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Customizing the Visual Studio Debugger Display of Your Data


Calvin Hsia
Microsoft Corporation

June 2006

Applies to:
   Visual Studio 2005
   Visual Studio .NET 2003
   Visual Studio 7.0

Summary: Illustrates ways to customize the Visual Studio 2005 debugger to get the most out of your debugging time. (6 printed pages)

As a software developer, I spend much of my time looking at code, learning how it works, and figuring out how to modify or fix it. A very good tool to help examine code is the Visual Studio debugger.

(Even if you're not a hard core programmer, the following tutorial shows some of the power of the Visual Studio components—for example, the project system, build system, and debugger—working together.)

At a breakpoint, I can examine local variables in the Watch, Auto, or Locals window to see their values and types. If it's a class or structure, the debugger will show a plus sign (+), indicating that it can be expanded, and the first couple members of that structure. Structures' submembers or inherited values can be examined. These structures can get very deep. Sometimes I need to inspect a value that's dozens of levels down in a hierarchy. That's a lot of complicated tree navigation in the debugger. Other times, I need to take a local variable name (or a member of that variable if it's a structure/class), drag and drop it to a new line in the Watch window, and then typecast it to a value that's more meaningful. As I step through the code, the variable might go out of scope, or it might have a different name in a subroutine, so I'd have to repeat the typecasting steps in the Watch window with the different variable name.

For example, suppose that one of the variables is called VBLine, and that it is an internal representation of a line of Visual Basic .NET code. It's much more meaningful to see Dim MyVar As String than a bunch of hex numbers in the debugger. I drag and drop it to the Watch window, typecast it to a DIM statement, and expand/navigate the results to find MyVar. Then, I step into the next called function, with VBLine passed as an argument. The receiving function names the parameter VBStatement, so my Watch window drilldown needs to be modified to use the different variable name.

This gets very cumbersome. Let's improve it!

Here's a simple demonstration of how you can control what the debugger displays.

  1. Start Visual Studio 2005 or 2003. (It also works in Visual Studio 7, although the steps might be slightly different.)

  2. Click File > New > Projects.

  3. Select Visual C++, and then Win32 Console Application, and name it Test.

  4. Click Finish in the wizard.

  5. Paste in some sample code to debug.

    #include "windows.h"
    int _tmain(int argc, _TCHAR* argv[])
    OSVERSIONINFOEX osinfo;   // Declare a structure
    ZeroMemory(&osinfo, sizeof(osinfo));   // init it to 0
    osinfo.dwOSVersionInfoSize = sizeof(OSVERSIONINFOEX);   // set the size
    GetVersionEx((LPOSVERSIONINFO) &osinfo);   // call WinAPI to fill it in

WIN32_FIND_DATA FAR ffd; // Declare a structure FindFirstFile("c:\windows\system32\k*.exe",&ffd); // Find the first file starting with "k"

return 0; //set bpt here }

This sample code just calls the Windows API functions [GetVersionEx]( and [FindFirstFile](, which fill structures that we can examine in the debugger.
  1. To make things simple, let's use ANSI rather than Unicode characters (Visual Studio 2005 defaults to Unicode):

    1. Click Project > Properties > Configuration Properties > General > Character Set.
    2. Change the character set from Use Unicode Character Set to Use Multi-Byte Character Set.
  2. Let's also remove the check for 64-bit portability issues:

    1. Click Project > Properties > Configuration Properties > C++ > General.
    2. Clear Detect 64 bit portability issues.
  3. Press F9 on the return line to set a breakpoint, and then press F5 to build and run the project.

When the breakpoint hits, the Debug window shows the following.

+      osinfo   {dwOSVersionInfoSize=284 dwMajorVersion=5 dwMinorVersion=1 ...}   _OSVERSIONINFOEXA
+      ffd   {dwFileAttributes=32 ftCreationTime={...} ftLastAccessTime={...} ...}   _WIN32_FIND_DATAA

Now, let's control the string displayed for a given type:

  1. Open the file called AutoExp.dat (which installs with Visual Studio) in the Visual Studio editor:

    1. Click File > Open > File.
    2. Locate the AutoExp.dat file. On my machine, it's at c:\Program Files\Microsoft Visual Studio 8\Common7\Packages\Debugger\Autoexp.dat.

