Delphi Tutorial : Send Param To previous instance and Close last instance of your program

This tutorial will show you how to send param into your program and
close the last instance of your program when finished sending param.

examples like this illustration .
when you execute your application the first instance will be created.
and when you execute your application a second time , then a second
instance will be created . so there are two applications run.
we are going to apply here is , when you execute a second application,
the second application will not run but only send params.

The first time your application run goes like this :
C:/program files/myapp/myapp.exe

The second time your application run goes like this :
C:/program files/myapp/myapp.exe -Hello " how are you ? "

where:
myapp.exe      is param(1).
-hello         is param(2).
"how are you"  is Param(3)

so the second instance your application send param(2) : " how are you "
and the first instance your applications will receive param(2) is " how are you "


here is the code
add this code to your drp program

    program Project1;

    uses
      Forms,
      rxvclutils,//or vclutils
      messages,
      windows,
      Unit1 in 'Unit1.pas' {Form1};

    {$R *.res}

    var
    previnstance:HWND;
    cds         :TCopyDataStruct;
    s           :string;
    title       :string ='Test Param';
    begin
      previnstance:= FindPrevInstance(TForm1.ClassName,title);
      CloseHandle(previnstance);
      if previnstance<>0 then begin

        if ParamStr(1)='-halo' then begin
          with cds do begin
              s:= paramstr(2);
              cbdata:=length(s)+1;
              lpData:=@s[1];
              end;
      SendMessage(previnstance,wm_CopyData,0,integer(@cds));
      WaitForSingleObject(previnstance,INFINITE);
      ActivatePrevInstance(TForm1.ClassName,title);
      CloseHandle(previnstance);
      end;
      exit;
      end;

      Application.Initialize;
      Application.CreateForm(TForm1, Form1);
      form1.Caption:= title;
      Application.Run;
      CloseHandle(previnstance);
    end.




and add this to your unit

....
public
procedure wmcopydata(var theMsg : TWMCopyData); message WM_COPYDATA;
......

/////////////
procedure Tform1.wmcopydata(var theMsg : TWMCopyData);
var
s:string;
begin
with theMsg.CopyDataStruct^ do
begin
s:=pchar(lpData);
showmessage(s);
end;
//////////////



on create
/////////////
procedure TForm1.FormCreate(Sender: TObject);
begin
if ParamStr(1)= '-halo' then begin
   if ParamStr(2) <> '' then
      showmessage(paramstr(2));
end;
end;