Neat method for blocking multiple instances

Here is a neat method for blocking multiple instances of an application from running.  I came up with this while answering another question on EE, and thought it might be nice to post for all to see.  I have never seen this method anywhere else and it seems like it might possibly be something other members of EE could use or improve upon. Anyway here it is


unit PREVINST;

{Unit PREVINST.pas -
      Include this unit in your project file (.dpr) to keep multiple instances
  of your application from running.  Works the same way as the standard method
  of using mutexes, except uses a semaphore and stores the application handle in
  the semaphore's count so we can get it to show the original instance.
  You would call it like this:

  program Project1;

      uses
        Forms,
        Unit1 in 'Unit1.pas',
        PREVINST in 'PREVINST.pas';

      begin
    if not HavePrevInstance(Application.Handle, 'SOmeWEIRDnameFoRTheSemaphOre') then
    begin
          Application.Initialize;
              Application.CreateForm(TForm1, Form1);
              Application.Run;
    end;
  end.}

interface

uses Windows;

function HavePrevInstance(AppHandle: THandle; UniqueName: String): boolean;

implementation

function HavePrevInstance(AppHandle: THandle; UniqueName: String): boolean;
const
  ERROR_ALREADY_EXISTS = 183; //Not in Windows.pas - dont know why
  SEMAPHORE_ALL_ACCESS = STANDARD_RIGHTS_REQUIRED or SYNCHRONIZE or 3; //Ditto
  WM_SYSCOMMAND = 274; //Define here so dont have to include Messages.pas
var
  MyHandle: THandle;
begin
  Result := False; //Set Initial Result to false
  //Now we create the semaphore and store the applications handle as its count
  MyHandle := CreateSemaphore(nil, AppHandle, AppHandle, PChar(UniqueName));
  //The following will be true if an instance of out app is already running
  if (MyHandle <> 0) and (GetLastError = ERROR_ALREADY_EXISTS) then
  begin
    Result := True; //Set to result to true
    //Now we open the semaphore to get its handle
    MyHandle := OpenSemaphore(SEMAPHORE_ALL_ACCESS, False, PChar(UniqueName));
    //This replaces the value of MyHandle with the Application handle of the prev instance
    ReleaseSemaphore(MyHandle, 0, @MyHandle);
    //now we have the handle to the previous instance - do whatever you want
    //This is what I usually do but you could pop up a messagebox or whatever
    if IsIconic(MyHandle) then
          PostMessage(MyHandle, WM_SYSCOMMAND, SC_RESTORE, 0)
    else
          SetForeGroundWindow(MyHandle);
  end;
end;

end.




Benefits of this method.

1. Same as using a mutex - can create using a name that is very unlikely to exist anywhere else on the system.

2. Stores the apps handle so the findwind calls to get it are not needed.

3. Very simple to add to existing code and keeps the app from starting at all if a previous instance is found.



Any suggestions on how this might be improved upon are welcome.

Heath