HOWTO: Determining Whether a User Is Subscribed to a Channel

• Internet Client SDK, versions 4.0, 4.01
• Microsoft Internet Explorer (Programming), versions 4.0, 4.01

SUMMARY

With the introduction of Active Channels in Internet Explorer 4.0, it may be desirable to determine whether a user is already subscribed to a channel, so that a Web page or an application does not redundantly offer users the ability to subscribe to a channel they've already subscribed to.

MORE INFORMATION

This article describes how to check a subscription from C++, Visual Basic, and script.

C++ - using ISubscriptionMgr::IsSubscribed

The code below demonstrates how this can be done using C++, from a console application:

   #include <windows.h>
   #include <tchar.h>
   #include <iostream.h>
   #include <subsmgr.h>
   #include <stdio.h>

   void main()
   {
      ISubscriptionMgr *pSubscriptionMgr;
      CoInitialize(NULL);

      HRESULT hr = CoCreateInstance(CLSID_SubscriptionMgr,
                                    NULL,
                                    CLSCTX_INPROC_SERVER,
                                    IID_ISubscriptionMgr,
                                    (void**)&pSubscriptionMgr);
      if (SUCCEEDED(hr))
      {
         BOOL Bool;
         BSTR bstrURL =
           SysAllocString(L"http://test.microsoft.com/tst.cdf");
         hr = pSubscriptionMgr->IsSubscribed(bstrURL, &Bool);

         pSubscriptionMgr->Release();

         char szBuffer[250];
         TCHAR szURL[200];
         WideCharToMultiByte(CP_ACP, 0, bstrURL, -1, szURL, 200, NULL,
                             NULL);
         if (Bool)
             wsprintf (szBuffer,"YES, you are subscribed to %s\n",szURL);
         else
             wsprintf (szBuffer,"NOPE, you are NOT subscribed to %s\n",
                        szURL);
         cout << szBuffer;
         SysFreeString (bstrURL);
      }

      CoUninitialize();
   }

The code below demonstrates how this can be done in a standard Visual Basic application. To use the ShellUIHelper object, use Project->References to add a reference to "Microsoft Internet Controls."

   Private Sub Form_Load()
       Dim ShlHlpr As ShellUIHelper

       Set ShlHlpr = New ShellUIHelper

       If ShlHlpr.IsSubscribed("http://test.microsoft.com/tst.cdf") Then
           MsgBox "You're Subscribed!"
       Else
           MsgBox "You're not Subscribed!"
       End If
   End Sub

Script - Using window.external.isSubscribed

The code below demonstrates how this can be done, using JavaScript in an .htm page:

   <SCRIPT>
     if (window.external.isSubscribed("http://test.microsoft.com/tst.cdf"))
        alert ("You are subscribed!");
     else
        alert ("You are NOT subscribed!");
   </SCRIPT>

REFERENCES

Internet Client SDK documentation

Keywords          : iegeneral kbcode
Technology        : kbInetDev
Version           : WINDOWS:4.0,4.01
Platform          : WINDOWS
Issue type        : kbhowto