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This chapter is from the book

Office Add-Ins

The second pattern used in Office development is the add-in pattern. This book will cover several types of Office add-ins. These include VSTO add-ins for Outlook, COM add-ins for Excel and Word, and Automation add-ins for Excel:

  • VSTO add-ins for Outlook. This new VSTO 2005 feature makes it extremely easy to create an add-in for Outlook 2003. The model is the most ".NET" of all the add-in models and is very similar to the VSTO 2005 code behind model for documents. Chapter 23 describes this model in detail.
  • COM add-ins for Excel and Word. A C# class in a class library project can implement the IDTExtensibility2 interface and register in the registry as a COM object and COM add-in. Through COM interop, Office creates the C# class and talks to it. Chapter 24 describes the creation of COM add-ins and some issues that make COM add-in development problematic.
  • Automation add-ins for Excel. These managed classes expose public functions that Excel can use in formulas. The C# class must register in the registry as a COM object. Through COM interop, Excel can create an automation add-in and use its public methods in formulas. Automation add-ins and their use in Excel formulas are discussed in Chapter 3.

There are some Office add-in technologies that this book will not discuss. Application level Smart Tags add-ins and Smart Documents add-ins are not discussed because VSTO provides a much easier way of accessing Smart Tag and Smart Document functionality, albeit at the document or template level rather than at the application level. For more information on VSTO's support for Smart Tags and Smart Documents, see Chapter 15 and Chapter 16.

Creating an Outlook Add-in in VSTO

To create an Outlook add-in project in VSTO, choose Project... from the New menu of the File menu in Visual Studio. Select the Visual C# node from the list of project types and select the Office node under the Visual C# node. The Outlook Add-in project appears in the list of templates. Type a name for your new Outlook add-in project and pick a location for the project. Then press the OK button.

Figure 5

Figure 2-5: Creating a new Outlook add-in project.

VSTO creates a project with references to the Outlook 2003 PIA, the core Office PIA, and other needed references as shown in Figure 2-6. VSTO also adds a project item to the project called ThisApplication.cs. This project item contains a C# class that you will add to when implementing your Outlook add-in.

Figure 6

Figure 2-6: The Outlook add-in project in Solution Explorer.

If you double click on the ThisApplication.cs project item, you will see the code shown in Listing 2-4. There is a simple Startup and Shutdown event handler where you can write code that executes on the startup and shutdown of the add-in. The ThisApplication class derives from an aggregate of the Outlook Application object. This allows you to access properties and methods of the Outlook Application object by writing code like this.Inspectors.Count in the ThisApplication class.

Listing 2-4: The initial code in the ThisApplication class in an Outlook add-in project.

using System;
using System.Windows.Forms;
using Microsoft.VisualStudio.Tools.Applications.Runtime;
using Outlook = Microsoft.Office.Interop.Outlook;

namespace OutlookAddin1
{
 public partial class ThisApplication
 {
  private void ThisApplication_Startup(object sender, System.EventArgs e)
  {
  }

  private void ThisApplication_Shutdown(object sender, System.EventArgs e)
  {
  }

  #region VSTO Designer generated code
  private void InternalStartup()
  {
   this.Startup += new System.EventHandler(ThisApplication_Startup);
   this.Shutdown += new System.EventHandler(ThisApplication_Shutdown);
  }
  #endregion
 }
}

Looking at Listing 2-4 you may wonder about the use of "partial" in the class definition. VSTO uses partial classes which are a new feature of .NET that allows you to define part of a class in one file and another part of a class in a second file and then compile them together as one class. VSTO uses this feature to hide some additional generated code associated with the ThisApplication class from you to reduce the complexity of the class where you write your code. The final ThisApplication class will be compiled from the partial class in Listing 2-4 and additional code in a partial class generated by VSTO that is hidden from you.

