Guide to BSTR and C String Conversions

Introduction

One of the confusing aspects of Windows programming is managing the conversion of Visual Basic style strings to/from C language style strings. It isn't that it is so difficult, it is just difficult to remember the details. It is usually not done often, and the MSDN documentation is so voluminous that it is difficult to find answers to your questions. But, the worst part is that you could perform some typecast that compiles fine, but doesn't work the way you expect. This results in code that doesn't work, and the bugs are hard to track down. After some experience, you learn to make sure your string conversions are doing what you expect.

C strings are arrays of characters terminated by a NULL character. Visual Basic strings differ in that the length of the string precede the characters in the string. So, a VB string knows its own length. In addition, all VB strings are Unicode (16 bits per character).

String Types

BSTR/C String conversions are required if:

• You are doing COM programming in C/C++
• You are writing multiple language applications, such as C++ DLL's accessed by Visual Basic applications.

C Language String Types and Classes

This article deals with the following C/MFC/ATL string types:

Demo Project

The demo project is just an MFC dialog-based application with buttons for each type of conversion. It is built using VC++ 6.0. It uses a couple of support functions you may find helpful:

BSTR GetBSTR()
{
  _bstr_t bstr1(_T("This is the test string."));

  BSTR bstr;

  bstr = bstr1.copy();

  return bstr;
}




CComBSTR GetComBSTR()
{
  CComBSTR bstr("This is the test string.");

  return bstr;
}


void CVbsDlg::ShowBSTR(BSTR bstr)
{
  _bstr_t bstrStart(bstr); 

  CString s;

  s.Format(_T("%s"), (LPCTSTR)bstrStart);

  AfxMessageBox(s);

}

Conversions

So, let's get to it. Here are the conversion techniques:

1. Converting BSTR to _bstr_t

  // BSTR to _bst_t

  BSTR bstrStart = GetBSTR();

  // use the constructor
  _bstr_t bstrFinal(bstrStart);

  ShowBSTR(bstrFinal);

  // Use the = operator
  bstrFinal = bstrStart;

  ShowBSTR(bstrFinal);

2. Converting a _bstr_t to BSTR

   You may want to get a BSTR from a _bstr_t class.

     // _bstr_t to BSTR
   
     _bstr_t bstrStart(_T("This is the test string."));
   
     BSTR bstrFinish;
   
     // use _bstr_t::copy member function
     bstrFinish = bstrStart.copy();
   
     ShowBSTR(bstrFinish);
   
     // use = operator
     bstrFinish = bstrStart;
   
     ShowBSTR(bstrFinish);
   

3. Converting a CComBSTR to BSTR

   You may want to get a BSTR from a CComBSTR class.

     // CComBSTR to BSTR
     CComBSTR bstrStart(_T("This is the test string."));
   
     BSTR bstrFinish;
   
     // use the = operator
     bstrFinish = bstrStart;
   
     ShowBSTR(bstrFinish);
   
     // use the Copy member function
     bstrFinish = bstrStart.Copy();
   
     ShowBSTR(bstrFinish);
   

4. Converting _bstr_t to CComBSTR

     // _bstr_t to CComBSTR
     _bstr_t bstrStart(_T("This is the test string."));
   
     CComBSTR bstrFinish;
   
     bstrFinish.AppendBSTR(bstrStart);
   
     ShowBSTR(bstrFinish);
   

5. Converting BSTR to C String

   (Note: conversion that only works in Unicode)

     // BSTR to C String
   
     BSTR bstrStart;
   
     bstrStart = GetBSTR();
   
     TCHAR szFinal[255];
   
     // direct conversion from BSTR to LPCTSTR only works
     // in Unicode
     _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrStart);
     AfxMessageBox(szFinal);
   
     _bstr_t bstrIntermediate(bstrStart);    // convert to
                                             // _bstr_t
     CString strFinal;
   
     // you have to go through _bstr_t to have it work in ANSI
     // and Unicode
     _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrIntermediate);
   
     // Or, using MFC
   
     strFinal.Format(_T("%s"), (LPCTSTR)bstrIntermediate);
   
     AfxMessageBox(strFinal);
   

6. Converting _bstr_t to C String

   (this works in both ANSI and Unicode)

     _bstr_t bstrStart(_T("This is the test string."));
     TCHAR szFinal[255];
   
     _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrStart);
   
     AfxMessageBox(szFinal);
   

7. Converting CComBSTR to LPCTSTR

   (not possible; must go through _bstr_t)

     // CComBSTR to C String
     CComBSTR bstrStart("This is the test string.");
   
     _bstr_t bstrIntermediate(bstrStart);
   
     TCHAR szFinal[255];
   
     _stprintf(szFinal, _T("%s"),
               (LPCTSTR)bstrIntermediate);
   
     AfxMessageBox(szFinal);
   

8. Converting LPCTSTR to _bstr_t

   Use a constructor or = operator

     // LPCTSTR to _bstr_t
   
     LPCTSTR szStart = _T("This is the text string");
   
     // Use the constructor
     _bstr_t bstrFinal(szStart);
   
     ShowBSTR(bstrFinal);
   
     // or use = operator
     bstrFinal = szStart;
   
     ShowBSTR(bstrFinal);
   

9. Converting LPCTSTR to CComBSTR

   Use a constructor or CComBSTR::Append function

     // LPCTSTR to CComBSTR
   
     // Use a constructor
   
     LPCTSTR szStart = _T("This is the text string");
   
     // Use the constructor
     CComBSTR bstrFinal(szStart);
   
     ShowBSTR(bstrFinal);
   
     // Or use the Append function
     bstrFinal.Empty();
     bstrFinal.Append(szStart);
   
     ShowBSTR(bstrFinal);
   

Well, I tested all of the conversions in the demo project. If you need to try others, download the demo for easy modification. I am sure I will hear if there are any mistakes!

Downloads