How to modify 64 bit registry? Getting UnauthrizedAccessException

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C#

Windows

Win7

Hi, i need some codes or another way to change the registry values under HKLM.
When i try it throws exceptions about authority. However my program is running under admin credentials. Here is my code:

void test()
{
    string reg_key = @"SOFTWARE\Microsoft\Windows\CurrentVersion\abc";

    RegistryKey reg = Registry.LocalMachine.OpenSubKey(reg_key);

    reg.OpenSubKey("test").SetValue("State", "1"); 
}

I can do it manually but i need to do it programmatically. So how can i do?