A sample of where the TODO comes from, although not really how it all happens, can be viewed at How to write a BHO
This page gives "sample code" anyone can copy and paste into their "design." Only fragments are here, and they had to be htmlized since the 'cat doesn't like tabs. Get the original if you want to use it. No guarantees about the code, as it's a "random pick."

A line that begins with "//" is "commented out" and should be ignored by any program that "runs" the code. Some other sites aren't as careful about the notation.

Note lines 24, 47, 52, 61.

From: www.pudn.com > BHO_enum.rar > BHO.cpp, change:2007-05-09,size:2221b
  1    // BHO.cpp : Defines the class behaviors for the application. 
  2    // 
  3    #include "stdafx.h" 
  4    #include "BHO.h" 
  5    #include "BHODlg.h" 
  6    #ifdef _DEBUG 
  7    #define new DEBUG_NEW 
  8    #undef THIS_FILE 
  9    static char THIS_FILE[] = __FILE__; 
  10    #endif 
  11    ///////////////////////////////////////////////////////////////////////////// 
  12    // CBHOApp 
  13    BEGIN_MESSAGE_MAP(CBHOApp, CWinApp) 
  14    //{{AFX_MSG_MAP(CBHOApp) 
  15    // NOTE - the ClassWizard will add and remove mapping macros here. 
  16    // DO NOT EDIT what you see in these blocks of generated code! 
  17    //}}AFX_MSG 
  18    ON_COMMAND(ID_HELP, CWinApp::OnHelp) 
  19    END_MESSAGE_MAP() 
  20    ///////////////////////////////////////////////////////////////////////////// 
  21    // CBHOApp construction 
  22    CBHOApp::CBHOApp() 
  23    { 
  24    // TODO: add construction code here, 
  25    // Place all significant initialization in InitInstance 
  26    } 
  27    //////////////////////////////////////////////////////////////////////////
  
  … …
  
  45.    if (nResponse == IDOK) 
  46.    { 
  47.    // TODO: Place code here to handle when the dialog is 
  48.    // dismissed with OK 
  49.    } 
  50.    else if (nResponse == IDCANCEL) 
  51.    { 
  52.    // TODO: Place code here to handle when the dialog is 
  53.    // dismissed with Cancel 
  54.    } 
  55.    // Since the dialog has been closed, return FALSE so that we exit the 
  56.    // application, rather than start the application's message pump. 
  57.    return FALSE; 
  58.    } 
  59.    int CBHOApp::ExitInstance() 
  60.    { 
  61.    // TODO: Add your specialized code here and/or call the base class 
  62.    ExitSkin();

John