How can I read a key value of Registry and print it to the screen with the MessageBox()

c++, winapi

Solution

Navigating the Win32 API can be a tricky business. The registry APIs are some of the more complicated. Here's a short program to demonstrate how to read a registry string.

#include <Windows.h>
#include <iostream>
#include <string>

using namespace std;

wstring ReadRegValue(HKEY root, wstring key, wstring name)
{
    HKEY hKey;
    if (RegOpenKeyEx(root, key.c_str(), 0, KEY_READ, &hKey) != ERROR_SUCCESS)
        throw "Could not open registry key";

    DWORD type;
    DWORD cbData;
    if (RegQueryValueEx(hKey, name.c_str(), NULL, &type, NULL, &cbData) != ERROR_SUCCESS)
    {
        RegCloseKey(hKey);
        throw "Could not read registry value";
    }

    if (type != REG_SZ)
    {
        RegCloseKey(hKey);
        throw "Incorrect registry value type";
    }

    wstring value(cbData/sizeof(wchar_t), L'\0');
    if (RegQueryValueEx(hKey, name.c_str(), NULL, NULL, reinterpret_cast<LPBYTE>(&value[0]), &cbData) != ERROR_SUCCESS)
    {
        RegCloseKey(hKey);
        throw "Could not read registry value";
    }

    RegCloseKey(hKey);

    size_t firstNull = value.find_first_of(L'\0');
    if (firstNull != string::npos)
        value.resize(firstNull);

    return value;
}

int wmain(int argc, wchar_t* argv[])
{
    wcout << ReadRegValue(HKEY_LOCAL_MACHINE, L"SOFTWARE\\Microsoft\\Windows\\CurrentVersion", L"CommonFilesDir");
    return 0;
}

Notes:

- I don't have CE so this is a plain Win32 app, compiled for Unicode. I took that route because CE doesn't do ANSI characters.

- I've taken advantage of a number of C++ features. Most significantly `std::wstring`. This makes string handling a cinch.

- I've used exceptions for error handling. You could replace that with some other mechanism, but it served my purpose of keeping the error handling issues in the background.

- Using exceptions makes closing the registry key slightly messy. A better solution would be to use an RAII class to wrap up the lifetime of the registry key. I've omitted that for simplicity, but in production code you would want to take that extra step.

- Usually, `RegQueryValueEx` returns `REG_SZ` data that is null-terminated. This code deals with that by truncating beyond the first null character. In case the value returned is not null-terminated, that truncation won't happen, but the value will still be fine.

- I've just printed to my console, but it would be trivial for you to call `MessageBox`. Like this: `MessageBox(0, value.c_str(), L"Caption", MB_OK)`

Problem

Original source