Question

From what I understand in the documentation, it returns null if it fails, but it seems to me there is an exception ready for every fail scenario.

In what standard scenario will this function actually return a null value?

Answer 1

If you look at the code through Reflector, you will see that if the return from the native method Win32Native.RegCreateKeyEx() returns anything other than an error code of 0 or if the result of the operation is null, it will then return null back to you.

A possible failure here is if the incorrect permissions are used, causing an access denied error code to be returned.

The code behind this method is as follows:

[ComVisible(false)]
public unsafe RegistryKey CreateSubKey(string subkey, RegistryKeyPermissionCheck permissionCheck, RegistrySecurity registrySecurity)
{
    ValidateKeyName(subkey);
    ValidateKeyMode(permissionCheck);
    this.EnsureWriteable();
    subkey = FixupName(subkey);
    if (!this.remoteKey)
    {
        RegistryKey key = this.InternalOpenSubKey(subkey, permissionCheck != RegistryKeyPermissionCheck.ReadSubTree);
        if (key != null)
        {
            this.CheckSubKeyWritePermission(subkey);
            this.CheckSubTreePermission(subkey, permissionCheck);
            key.checkMode = permissionCheck;
            return key;
        }
    }
    this.CheckSubKeyCreatePermission(subkey);
    Win32Native.SECURITY_ATTRIBUTES structure = null;
    if (registrySecurity != null)
    {
        structure = new Win32Native.SECURITY_ATTRIBUTES();
        structure.nLength = Marshal.SizeOf(structure);
        byte[] securityDescriptorBinaryForm = registrySecurity.GetSecurityDescriptorBinaryForm();
        byte* pDest = stackalloc byte[1 * securityDescriptorBinaryForm.Length];
        Buffer.memcpy(securityDescriptorBinaryForm, 0, pDest, 0, securityDescriptorBinaryForm.Length);
        structure.pSecurityDescriptor = pDest;
    }
    int lpdwDisposition = 0;
    SafeRegistryHandle hkResult = null;
    int errorCode = Win32Native.RegCreateKeyEx(this.hkey, subkey, 0, null, 0, GetRegistryKeyAccess(permissionCheck != RegistryKeyPermissionCheck.ReadSubTree), structure, out hkResult, out lpdwDisposition);
    if ((errorCode == 0) && !hkResult.IsInvalid)
    {
        RegistryKey key2 = new RegistryKey(hkResult, permissionCheck != RegistryKeyPermissionCheck.ReadSubTree, false, this.remoteKey, false);
        this.CheckSubTreePermission(subkey, permissionCheck);
        key2.checkMode = permissionCheck;
        if (subkey.Length == 0)
        {
            key2.keyName = this.keyName;
            return key2;
        }
        key2.keyName = this.keyName + @"\" + subkey;
        return key2;
    }
    if (errorCode != 0)
    {
        this.Win32Error(errorCode, this.keyName + @"\" + subkey);
    }
    return null;
}