C#

private const int TCN_SELCHANGING = -552;
protected override void WndProc(ref Message m)
{
  base.WndProc(ref m);
  if (m.Msg == TCN_SELCHANGING)
  {
    // Uses made-up variable below - use whatever condition you need.
    if (!allowChange) m.Result = new IntPtr(1); // TRUE
  }
}

#define TCN_FIRST               (0U-550U)       // tab control
...
#define TCN_SELCHANGING         (TCN_FIRST - 2)

LRESULT CALLBACK WindowProc(          HWND hwnd,
    UINT uMsg,
    WPARAM wParam,
    LPARAM lParam
);

protected override void WndProc(ref Message m) {
   Console.WriteLine(m.Msg);
   base.WndProc (ref m);
}

const int TCN_SELCHANGING = -552;
protected override void WndProc(ref Message m) {
  Debug.Assert(m.Msg != TCN_SELCHANGING); //Never fires
  base.WndProc (ref m);
}

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;
public class Test : Form
{
  public static void Main()
  {
    Application.Run(new Test());
  }
  public Test()
  {
    TabControl tab = new TabControl();
    
    this.SuspendLayout();
    this.Text = "Test";
    this.Controls.Add(tab);
	tab.TabPages.Add(new TabPage("Page 1"));
	tab.TabPages.Add(new TabPage("Page 2"));
    tab.Dock = DockStyle.Fill;
    this.ResumeLayout();
  }
  private const int WM_NOTIFY = 0x004e;
  private const int TCN_SELCHANGING = -552;
  private struct NMHDR
  {
    public IntPtr hwndFrom;
    public int idFrom;
    public int code;
  }
  protected override void WndProc(ref Message m)
  {
    base.WndProc(ref m);
    if (m.Msg == WM_NOTIFY)
    {
      NMHDR hdr = (NMHDR)Marshal.PtrToStructure(m.LParam, typeof(NMHDR));
      Console.WriteLine(hdr.code);
      if (hdr.code == TCN_SELCHANGING)
        m.Result = new IntPtr(1);
    }
  }
}

public class MyListBox : ListBox
{
  // ...
  private MyListBox.MyObjectCollection items;
  public new MyListBox.MyObjectCollection Items
  {
    if (items == null)
      items = new MyListBox.MyObjectCollection();
    return items;
  }
  private class MyObjectCollection : CollectionBase // CollectionBase helps,
                                                    // but is not required
  {
    // Implement methods like Add with Typed parameters.
  }
}