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How to code a DLL and Unit Interface


Submitted on: 6/18/2002 2:33:58 PM
By: ShadowMaster  
Level: Advanced
User Rating: By 1 Users
Compatibility:Delphi 5

Users have accessed this article 5543 times.
(About the author)
     This article will show you how to code a DLL (Dynamic Link Library) and call its method and/or functions from the unit interface.

Terms of Agreement:   
By using this article, you agree to the following terms...   
1) You may use this article in your own programs (and may compile it into a program and distribute it in compiled format for languages that allow it) freely and with no charge.   
2) You MAY NOT redistribute this article (for example to a web site) without written permission from the original author. Failure to do so is a violation of copyright laws.   
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First lets start a new delphi project. For the purpose of this exercise,
we will select the DLL Wizard.

2. Save the project as whatever you want, but remember the name as you
will need this for later use when you $define the library for conditional compilation.

3. add a unit to the DLL project, this is your DLL interface unit, name it something like myDLLInt.pas

4. No you are ready to code your DLL and interface.

Let start with 2 simple add/subtract functions that can be called from the DLL via the interface.
Here is the source to the actual DLL that we will use:

library adder;
uses SysUtils,Classes,adderInt;
{$R *.res}

function addNumbers(num1:integer;num2:integer):integer;stdcall;
Result := num1+num2;

function subNumbers(num1:integer;num2:integer):integer;stdcall;


Notice that my units uses has the adderInt specified as I named my interface unit adderInt.pas.
These are the actual functions that will be called.

NEXT: We must write an interface that will allow the calling of our DLL
functions form other applications namely C/C++ apps and of course Delphi apps.
The source code for your interface will look like this:

unit adderInt;
function addNumbers(num1:integer;num2:integer):integer;stdcall;
function subNumbers(num1:integer;num2:integer):integer;stdcall;
function addNumbers; external 'ADDER.DLL' name 'addNumbers';
function subNumbers; external 'ADDER.DLL' name 'subNumbers';

Thats it! Compile the project and now you should have a DLL called yourname.dll
To use this DLL you must do 3 things:

1. Place the yourfile.dll into the same directory as your EXE or Project you are writing.
2. Add the interface unit to your current project. eg mylibInt.pas
3. Add the interface to your uses in your project. eg. uses
there ya go, you just coded a DLL
Email me at support@cttech.net with any questions.

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Other User Comments
1/15/2003 8:05:50 AM:
Please, give me a little concept about client/server and more database
Keep the Planet clean! If this comment was disrespectful, please report it:

5/2/2003 9:01:51 AM:
I have created the DLL. Howw do you call it from other projects?
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