Create a custom selection extension by extending ArcObjects
ToggleDockWinBtn.cs
// Copyright 2012 ESRI
// 
// All rights reserved under the copyright laws of the United States
// and applicable international laws, treaties, and conventions.
// 
// You may freely redistribute and use this sample code, with or
// without modification, provided you include the original copyright
// notice and use restrictions.
// 
// See the use restrictions.
// 

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using ESRI.ArcGIS.ADF.BaseClasses;
using ESRI.ArcGIS.ADF.CATIDs;
using ESRI.ArcGIS.Framework;
using ESRI.ArcGIS.ArcMapUI;

namespace SelectionCOMSample
{
  /// <summary>
  /// Summary description for ToggleDockWinBtn.
  /// </summary>
  [Guid("26cd6ddc-c2d0-4102-a853-5f7043c6e797")]
  [ClassInterface(ClassInterfaceType.None)]
  [ProgId("SelectionCOMSample.ToggleDockWinBtn")]
  public sealed class ToggleDockWinBtn : BaseCommand
  {
    #region COM Registration Function(s)
    [ComRegisterFunction()]
    [ComVisible(false)]
    static void RegisterFunction(Type registerType)
    {
      // Required for ArcGIS Component Category Registrar support
      ArcGISCategoryRegistration(registerType);

      //
      // TODO: Add any COM registration code here
      //
    }

    [ComUnregisterFunction()]
    [ComVisible(false)]
    static void UnregisterFunction(Type registerType)
    {
      // Required for ArcGIS Component Category Registrar support
      ArcGISCategoryUnregistration(registerType);

      //
      // TODO: Add any COM unregistration code here
      //
    }

    #region ArcGIS Component Category Registrar generated code
    /// <summary>
    /// Required method for ArcGIS Component Category registration -
    /// Do not modify the contents of this method with the code editor.
    /// </summary>
    private static void ArcGISCategoryRegistration(Type registerType)
    {
      string regKey = string.Format("HKEY_CLASSES_ROOT\\CLSID\\{{{0}}}", registerType.GUID);
      MxCommands.Register(regKey);

    }
    /// <summary>
    /// Required method for ArcGIS Component Category unregistration -
    /// Do not modify the contents of this method with the code editor.
    /// </summary>
    private static void ArcGISCategoryUnregistration(Type registerType)
    {
      string regKey = string.Format("HKEY_CLASSES_ROOT\\CLSID\\{{{0}}}", registerType.GUID);
      MxCommands.Unregister(regKey);

    }

    #endregion
    #endregion

    private IApplication m_application;
    private SelectionExtension m_mainExtension;
    private IDockableWindow m_dockWindow;

    public ToggleDockWinBtn()
    {
      base.m_category = "Developer Samples"; 
      base.m_caption = "Toggle Dockable Window C#";  
      base.m_message = "Toggle dockable window C#.";  
      base.m_toolTip = "Toggle dockable window C#.\r\nSelection Sample Extension needs to be turned on in Extensions dialog.";  
      base.m_name = "ESRI_SelectionCOMSample_ToggleDockWinBtn";  

      try
      {
        base.m_bitmap = new Bitmap(GetType().Assembly.GetManifestResourceStream("SelectionCOMSample.Images.ToggleDockWinBtn.png")); 
      }
      catch (Exception ex)
      {
        System.Diagnostics.Trace.WriteLine(ex.Message, "Invalid Bitmap");
      }
    }

    #region Overriden Class Methods

    /// <summary>
    /// Occurs when this command is created
    /// </summary>
    /// <param name="hook">Instance of the application</param>
    public override void OnCreate(object hook)
    {
      if (hook == null)
        return;

      m_application = hook as IApplication;

      //Disable if it is not ArcMap
      if (hook is IMxApplication)
        base.m_enabled = true;
      else
        base.m_enabled = false;

      m_mainExtension = SelectionExtension.GetExtension();

      if (m_mainExtension != null)
        m_dockWindow = m_mainExtension.GetSelectionCountWindow;

    }

    /// <summary>
    /// Occurs when this command is clicked
    /// </summary>
    public override void OnClick()
    {
      if (m_dockWindow == null)
        return;

      m_dockWindow.Show(!m_dockWindow.IsVisible());

    }

    public override bool Enabled
    {
      get
      {
        if (m_mainExtension == null || m_dockWindow == null)
          return false;
        else
          return m_mainExtension.IsExtensionEnabled;
      }
    }
    #endregion
  }
}