C#

public class Singleton
{
    // private constructor to prevent instantiation
    private Singleton() {}


    private static volatile Singleton _singleton;
    private static object _lock = new object();

    public static Singleton Value
    {
        get
        {
            if (_singleton == null)
            {
                lock(_lock);
                {
                    if (_singleton == null) _singleton = new Singleton();
                }
            }
            return _singleton;
         }
     }
}

private void button1_Click(object sender, System.EventArgs e)
{
  if( treeView1.SelectedNode != null && treeView2.SelectedNode != null )
  {
    TreeNode n = treeView1.SelectedNode;
    treeView1.SelectedNode.Remove();
    treeView2.SelectedNode.Nodes.Add( n );
  }
}

// In your constructor after InitializeComponent()
treeView1.MouseDown += new MouseEventHandler( SourceTreeMD );
treeView1.MouseUp += new MouseEventHandler( SourceTreeMU );
treeView1.MouseLeave += new EventHandler( SourceTreeML );
treeView2.DragDrop += new DragEventHandler( TargetDrop );
treeView2.DragEnter += new DragEventHandler( TargetEnter );

private void SourceTreeMD( object sender, MouseEventArgs mea )
{
  // Remember the node on the mouse down.
  draggedNode = treeView1.GetNodeAt( mea.X, mea.Y );
}

private void SourceTreeMU( object sender, MouseEventArgs mea )
{
  // Forget the node.
  draggedNode = null;
}

private void SourceTreeML( object sender, EventArgs ea )
{
  // Do the drag and drop.
  if( draggedNode != null )
  {
    treeView1.DoDragDrop( draggedNode, DragDropEffects.Move );
  }
}

private void TargetDrop( object sender, DragEventArgs dea )
{
  // Drop the node.
  Point p = treeView2.PointToClient( new Point( dea.X, dea.Y ) );
  TreeNode dropTargetNode = treeView2.GetNodeAt( p );
  TreeNode sourceNode = dea.Data.GetData( typeof( TreeNode ) ) as TreeNode;
  if( dropTargetNode != null && sourceNode != null )
  {
    sourceNode.Remove();
    dropTargetNode.Nodes.Add( sourceNode );
  }
  else if( sourceNode != null )
  {
    sourceNode.Remove();
    treeView2.Nodes.Add( sourceNode );
  }
}

private void TargetEnter( object sender, DragEventArgs dea )
{
  // Notify the target that the drop is good.
  if( dea.Data.GetDataPresent( typeof( TreeNode ) ) )
  {
    dea.Effect = DragDropEffects.Move;
  }
}

// The node getting dragged.
private TreeNode draggedNode;