Delphi Tips And Tricks: CoInitialize/CoUninitialize Part II

In Part I of CoInitialize/Uninitialize of Delphi Tips and Tricks I talked about encountering problems with CoUninitialize and how the Delphi compiler generates code to IUnknown._Release after a function block. In this part I want to discuss another common problem having to do with making multiple calls to CoInitialize and CoUninitialize.

Here are the rules to calling CoIninitialize and CoUninitialize:

• Each thread must initialize the COM library by calling CoInitialize
  
• Each call to CoInitialize with a return value of S_OK and S_FALSE must be paired with a call to CoUninitialize
  
• Each thread can only be initialized with one threading model (remember GUI code is appartment threaded)
  
• The first thread that calls CoInitialize must be the last thread to call CoUninitialize.
  
• Do not call CoInitialize in DllMain
  

If you do not follow these five basic rules your application will not function properly. You can find these the documentation with these five basic rules on MSDN here. There is more information but the simple list above is much easier to read.

Below is a simple example that demonstrates correct usage and handling of the return value from CoInitialize and CoInitializeEx in a single threaded application. If there is any interested I will expand this to a multithreaded application?

program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils, ActiveX, Windows;

procedure CoInitStatus(Success: HResult);
begin
  case Success of
    S_OK, S_FALSE: WriteLn('success');
    RPC_E_CHANGED_MODE, E_INVALIDARG,
    E_OUTOFMEMORY, E_UNEXPECTED: WriteLn('failed');
  end;
end;

procedure CoUninit(Success: HResult);
begin
  case Success of
    S_OK, S_FALSE: CoUninitialize;
  end;
end;

var
  Success1, Success2, Success3: HResult;
begin
  Success1 := CoInitialize(nil);
  CoInitStatus(Success1);

  Success2 := CoInitialize(nil);
  CoInitStatus(Success2);

  Success3 := CoInitializeEx(nil, COINIT_MULTITHREADED);
  CoInitStatus(Success3);

  CoUninit(Success3);
  CoUninit(Success2);
  CoUninit(Success1);
end.

The output from running this program should be:

success
success
failed