    This file describes how to customize the output of the Debug Watch, Locals, and Auto windows. It's formatted like an INI file.

  2. Add the following line to the AutoExp.dat file, in the [AutoExpand] section.

    _OSVERSIONINFOEXA = Hi there <szCSDVersion> Build number = <dwBuildNumber>

  3. Press F5 to go to the breakpoint.

Now the Watch window shows the following.

+      osinfo   {Hi there 0x0013fe58 "Service Pack 2" Build number = 2600}   _OSVERSIONINFOEXA

This is a big improvement: we've told the debugger which members of the structure to show, and how to format them! We can still click the + to drill down the member hierarchy.

When starting a debug session, the debugger reads the AutoExpand file, and if the left of the equal sign matches the type in the Type column of the Locals/Watch/Auto window, the right side will direct how to format the displayed string. The comments at the beginning of AutoExp.dat give more details, including more formatting options.

This is great, but it's nothing compared to what we'll do next!

You can write code that executes in the debugger process, and that can read the memory of the debugee! AutoExp.dat controls this feature too:

  • In the [AutoExpand] section of the AutoExp.dat file, replace the line that you added in Step 2 above with the following three lines.

The $ADDIN(DllName,FunctionName) syntax means that the DLL that is named will be loaded, and the FunctionName export in the DLL will be called. (Ignore the gobbledygook: it's just C++ name decorating, indicating the calling convention, the parameters, and so on.) If any error occurs—for example, the DLL can't be found, the export can't be found, or the DLL caused an exception, the displayed string will be {???}.

Now, let's create the project that will build MyDbg.DLL, and add it to the current solution:

  1. Click File > New > Project > Visual C++ Win32 Project.
  2. Name the project MyDbgEE, and select Add to Solution (rather than Create New Solution).
  3. In the Win32 App Wizard that appears, change the application type to a DLL.
  4. Change the project properties as above to non-Unicode and no 64-bit issues.
  5. Add the following lines.
    #define ADDIN_API    __declspec(dllexport)

typedef struct tagDEBUGHELPER { DWORD dwVersion; BOOL (WINAPI *ReadDebuggeeMemory)( struct tagDEBUGHELPER pThis, DWORD dwAddr, DWORD nWant, VOID pWhere, DWORD *nGot ); // from here only when dwVersion >= 0x20000 DWORDLONG (WINAPI *GetRealAddress)( struct tagDEBUGHELPER *pThis ); BOOL (WINAPI *ReadDebuggeeMemoryEx)( struct tagDEBUGHELPER pThis, DWORDLONG qwAddr, DWORD nWant, VOID pWhere, DWORD *nGot ); int (WINAPI *GetProcessorType)( struct tagDEBUGHELPER *pThis ); } DEBUGHELPER;

ADDIN_API HRESULT WINAPI EE_OSVERSIONINFOEXA( DWORD dwAddress, DEBUGHELPER *pHelper, int nBase, BOOL bUniStrings, char *pResult, size_t max, DWORD reserved ) { wsprintf(pResult,"Testing Addr = %x Uni = %d base = %d %x",dwAddress,bUniStrings, nBase, *(DWORD *)dwAddress); return S_OK; }

ADDIN_API HRESULT WINAPI EE_WIN32_FIND_DATAA( DWORD dwAddress, DEBUGHELPER *pHelper, int nBase, BOOL bUniStrings, char *pResult, size_t max, DWORD reserved ) { WIN32_FIND_DATA FAR ffd; DWORD nGot=0; pHelper->ReadDebuggeeMemory(pHelper,dwAddress,sizeof(ffd),&ffd,&nGot); wsprintf(pResult,"FindData found file '%s' DBG Process ID = %d",ffd.cFileName, GetCurrentProcessId()); return S_OK; }

