1;
...

// add selected items from treeview to the
// listview on the right hand side
foreach (TreeNode n in treeViewMS1.SelectedNodes)
{
   listView1.Items.Add( n.Text, n.ImageIndex );
}

SelectedNodes is also a read-write property. What follows is a sample code that forces the selection of given tree items:

// valid item from the sample tree
TreeNode n1 = treeViewMS1.Nodes[0].Nodes[0].Nodes[0];
// valid item from the sample tree
TreeNode n2 = treeViewMS1.Nodes[0].Nodes[0].Nodes[2];
ArrayList coll = new ArrayList();
coll.Add(n1);
coll.Add(n2);
treeViewMS1.SelectedNodes = coll;

Technical details

Of course, this control is derived from the base TreeView control:

public class TreeViewMS : System.Windows.Forms.TreeView
{
    // class members
    protected ArrayList     m_coll;
    protected TreeNode      m_lastNode, m_firstNode;

    ...

    /* API method */ public ArrayList SelectedNodes
    {
        get
        {
            return m_coll;
        }
        set
        {
            removePaintFromNodes();
            m_coll.Clear();
            m_coll = value;
            paintSelectedNodes();
        }
    }

}

What we need to control item selection is events such like BEFORESELECT and AFTERSELECT to get the current selected node. BEFORESELECT is only useful to undo a node selection, for instance when you select twice a node with CTRL down.

Because Microsoft has clearly figured out that TreeView s were likely to be derived, they have overridable methods BeforeSelect(...) and AfterSelect(...) that don't interfere with the events named the same. All what we need is override the 2 methods and not forget to call the base class in the implementation:

protected override void OnBeforeSelect(TreeViewCancelEventArgs e)
{
    // e.Node is the current node exposed by the base TreeView control
    base.OnBeforeSelect(e);

    bool bControl = (ModifierKeys==Keys.Control);
    bool bShift = (ModifierKeys==Keys.Shift);

    // selecting twice the node while pressing CTRL ?
    if (bControl && m_coll.Contains( e.Node ) )
    {
        // unselect it
        // (let framework know we don't want selection this time)
        e.Cancel = true;

        // update nodes
        removePaintFromNodes();
        m_coll.Remove( e.Node );
        paintSelectedNodes();
        return;
    }

    m_lastNode = e.Node;
    if (!bShift) m_firstNode = e.Node; // store begin of shift sequence
}

protected override void OnAfterSelect(TreeViewEventArgs e)
{
    // e.Node is the current node exposed by the base TreeView control

    base.OnAfterSelect(e);

    bool bControl = (ModifierKeys==Keys.Control);
    bool bShift = (ModifierKeys==Keys.Shift);

    if (bControl)
    {
        if ( !m_coll.Contains( e.Node ) ) // new node ?
        {
            m_coll.Add( e.Node );
        }
        else  // not new, remove it from the collection
        {
            removePaintFromNodes();
            m_coll.Remove( e.Node );
        }
        paintSelectedNodes();
    }
    else 
    {
        if (bShift)
        {
            Queue myQueue = new Queue();

            TreeNode uppernode = m_firstNode;
            TreeNode bottomnode = e.Node;

            // case 1 : begin and end nodes are parent
            bool bParent = isParent(m_firstNode, e.Node);
            if (!bParent)
            {
                bParent = isParent(bottomnode, uppernode);
                if (bParent) // swap nodes
                {
                    TreeNode t = uppernode;
                    uppernode = bottomnode;
                    bottomnode = t;
                }
            }
            if (bParent)
            {
                 TreeNode n = bottomnode;
                 while ( n != uppernode.Parent)
                 {
                     if ( !m_coll.Contains( n ) ) // new node ?
                         myQueue.Enqueue( n );

                      n = n.Parent;
                 }
            }
            // case 2 : nor the begin nor the
            // end node are descendant one another
            else
            {
                 // are they siblings ?                 

                 if ( (uppernode.Parent==null && bottomnode.Parent==null) 
                       || (uppernode.Parent!=null && 
                       uppernode.Parent.Nodes.Contains( bottomnode )) )
                 {
                      int nIndexUpper = uppernode.Index;
                      int nIndexBottom = bottomnode.Index;
                      if (nIndexBottom < nIndexUpper) // reversed?
                      {
                           TreeNode t = uppernode;
                           uppernode = bottomnode;
                           bottomnode = t;
                           nIndexUpper = uppernode.Index;
                           nIndexBottom = bottomnode.Index;
                      }

                      TreeNode n = uppernode;
                      while (nIndexUpper <= nIndexBottom)
                      {
                           if ( !m_coll.Contains( n ) ) // new node ?
                               myQueue.Enqueue( n );

                           n = n.NextNode;

                           nIndexUpper++;
                      } // end while

                  }
                  else
                  {
                      if ( !m_coll.Contains( uppernode ) ) 
                          myQueue.Enqueue( uppernode );
                      if ( !m_coll.Contains( bottomnode ) ) 
                          myQueue.Enqueue( bottomnode );
                  }

             }

             m_coll.AddRange( myQueue );

             paintSelectedNodes();
             // let us chain several SHIFTs if we like it
             m_firstNode = e.Node; 

         } // end if m_bShift
         else
         {
              // in the case of a simple click, just add this item
              if (m_coll!=null && m_coll.Count>0)
              {
                   removePaintFromNodes();
                   m_coll.Clear();
              }
              m_coll.Add( e.Node );
          }
     }
}


// Helpers
//
//


protected bool isParent(TreeNode parentNode, TreeNode childNode)
{
    if (parentNode==childNode)
        return true;

    TreeNode n = childNode;
    bool bFound = false;
    while (!bFound && n!=null)
    {
        n = n.Parent;
        bFound = (n == parentNode);
    }
    return bFound;
}


protected void paintSelectedNodes()
{
    foreach ( TreeNode n in m_coll )
    {
        n.BackColor = SystemColors.Highlight;
        n.ForeColor = SystemColors.HighlightText;
    }
}

protected void removePaintFromNodes()
{
    if (m_coll.Count==0) return;

    TreeNode n0 = (TreeNode) m_coll[0];
    Color back = n0.TreeView.BackColor;
    Color fore = n0.TreeView.ForeColor;

    foreach ( TreeNode n in m_coll )
    {
        n.BackColor = back;
        n.ForeColor = fore;
    }
}

As you may note, SelectedNodes returns a System.Collections.ArrayList instance of underlying TreeView items, not the more natural System.Windows.Forms.TreeNodeCollection instance. Why this? In fact, that's not up to developers, the TreeNodeCollection is deliberately provided by the .NET framework with a hidden constructor, hence it is not possible to reuse it. This has raised some concern on public newsgroups, but Microsoft people have not agreed to change this anytime soon.

Simpler reuse

What if you don't want to redistribute the TreeViewMS.dll library (I admit that separate libraries are always bottlenecks)? All you have to do is take the code for OnBeforeSelect() and OnAfterSelect() as described above and attach it to the standard TreeView events.

Thanks to David Sleeckx for the bug hunting.

History

• August 8, 2002 - First version.
• Updated August 18, 2002.