VariableHeightItems.cs

static class VariableHeightItems
{
	[DllImport("user32")]
	static extern IntPtr SendMessage(IntPtr hwnd, uint msg, IntPtr wp, IntPtr lp);

	[StructLayout(LayoutKind.Sequential)]
	struct TVITEMEX
	{
		public Mask mask;
		public IntPtr item;
		public uint state;
		public uint stateMask;
		public IntPtr pszText;
		public int cchTextMax;
		public int iImage;
		public int iSelectedImage;
		public int iChildren;
		public IntPtr lParam;
		public int iIntegral;

		// ... plus some windows 6+ crap
	}

	[Flags]
	enum Mask : uint
	{
		Text = 1,
		Image = 2,
		Param = 4,
		State = 8,
		Handle = 16,
		SelectedImage = 32,
		Children = 64,
		Integral = 128,
	}

	const int TVM_GETITEM = 0x1100 + 62;
	const int TVM_SETITEM = 0x1100 + 63;

	public static int GetHeight(this TreeNode tn)
	{
		TVITEMEX tvix;
		tvix.mask = Mask.Handle | Mask.Integral;
		tvix.item = tn.Handle;
		tvix.iIntegral = 0;
		unsafe { SendMessage(tn.TreeView.Handle, TVM_GETITEM, IntPtr.Zero, new IntPtr((void*)&tvix)); }
		return tvix.iIntegral;
	}

	public static void SetHeight(this TreeNode tn, int height)
	{
		TVITEMEX tvix;
		tvix.mask = Mask.Handle | Mask.Integral;
		tvix.item = tn.Handle;
		tvix.iIntegral = height;
		unsafe { SendMessage(tn.TreeView.Handle, TVM_SETITEM, IntPtr.Zero, new IntPtr((void*)&tvix)); }
	}
}