Now, we need to tell Visual Studio where to put the built DLL, so that the debugger can find it. We can use the build events in the project:

  1. For the DLL project, click Project > Properties > Configuration Properties > Build Events > Post Build Event > Command Line.
  2. Enter copy $(TargetPath) "$(DevEnvDir)". Make sure that you have the quotation marks and parentheses right. If you put in a description string, that string will be echoed to the Output window when building. Now, when you rebuild, the debug DLL will be copied to the same directory as Devenv.exe.
  3. Press F5 and see the values in the Debug window! Bring up the Task Manager and notice that the Process ID shown is the same as that of the Devenv.exe debugger process.

To make things more interesting, let's see how our debug code can read the debugger memory. We'll add some code to obscure a desired value, but we'll dig for it in the debug DLL.

  1. Add the following code after the #include "windows.h" line in the main Test code.

    struct MyClass {// normally this will go in #include file
    int mymem1;   // make the 1st few members irrelevant, so debugger won't show interesting info
    int mymem2;
    int mymem3;
    int mymem4;
    int mymem5;
    short *str;   // make this not a string, so debugger won't show it as a string
    MyClass * m_pNextClass;   // self referential, perhaps like a linklist

  2. Add the following code to just before the return statement.

       MyClass * pMyClass = new MyClass();   // declare a new instance of MyClass
    pMyClass->str = new short(8);      // create a heap allocated byte array
    memcpy(pMyClass->str,"NotMe!",7);   // desired value to see in debugger

pMyClass->m_pNextClass = new MyClass(); // make a submember instance pMyClass->m_pNextClass->str = new short(8); // heap allocated submember string memcpy(pMyClass->m_pNextClass ->str,"Bingo!",7); // desired value to see in pMyClass

This code creates a class, **MyClass**, with a pointer to another instance of **MyClass** that contains the desired debug display value.

Now, we need to modify the debug DLL to dig for the value.

  1. Copy the same structure definition as above into the debug DLL code. (Typically, these definitions will be in a shared #include file.)

  2. Add the following code.

    ADDIN_API  HRESULT WINAPI EE_MyClass( DWORD dwAddress, DEBUGHELPER *pHelper, int nBase, BOOL bUniStrings, char *pResult, size_t max, DWORD reserved )
    DWORD nGot=0;
    MyClass oMyClass;
    pHelper->ReadDebuggeeMemory(pHelper,dwAddress,sizeof(oMyClass),&oMyClass,&nGot); // read the debuggee's structure
    char szMainStr[100];
    char szMemberStr[100];
    *szMemberStr=0;   // init to empty string
    if (oMyClass.m_pNextClass)   // if there's a sub member
    MyClass oNextClass;
    pHelper->ReadDebuggeeMemory(pHelper,(DWORD)oMyClass.m_pNextClass,sizeof(oNextClass),&oNextClass,&nGot); // read it
    pHelper->ReadDebuggeeMemory(pHelper,(DWORD)oNextClass.str,sizeof(szMemberStr),&szMemberStr,&nGot);      // read it's string
    pHelper->ReadDebuggeeMemory(pHelper,(DWORD)oMyClass.str,sizeof(szMainStr),szMainStr,&nGot);   // read the string of the main struct
    wsprintf(pResult,"MyClass string is '%s'. Submem = '%s'",szMainStr,szMemberStr);
    return S_OK;

  3. Press F5, and Bingo! You can still drill down into the class manually as before, so you haven't lost any functionality.

The debug DLL can be rebuilt even while debugging: it's loaded/unloaded as needed by the debugger. This means that persisting values might be cumbersome. I've used custom registry keys for persisting values, such as global variables.

I've been using this debug expression evaluator architecture for years for huge projects, including Visual Foxpro and Visual Basic.NET, and I find it indispensable and a huge time saver.

See also:

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