We are going to add to the code in Listing 2-4 to create an add-in to that will solve an annoying problem—people replying inadvertently to an e-mail sent out to a mailing alias that contains a large number of people. Unless you have "Vice President" in your title, you probably do not want to be sending e-mail to more than, say, 25 people at any given time. We are going to create an add-in that will warn you if you do this and give you the "This is a potentially career limiting move. Are you sure you want to send this e-mail to 25,000 people?" message.

Outlook's Application object has an ItemSend event that fires whenever a user sends an email. We will add additional code to the Startup method of the ThisApplication class to hookup an event handler for the ItemSend event as shown in Listing 2-5. Because the ThisApplication class derives from an aggregate of Outlook's Application object, we can write the code "this.ItemSend" because ItemSend is an event raised by the ThisApplication base class. The ItemSend event handler takes an object parameter called Item which is the Outlook item being sent. Because Item could be any of a number of things such as a meeting request or an e-mail message, Item is passed as an object instead of as a specific type. The ItemSend event handler also has a bool parameter passed by reference called Cancel that can be set to true to prevent the Outlook item from being sent.

In our ItemSend event handler we need to check to see if the Item parameter which is passed as an object is actually an e-mail. The easiest way to achieve this is to use the as keyword to try to cast the Item parameter to an Outlook.MailItem. If the cast succeeds, the resulting value will be non-null and we will know that the item being sent is an Outlook.MailItem and therefore an e-mail message. We can then iterate through the Recipients collection on the MailItem object and check to see if we are sending to any recipient lists that include more than 25 people. Each Recipient object in the Recipients collection has an AddressEntry property that returns an AddressEntry object. The AddressEntry object has a Members property that returns a collection that we can check the count of. If we find the count to be more than 25, we will show a dialog and ask the user if they really want to send the mail. If the user clicks the No button, we will set the Cancel parameter of the ItemSend event to true to cancel the sending of career limiting e-mail.

Listing 2-5: A ThisApplication Class in an Outlook add-in project that handles the ItemSend event and checks for more than 25 recipients.

using System;
using System.Windows.Forms;
using Microsoft.VisualStudio.Tools.Applications.Runtime;
using Outlook = Microsoft.Office.Interop.Outlook;

namespace OutlookAddin1
{
 public partial class ThisApplication
 {
  private void ThisApplication_Startup(object sender, System.EventArgs e)
  {
   this.ItemSend += new Outlook.ApplicationEvents_11_ItemSendEventHandler(ThisApplication_ItemSend);
  }

  void ThisApplication_ItemSend(object Item, ref bool Cancel)
  {
   Outlook.MailItem myItem = Item as Outlook.MailItem;

   if (myItem != null)
   {
    foreach (Outlook.Recipient recip in myItem.Recipients)
    {
     if (recip.AddressEntry.Members.Count > 25)
     {
      // Ask the user if they really want to send this email
      string message = "Are you sure that you want to send mail to " +
       recip.AddressEntry.Name + " " + " which includes " +
       recip.AddressEntry.Members.Count + " people?";

      string caption = "More than 25 recipients";
      MessageBoxButtons buttons = MessageBoxButtons.YesNo;
      DialogResult result;

      result = MessageBox.Show(message, caption, buttons);

      if (result == DialogResult.No)
      {
       Cancel = true;
       break;
      }
     }
    }
   }
  }

  private void ThisApplication_Shutdown(object sender, System.EventArgs e)
  {
  }

  #region VSTO Designer generated code
  private void InternalStartup()
  {
   this.Startup += new System.EventHandler(ThisApplication_Startup);
   this.Shutdown += new System.EventHandler(ThisApplication_Shutdown);
  }
  #endregion
 }
}

When you run the project with the code shown in Listing 2-4, Outlook launches and the add-in loads. Try sending a mail to an alias that includes more than 25 people—you might want to go offline first in case you mistyped the code. If all works right, the add-in will display a dialog box warning you that you are sending an e-mail to more than 25 people and you will be able to cancel the send of the e-mail. Exit Outlook to end your debugging session.

VSTO Outlook add-ins are discussed in more detail in chapter 23. The Outlook object model is discussed in Chapters 9 through 11.

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