Dynamic Data Exchange (DDE) is a form of interprocess communication (IPC) under Microsoft Windows or OS/2. The DdeCallback function is a critical component in this mechanism, allowing applications to communicate and share data. While DDE is somewhat outdated and has been largely replaced by more modern IPC methods like COM, OLE, and .NET Remoting, understanding DdeCallback can still be valuable for maintaining legacy systems or integrating with older software.

In this article, we will explore how to implement DdeCallback in a Windows environment using C++. We will cover the basics of setting up a DDE server and client, and provide practical examples to illustrate the process.

Examples:

1. Setting Up a DDE Server:

   #include <windows.h>
   #include <ddeml.h>
   
   // DdeCallback function prototype
   HDDEDATA CALLBACK DdeCallback(
      UINT uType,
      UINT uFmt,
      HCONV hconv,
      HSZ hsz1,
      HSZ hsz2,
      HDDEDATA hdata,
      DWORD dwData1,
      DWORD dwData2
   );
   
   int main() {
      DWORD idInst = 0;
      UINT result = DdeInitialize(&idInst, (PFNCALLBACK)DdeCallback, APPCMD_CLIENTONLY, 0);
      if (result != DMLERR_NO_ERROR) {
          printf("DdeInitialize failed: %u\n", result);
          return 1;
      }
   
      HSZ hszService = DdeCreateStringHandle(idInst, "MyService", CP_WINANSI);
      HSZ hszTopic = DdeCreateStringHandle(idInst, "MyTopic", CP_WINANSI);
   
      // Register the service
      HCONVLIST hConvList = DdeConnectList(idInst, hszService, hszTopic, NULL, NULL);
      if (hConvList == NULL) {
          printf("DdeConnectList failed\n");
          DdeFreeStringHandle(idInst, hszService);
          DdeFreeStringHandle(idInst, hszTopic);
          DdeUninitialize(idInst);
          return 1;
      }
   
      // Main message loop
      MSG msg;
      while (GetMessage(&msg, NULL, 0, 0)) {
          TranslateMessage(&msg);
          DispatchMessage(&msg);
      }
   
      // Cleanup
      DdeDisconnectList(hConvList);
      DdeFreeStringHandle(idInst, hszService);
      DdeFreeStringHandle(idInst, hszTopic);
      DdeUninitialize(idInst);
   
      return 0;
   }
   
   HDDEDATA CALLBACK DdeCallback(
      UINT uType,
      UINT uFmt,
      HCONV hconv,
      HSZ hsz1,
      HSZ hsz2,
      HDDEDATA hdata,
      DWORD dwData1,
      DWORD dwData2
   ) {
      // Handle DDE messages here
      switch (uType) {
          case XTYP_CONNECT:
              return (HDDEDATA)TRUE;
          case XTYP_DISCONNECT:
              // Handle disconnection
              break;
          // Add more cases as needed
      }
      return (HDDEDATA)NULL;
   }

2. Setting Up a DDE Client:

   #include <windows.h>
   #include <ddeml.h>
   
   // DdeCallback function prototype
   HDDEDATA CALLBACK DdeCallback(
      UINT uType,
      UINT uFmt,
      HCONV hconv,
      HSZ hsz1,
      HSZ hsz2,
      HDDEDATA hdata,
      DWORD dwData1,
      DWORD dwData2
   );
   
   int main() {
      DWORD idInst = 0;
      UINT result = DdeInitialize(&idInst, (PFNCALLBACK)DdeCallback, APPCMD_CLIENTONLY, 0);
      if (result != DMLERR_NO_ERROR) {
          printf("DdeInitialize failed: %u\n", result);
          return 1;
      }
   
      HSZ hszService = DdeCreateStringHandle(idInst, "MyService", CP_WINANSI);
      HSZ hszTopic = DdeCreateStringHandle(idInst, "MyTopic", CP_WINANSI);
   
      // Connect to the service
      HCONV hConv = DdeConnect(idInst, hszService, hszTopic, NULL);
      if (hConv == NULL) {
          printf("DdeConnect failed\n");
          DdeFreeStringHandle(idInst, hszService);
          DdeFreeStringHandle(idInst, hszTopic);
          DdeUninitialize(idInst);
          return 1;
      }
   
      // Send a request
      HSZ hszItem = DdeCreateStringHandle(idInst, "MyItem", CP_WINANSI);
      HDDEDATA hData = DdeClientTransaction(NULL, 0, hConv, hszItem, CF_TEXT, XTYP_REQUEST, 1000, NULL);
      if (hData != NULL) {
          char* data = (char*)DdeAccessData(hData, NULL);
          printf("Received data: %s\n", data);
          DdeUnaccessData(hData);
          DdeFreeDataHandle(hData);
      }
   
      // Cleanup
      DdeDisconnect(hConv);
      DdeFreeStringHandle(idInst, hszService);
      DdeFreeStringHandle(idInst, hszTopic);
      DdeFreeStringHandle(idInst, hszItem);
      DdeUninitialize(idInst);
   
      return 0;
   }
   
   HDDEDATA CALLBACK DdeCallback(
      UINT uType,
      UINT uFmt,
      HCONV hconv,
      HSZ hsz1,
      HSZ hsz2,
      HDDEDATA hdata,
      DWORD dwData1,
      DWORD dwData2
   ) {
      // Handle DDE messages here
      return (HDDEDATA)NULL;
   }