Read/Write Windows Registry on Windows 64-Bits Platforms (C# Version)

Abstraction

How does a 32-bits(x64) program running on Windows 64-bits(x64) access the 64-bits registry? In C#, you can use P/Invoke to call RegOpenKeyEx function with KEY_WOW64_KEY to access the 64-bits registry.

Code Snippet

//
// This is a C# example to get the version of Windows Media Center on Windows 7 64-bits.
//
class RegQueryValueDemo
{
  [DllImport("advapi32.dll", CharSet = CharSet.Unicode, EntryPoint = "RegOpenKeyExW", SetLastError = true)]
  public static extern int RegOpenKeyEx(UIntPtr hKey, string subKey, uint options, int sam, out UIntPtr phkResult);
  public static UIntPtr HKEY_CURRENT_USER = (UIntPtr)0x80000001;
  public static UIntPtr HKEY_LOCAL_MACHINE = (UIntPtr)0x80000002;
  public static int KEY_QUERY_VALUE = 0x0001;
  public static int KEY_SET_VALUE = 0x0002;
  public static int KEY_CREATE_SUB_KEY = 0x0004;
  public static int KEY_ENUMERATE_SUB_KEYS = 0x0008;
  public static int KEY_WOW64_64KEY = 0x0100;
  public static int KEY_WOW64_32KEY = 0x0200;

  [DllImport("advapi32.dll", CharSet = CharSet.Unicode, EntryPoint = "RegQueryValueExW", SetLastError = true)]
  public static extern int RegQueryValueEx(UIntPtr hKey, string lpValueName, int lpReserved, out uint lpType,
    StringBuilder lpData, ref int lpcbData);

  public string GetMCEIdent()
  {
    UIntPtr regKeyHandle;
    if (RegOpenKeyEx(
    HKEY_LOCAL_MACHINE,
    "SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Media Center",
    0, KEY_QUERY_VALUE | KEY_WOW64_64KEY, out regKeyHandle) == 0)
    {
      uint type;

      StringBuilder stringBuilder = new StringBuilder(2048);
      int cbData = stringBuilder.Capacity;
      if (RegQueryValueEx(regKeyHandle, "Ident", 0, out type, stringBuilder, ref cbData) == 0)
      {
        return stringBuilder.ToString();
      }
    }

    return string.Empty;
  }
}

Find a C++ version?
Read/Write Windows Registry on Windows 64-Bits Platforms

Reference on MSDN:
RegOpenKeyEx Function
32-bit and 64-bit Application Data in the Registry

Read/Write Windows Registry on Windows 64-Bits Platforms

On Windows Vista 64-bits, there's a 32-bits emulator called WOW64 for 32-bits applications. A 32-bits application running on WOW64 uses the registry redirector, because the 64-bit version of an application may use different registry keys and values than the 32-bit version.

For 32-bits application:
On 64-bit platforms, it runs on WOW64 and accesses 32-bit registry (inside Wow6432Node).

For 64-bits application:
On 64-bit platforms, it runs natively and accesses 64-bit registry (not inside Wow6432Node).

Whatever, you can explicitly indicate which version of registry you want to access, just use RegOpenKeyEx function with KEY_WOW64_32KEY or KEY_WOW64_64KEY in the desired access rights parameter.

Example: A 32-bit application running on WOW64 accesses the 64-bit registry

HKEY hkResult = NULL;
REGSAM samDesired = KEY_READ | KEY_WOW64_64KEY;
LONG lRetOpen = ::RegOpenKeyExW(
  HKEY_LOCAL_MACHINE,
  L"SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Media Center",
  NULL, samDesired, &hkResult);

Reference on MSDN:
32-bit and 64-bit Application Data in the Registry