Google Calendar is a lovely looking and acting "free online shareable calendar service", allowing you to maintain your personal schedule, share it with others, and view public calendars.

Google has also provided an API such that your programs and websites can make use of it remotely; for instance to view, add or delete events. There are special client libraries to let you use it from the Java or C# .NET programming languages. It also supports communication via standard XML, to allow any other suitable language to get involved.

The Poorhouse decided to go down the Visual Studio 6 route; particularly Visual Basic 6. There aren't many, if any, examples around of how to use the Google Calendar API with VB6, but below is an illustration of how it can be done relatively simply with Visual Basic. This example shows how to add an event to a calendar from a VB program local to your computer. In that it communicates via pure XML, most of this could easily be abstracted any other similiarish language.

The transaction with Google happen via HTTP POST requests. There is a control available in VS6 that allows you to perform these simply; the Microsoft Internet Transfer Control. It is basically a wrapper around the WinInet Windows API, making it nice and easy to use. To add this to your project, start a new standard .exe project in VB6. Go to the Project menu, then Components and pick the "Microsoft Internet Transfer Control". Click OK, and a component will be added to the components toolbox ready for use. Drop one of these onto your form.

Full instructions as to how to use the control are available from Microsoft. For now though, as we are doing POST requests we will need the .execute method of the Internet Transfer control.

To add an event to Google Calendar, we need to know that the calendar has been set up (Go here to set one up if you don't have one), and that you have your Google account's username (i.e. your email address) and password.

As per the API instructions you need to send a HTTP POST request to https://www.google.com/accounts/ClientLogin
This must include the following parameters.

• Email: your email address
• Passwd: your password
• source: an identifier for your application in the form companyName-applicationName-versionID
• service: what Google service you want to use; for the calendar this should always be cl.

In Visual Basic, you can do it as below this. Imagine Inet1 is the name of the Internet Transfer Control you added above, and your email address, password and application ID are stored in the string variables "myEmail", "myPassword" and "mySource". Note the use of the form-urlencoded Content-Type header.

Your application needs to wait for Google to respond. Do this by looking out for the icResponseCompleted event in the Inet1_StateChanged event handler. When it does, store Google's reply in a variable, e.g.:

In that example, the Response variable now holds Google's response. What you need here, is the authorisation token given out in that response. You will then use that to prove that you are allowed to add an event to your calendar.

Therefore you should look for the phrase "Auth" in the response (outputString), and store the value for later. You can do it via:

The AuthCode variable will now hold Google's Authorisation token.

You now need to construct an event in XML. Check the Event Kind documentation from Google, or see the example on the Google API page. If you're doing it a lot, you'd probably want to use a XML tool to help you out, but as a one off you can just assemble it as a string. Make a string variable EventCode and assign the Event XML to it.

Now you must do a POST request to Google again; this time use the address http://www.google.com/calendar/feeds/default/private/full

Send your EventCode variable (the XML event) as the form data, and use the Content-Type and Authorization header. This should look like:

In VB, using the variables defined so far, you can do this by;

If you check the response to your request, you should find Google sends you your data back, plus some ID values, with the header "201 Created". Job done. If you go and view your calendar now you should see the event.

As a proof of concept, a VB program is attached below that allows you to add an event to your Google calendar by filling the appropriate values. Be sure to keep the date and time fields in the same format as they default to.

The full source code is available in the text attachment to this post. The only fancy bit is the use of Karl Peterson's lovely CTimeZone VB class. This was used to make sure the Google event that this program made matched the timezone of the local computer. By default, Google Calendar creates events based on the Greenwich Mean Time timezone.

Note: Using the above procedure, I found that every time I restarted the program, the second step didn't work, and gave a 401 authorization required error when I tried to enter an event. Subsequent times worked well though. An explanation for this and a solution can be found here.