一、首先創建一個DLL工程。
1） 在bcb集成環境中，通過菜單File | new | other打開new Items對話框，選中DLL Wizar項，選中C++，選中Use VCL，確定。系統會自動創建一個DLL工程，內容如下：
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
#include <vcl.h>
#include <windows.h>
#pragma hdrstop
//---------------------------------------------------------------------------
// Important note about DLL memory management when your DLL uses the
// static version of the RunTime Library:
//
// If your DLL exports any functions that pass String objects (or structs/
// classes containing nested Strings) as parameter or function results,
// you will need to add the library MEMMGR.LIB to both the DLL project and
// any other projects that use the DLL. You will also need to use MEMMGR.LIB
// if any other projects which use the DLL will be performing new or delete
// operations on any non-TObject-derived classes which are exported from the
// DLL. Adding MEMMGR.LIB to your project will change the DLL and its calling
// EXE's to use the BORLNDMM.DLL as their memory manager. In these cases,
// the file BORLNDMM.DLL should be deployed along with your DLL.
//
// To avoid using BORLNDMM.DLL, pass string information using "char *" or
// ShortString parameters.
//
// If your DLL uses the dynamic version of the RTL, you do not need to
// explicitly add MEMMGR.LIB as this will be done implicitly for you
//---------------------------------------------------------------------------
<?xml:namespace prefix = o />
#pragma argsused
int WINAPI DllEntryPoint(HINSTANCE hinst, unsigned long reason, void* lpReserved)
{
return 1;
}
//---------------------------------------------------------------------------
2） 現在可以通過菜單Project | Build Project1來編譯和連接當前的DLL工程。儘管還沒有添加任何一句代碼，DLL文件仍然可以順利創建。但這裏我們要繼續進行下去。新建一個目錄: D:/samples/mydll，保存資源文件爲mydllprj.cpp，保存工程文件爲mydllprj.bpr。
3） 新建的DLL中包含兩個函數，分別實現了兩個整數的加法運算和減法運算，具體代碼如下：
//---------------------------------------------------------------------------
#include <vcl.h>
#include <windows.h>
#pragma hdrstop
//---------------------------------------------------------------------------
// Important note about DLL memory management when your DLL uses the
// static version of the RunTime Library:
//
// If your DLL exports any functions that pass String objects (or structs/
// classes containing nested Strings) as parameter or function results,
// you will need to add the library MEMMGR.LIB to both the DLL project and
// any other projects that use the DLL. You will also need to use MEMMGR.LIB
// if any other projects which use the DLL will be performing new or delete
// operations on any non-TObject-derived classes which are exported from the
// DLL. Adding MEMMGR.LIB to your project will change the DLL and its calling
// EXE's to use the BORLNDMM.DLL as their memory manager. In these cases,
// the file BORLNDMM.DLL should be deployed along with your DLL.
//
// To avoid using BORLNDMM.DLL, pass string information using "char *" or
// ShortString parameters.
//
// If your DLL uses the dynamic version of the RTL, you do not need to
// explicitly add MEMMGR.LIB as this will be done implicitly for you
//---------------------------------------------------------------------------
#pragma argsused int WINAPI DllEntryPoint(HINSTANCE hinst, unsigned long reason, void* lpReserved)
{
return 1;
}
//往下爲新添加代碼
extern "C" __declspec(dllexport) int PlusNum(int X,int Y)
{
return X+Y;
}
extern "C" __declspec(dllexport) int MinusNum(int X,int Y)
{
return X-Y;
}
//---------------------------------------------------------------------------
4） 開始編譯和連接工程，編譯和連接成功後在D:/samples/mydll中可見到mydllprj.dll和mydllprj.lib。
二、建立客戶應用程序，調用mydllprj.dll。
1） 通過菜單File | Close all 關閉所有工程和文件，然後通過菜單File |new Application 打開一個新的工程，將資源和工程文件保存在D:/samples，分別命名爲mysmp.cpp和mysmpdll.bpr。
2） 設計窗體界面如下：
<?xml:namespace prefix = v />
調整好後如下：
3） 在ComboBox1的Items屬性中依次加入：”兩數相加”和”兩數相減”，並將Style屬性設置爲csDropDownList。
4） 在mysmp.h中的窗體類Tform1聲明之前添加下面的代碼：
//---------------------------------------------------------------------------
#include <Classes.hpp>
#include <Controls.hpp>
#include <StdCtrls.hpp>
#include <Forms.hpp>
//新加代碼
extern "C" __declspec(dllimport) int PlusNum(int X,int Y);
extern "C" __declspec(dllimport) int MinusNum(int X,int Y);
//extern "C" __declspec(dllimport) int MultiplyNum(int X,int Y);
//新加代碼結束
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published: // IDE-managed Components
TLabel *Label1;
TLabel *Label2;
TLabel *Label3;
5） 添加Button1控件的OnClick事件的處理代碼如下：
//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
if(ComboBox1->Text=="兩數相加")
Edit3->Text=IntToStr(PlusNum(StrToInt(Edit1->Text),StrToInt(Edit2->Text)));
else if(ComboBox1->Text=="兩數相減")
Edit3->Text=IntToStr(MinusNum(StrToInt(Edit1->Text),StrToInt(Edit2->Text)));
// else if(ComboBox1->Text=="兩數相乘")
// Edit3->Text=IntToStr(MultiplyNum(StrToInt(Edit1->Text),StrToInt(Edit2->Text)));
else
MessageBox(Form1->Handle,"請指定一個計算方法。","警告",MB_OK|MB_ICONWARNING);
}
//---------------------------------------------------------------------------
6） 現在編譯連接和運行程序，程序會提示錯誤信息，錯誤信息如下：
[Linker Error] Unresolved external '_PlusNum' referenced from D:/SAMPLES/MYSMP.OBJ
[Linker Error] Unresolved external '_MinusNum' referenced from D:/SAMPLES/MYSMP.OBJ
這是因爲編譯器沒有找到開始生成的mydllprj.dll這個連接庫文件，需要在bcb集成開發環境中加入，通過菜單Project | add to Project，把d:/samples/mydll/mydllprj.lib添加進來。然後編譯連接並運行，這時系統會提示”找不到mydllprj.dll運行庫”，這是因爲mydllprj.dll在d:/samples/mydll/，系統按指定的路徑找不到這個文件，我們這裏可以把mydllprj.dll複製到d:/samples下，然後編譯運行，這是你就可以看到期待已久的界面了。