How can I create and use DLL in Delphi?

Creating and using DLLs (Dynamic Link Libraries) in Delphi involves defining functions or procedures in a separate unit or project, compiling it into a DLL, and then using it in another Delphi application. Here’s a step-by-step guide:

Step 1: Create a DLL project

1. Open the Delphi IDE.
2. Create a new project: File -> New -> Other -> Dynamic Link Library.

Step 2: Define functions or procedures

In your DLL project, define the functions or procedures you want to export. For example:

In this example, the AddNumbers function is defined, and the exports section specifies which functions or procedures are accessible from outside the DLL.

Step 3: Build the DLL

Build your DLL project: Project -> Build MyDLL.

Step 4: Use the DLL in another Delphi project

1. Create a new Delphi project where you want to use the DLL.
2. Declare the functions from the DLL in your new project:
   
   Delphi/Pascal

   unit MyMainForm; interface uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs; type TForm1 = class(TForm) procedure FormCreate(Sender: TObject); procedure FormDestroy(Sender: TObject); private DLLHandle: THandle; AddNumbersFunc: function(a, b: Integer): Integer; stdcall; public { Public declarations } end; var Form1: TForm1; implementation {$R *.dfm} procedure TForm1.FormCreate(Sender: TObject); begin DLLHandle := LoadLibrary(‘MyDLL.dll’); if DLLHandle <> 0 then begin @AddNumbersFunc := GetProcAddress(DLLHandle, ‘AddNumbers’); if @AddNumbersFunc <> nil then begin // Use the function ShowMessage(‘Result: ‘ + IntToStr(AddNumbersFunc(5, 7))); end else ShowMessage(‘Function not found in DLL.’); end else ShowMessage(‘DLL not loaded.’); end; procedure TForm1.FormDestroy(Sender: TObject); begin if DLLHandle <> 0 then FreeLibrary(DLLHandle); end; end.

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

   unit MyMainForm;   interface   uses   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,   Dialogs;   type   TForm1 = class(TForm)     procedure FormCreate(Sender: TObject);     procedure FormDestroy(Sender: TObject);   private     DLLHandle: THandle;     AddNumbersFunc: function(a, b: Integer): Integer; stdcall;   public     { Public declarations }   end;   var   Form1: TForm1;   implementation   {$R *.dfm}   procedure TForm1.FormCreate(Sender: TObject); begin   DLLHandle := LoadLibrary(‘MyDLL.dll’);   if DLLHandle <> 0 then   begin     @AddNumbersFunc := GetProcAddress(DLLHandle, ‘AddNumbers’);     if @AddNumbersFunc <> nil then     begin       // Use the function       ShowMessage(‘Result: ‘ + IntToStr(AddNumbersFunc(5, 7)));     end     else       ShowMessage(‘Function not found in DLL.’);   end   else     ShowMessage(‘DLL not loaded.’); end;   procedure TForm1.FormDestroy(Sender: TObject); begin   if DLLHandle <> 0 then     FreeLibrary(DLLHandle); end;   end.

   
   In this example, the LoadLibrary function is used to load the DLL, and GetProcAddress is used to obtain the address of the exported function. The AddNumbersFunc variable is then assigned with the function address and can be used like a regular Delphi function.

   Step 5: Run your application

   Run your new Delphi application, and it should use the functions from the DLL.

   Remember to handle errors appropriately, check for nil after GetProcAddress, and free the library using FreeLibrary when you’re done.

   Make sure the DLL is in the same directory as your executable or is located in a directory listed in the system’s PATH environment variable.