try to change ActivePowerScheme: RegOpenKeyEx failed with error 0 - c++

Question

I need to set ActivePowerScheme by changing it in registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\User\PowerSchemes.
So I try to do it with winapi functions RegOpenKeyEx and RegSetValueEx
wchar_t *PowerScheme=TEXT("8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c");
HKEY hRootKey = HKEY_LOCAL_MACHINE;
PWCHAR sKey = TEXT("SYSTEM\\CurrentControlSet\\Control\\Power\\User\\PowerSchemes");
PWCHAR sActivePowerS = TEXT("ActivePowerScheme");
HKEY hKeyResult = NULL;
//open
if (RegOpenKeyEx(hRootKey,sKey,0,KEY_ALL_ACCESS,&hKeyResult)!=ERROR_SUCCESS) {
//it is always failing with error 0 !
DWORD dw = GetLastError();
}
But RegOpenKeyEx() is always failing with error 0, that means "Operation completed successfully". And RegSetValueEx() returns same value.
if(RegSetValueEx(hKeyResult,sActivePowerS,0,REG_SZ,
(BYTE *)PowerScheme,wcslen(PowerScheme))!=ERROR_SUCCESS) {
//it is always failing with error 0
DWORD dw = GetLastError();
}
And of course current power scheme doesn't change value. But according to msdn:
"If the function succeeds, the return value is ERROR_SUCCESS.
If the function fails, the return value is a nonzero error code".
I will be grateful to any your answers.
P.S. it compiled in Windows 7 and executed with rights of admin

Answer 1

You are going about this the wrong way. You RARELY need to change stuff in the registry yourself.
Read Power Scheme Management on the MSDN site for the proper way of doing it.

Answer 2

As documentation states, RegOpenKeyEx does not update GetLastError, and return value is the error code itself. Would you mind checking it?
I'd bet you have ERROR_ACCESS_DENIED error here.
UPD: While this perhaps answers your question, you should consider using API suggested by RedX in order to update power management settings. Permissions on this registry key are set (for a reason!) in a way that even Administrators have only read permissions, and not write.

Answer 3

In the comments you state that RegOpenKeyEx returns ERROR_ACCESS_DENIED. This is because you request write access to a key to which you do not have sufficient rights because of UAC. You will need to run your process elevated to write to this key.
As others have correctly pointed out, you should not call GetLastError since RegOpenKeyEx does not set the last error value and instead returns the error code directly. More importantly you should be using the power management API rather than hacking the registry.
Even when you switch to the power management API you will still require administrator rights. You can arrange this by setting requestedExecutionLevel to requireAdministrator in your application manifest.
In Visual Studio you can make this change in the project configuration under Linker | Manifest File | UAC Execution Level.