VariableHeightItems.cs

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

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;
    const int TVM_GETITEMHEIGHT = 0x1100 + 28;
    const int TVM_SETITEMHEIGHT = 0x1100 + 27;

    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)); }
    }

    public static int GetItemHeight(this TreeView tv)
    {
        var none = new IntPtr(0);
        unsafe { return SendMessage(tv.Handle, TVM_GETITEMHEIGHT, none, none).ToInt32(); }
    }

    public static void SetItemHeight(this TreeView tv, int height)
    {
        unsafe { SendMessage(tv.Handle, TVM_SETITEMHEIGHT, new IntPtr(height), new IntPtr(0)); }
    }

    public static void SetHeightInPixels(this TreeNode tn, int heightInPixels)
    {
        if (tn.TreeView.GetItemHeight() != 1)
            tn.TreeView.SetItemHeight(1);
        tn.SetHeight(heightInPixels);
    }
}