Question

I am attempting to write a value to the HKLM registry using TRegistry component in Delphi.

Since I am running on Windows 2000 as a standard user (or XP as a standard user, or Windows Vista as a standard user, or Windows 7 with a standard user), I fully expect that I will not be able to write to the HKEY_LOCAL_MACHINE portion of the registry:

reg := TRegistry.Create(KEY_WRITE);
try
   reg.Access := KEY_WRITE; //sure, set it again, why not
   reg.RootKey := HKEY_LOCAL_MACHINE;
   if not reg.OpenKey('\Software\Microsoft\SQMClient', True) then
      Exit;

   reg.WriteString('MachineId', s);
finally
   reg.Free;
end;

Unfortunately, the WriteString throws an ERegistryException:

Failed to set data for 'MachineId`

This is fully expected, which is why I’m trying to avoid the exception. I do not see any CanWriteString or TryWriteString in TRegistry.

How can I not trigger an exception when trying to write to HKLM?

---

Self-evident notes:

• if the user actually is an administrator then the write should be able to succeed

• wrapping the call to WriteString in a try-except:

  reg := TRegistry.Create(KEY_WRITE);
  try
    reg.RootKey := HKEY_LOCAL_MACHINE;
    if not reg.OpenKey('\Software\Microsoft\SQMClient', True) then
       Exit;
  
    try
       reg.WriteString('MachineId', s);
    except
       on E:ERegistryException do
          {nothing};
    end;
  finally
    reg.Free;
  end;
  

  doesn’t prevent the exception from being thrown in the first place.

Update: From RTL source:

KEY_WRITE          = (STANDARD_RIGHTS_WRITE or
                        KEY_SET_VALUE or
                        KEY_CREATE_SUB_KEY) and not
                        SYNCHRONIZE;

from MSDN:

KEY_WRITE (0x20006)  

Combines the STANDARD_RIGHTS_WRITE, KEY_SET_VALUE, and KEY_CREATE_SUB_KEY access rights.

Answer 1

You can’t get TRegistry to behave the way you want. There are no TryXXX methods and there are not parameters that disable exceptions. You can be sure that this is so because the TRegistry methods do not provide any error or status codes.

You will have to write your own wrappers around the Win32 registry API.

As an aside, I agree with your opinion, expressed in the comments, that TRegistry is lacking in functionality here. We expect registry operations to fail, and so we should not have to catch exceptions to deal with that.