Memory leak using WMI in Delphi 7

Question

I'm experiencing a memory leak when using WMI from Delphi 7 to query a (remote) pc. The memory leak only occurs on Windows 2003 (and Windows XP 64). Windows 2000 is fine, and so is Windows 2008. I'm wondering if anyone has experienced a similar problem.

The fact that the leak only occurs in certain versions of Windows implies that it might be a Windows issue, but I've been searching the web and haven't been able to locate a hotfix to resolve the issue. Also, it might be a Delphi issue, since a program with similar functionality in C# doesn't seem to have this leak. The latter fact has led me to believe that there might be another, better, way to get the information I need in Delphi without getting a memory leak.

I've included the source to a small program to expose the memory leak below. If the line sObject.Path_ below the { Leak! } comment is executed, the memory leak occurs. If I comment it out, there's no leak. (Obviously, in the "real" program, I do something useful with the result of the sObject.Path_ method call :).)

With a little quick 'n dirty Windows Task Manager profiling on my machine, I found the following:

                       Before  N=100  N=500  N=1000
With sObject.Path_     3.7M    7.9M   18.2M  31.2M
Without sObject.Path_  3.7M    5.3M    5.4M   5.3M

I guess my question is: has anyone else encountered this problem? If so, is it indeed a Windows issue, and is there a hotfix? Or (more likely) is my Delphi code broken, and is there a better way to get the information I need?

You'll notice on several occasions, nil is assigned to objects, contrary to the Delphi spirit... These are COM objects that do not inherit from TObject, and have no destructor I can call. By assigning nil to them, Windows's garbage collector cleans them up.

program ConsoleMemoryLeak;

{$APPTYPE CONSOLE}

uses
  Variants, ActiveX, WbemScripting_TLB;

const
  N = 100;
  WMIQuery = 'SELECT * FROM Win32_Process';
  Host = 'localhost';

  { Must be empty when scanning localhost }
  Username = '';
  Password = '';

procedure ProcessObjectSet(WMIObjectSet: ISWbemObjectSet);
var
  Enum: IEnumVariant;
  tempObj: OleVariant;
  Value: Cardinal;
  sObject: ISWbemObject;
begin
  Enum := (wmiObjectSet._NewEnum) as IEnumVariant;
  while (Enum.Next(1, tempObj, Value) = S_OK) do
  begin
    sObject := IUnknown(tempObj) as SWBemObject;

    { Leak! }
    sObject.Path_;

    sObject := nil;
    tempObj := Unassigned;
  end;
  Enum := nil;
end;

function ExecuteQuery: ISWbemObjectSet;
var
  Locator: ISWbemLocator;
  Services: ISWbemServices;
begin
  Locator := CoSWbemLocator.Create;
  Services := Locator.ConnectServer(Host, 'root\CIMV2',
                  Username, Password, '', '', 0, nil);
  Result := Services.ExecQuery(WMIQuery, 'WQL',
                  wbemFlagReturnImmediately and wbemFlagForwardOnly, nil);
  Services := nil;
  Locator := nil;
end;

procedure DoQuery;
var
  ObjectSet: ISWbemObjectSet;
begin
  CoInitialize(nil);
  ObjectSet := ExecuteQuery;
  ProcessObjectSet(ObjectSet);
  ObjectSet := nil;
  CoUninitialize;
end;

var
  i: Integer;
begin
  WriteLn('Press Enter to start');
  ReadLn;
  for i := 1 to N do
    DoQuery;
  WriteLn('Press Enter to end');
  ReadLn;
end.

Answer 1

you should store the return value of

sObject.Path_;

in a variable and make it SWbemObjectPath. This is necessary to make the reference counting right.

Answer 2

I can reproduce the behaviour, the code leaks memory on Windows XP 64 and does not on Windows XP. Interestingly this occurs only if the Path_ property is read, reading Properties_ or Security_ with the same code does not leak any memory. A Windows-version-specific problem in WMI looks like the most probable cause of this. My system is up-to-date AFAIK, so there probably isn't a hotfix for this either.

I'd like to comment on your resetting all variant and interface variables, though. You write

You'll notice on several occasions, nil is assigned to objects, contrary to the Delphi spirit... These are COM objects that do not inherit from TObject, and have no destructor I can call. By assigning nil to them, Windows's garbage collector cleans them up.

This is not true, and consequently there is no need to set the variables to nil and Unassigned. Windows does not have a garbage collector, what you are dealing with are reference-counted objects, which are immediately destroyed once the reference count reaches 0. The Delphi compiler does insert the necessary calls to increment and decrement the reference count as necessary. Your assignments to nil and Unassigned decrement the reference count, and free the object when it reaches 0.

A new assignment to a variable, or the exiting of the procedure take care of this as well, so additional assignments are (albeit not wrong) superfluous and decrease the clarity of the code. The following code is completely equivalent and does not leak any additional memory:

procedure ProcessObjectSet(WMIObjectSet: ISWbemObjectSet);
var
  Enum: IEnumVariant;
  tempObj: OleVariant;
  Value: Cardinal;
  sObject: ISWbemObject;
begin
  Enum := (wmiObjectSet._NewEnum) as IEnumVariant;
  while (Enum.Next(1, tempObj, Value) = S_OK) do
  begin
    sObject := IUnknown(tempObj) as SWBemObject;
    { Leak! }
    sObject.Path_;
  end;
end;

I'd say one should explicitly reset interfaces only if this does actually free the object (so the current ref count has to be 1) and the destruction itself should really happen exactly at this point. Examples for the latter are that a large chunk of memory can be freed, or that a file needs to be closed or a synchronization object to be released.