Microsoft Office and .NET Interoperability


Paul Cornell
Microsoft Corporation

November 1, 2001

By now, you have most likely heard about the new features that the Microsoft® .NET platform brings to solutions development. I am sure you are wondering how you can take advantage of these .NET features with your Microsoft Office solutions. In this month's column, I introduce you to how Office and .NET work together from a code perspective.

If you are new to .NET, please see last month's column, Introducing .NET to Office Developers, for more information on .NET and its relationship to Office.

How Do Office and .NET Interoperate?

Microsoft Office Visual Basic® for Applications (VBA, based on the Component Object Model (COM)) code and .NET cannot natively communicate with each other. However, a .NET feature called COM interop provides callable wrappers to allow .NET and COM to interoperate, as shown in Figure 1.

Figure 1. COM and .NET interoperability using callable wrappers

A runtime callable wrapper (RCW) allows a COM component (for example, an Office VBA object library) to be used by .NET (for example, a Visual Basic .NET application). In Microsoft Visual Studio® .NET, for example, when you click Add Reference from the Project menu, click the COM tab, double-click a type library file, and click OK, Visual Studio .NET converts the objects and members in the COM type library file into equivalent .NET assemblies (see Figure 2). After the .NET assemblies are generated, you should be able to instantiate classes and call members from Visual Studio .NET as if the COM objects and members were native .NET classes and members.

Figure 2. Adding a reference to a COM object library from Visual Studio .NET

A COM callable wrapper (CCW) allows a .NET assembly to be used by COM (for example, Office VBA). The Assembly Registration Tool (RegAsm.exe), a .NET Framework SDK tool, reads the metadata in a .NET assembly (equivalent to a COM dynamic-link library (DLL) or executable file (EXE)) and makes registry entries that allow COM to call the classes and members in the .NET assembly. After running RegAsm.exe, you should be able to set a reference to the generated COM type library file through the References command on the Tools menu in the Office Visual Basic Editor, and create COM objects and call members as usual.

To run RegAsm.exe, from the Microsoft Visual Studio Command Prompt (on the Start menu, point to Programs, point to Microsoft Visual Studio.NET 7.0, point to Visual Studio.NET tools, and click Visual Studio.NET Command Prompt), type the following:

regasm AssemblyPath /tlb:FileName.tlb

Where AssemblyPath is the path to the target assembly, and FileName is the name you want to give your type library. For example:

regasm "C:\WINNT\Microsoft.NET\Framework\v1.0.2914\mscorlib.dll"

This inserts information about the mscorlib.dll .NET assembly into the registry, and creates the mscorlib.tlb type library file so that it can be accessed from the References dialog box (Tools menu) in the Office Visual Basic Editor.

You will need to distribute the .NET assembly, and an Office solution based on that assembly, to your users. See Packaging and Deploying .NET Solutions below for more information.

An ActiveX callable wrapper allows .NET to use a Microsoft ActiveX® control (such as an Office Web Component) to be used by .NET. When you right-click the Visual Studio .NET Toolbox, click Customize Toolbox, select an entry on the COM Components tab, and click OK (as shown in Figure 3). Visual Studio then generates the code to use the ActiveX control from the Toolbox just like any other .NET control.

Figure 3. Adding an ActiveX control to the Toolbox in Visual Studio .NET

Calling the .NET Framework Class Libraries from Office VBA

Although Office VBA can make some calls into the functionality provided by the .NET Framework class libraries, there are currently some serious limitations with this approach because COM (on which Office VBA is based) does not recognize the following in the .NET Framework class libraries:

  • Parameterized constructors: Any class in the .NET Framework class library with a constructor that takes one or more parameters will generate a run-time error if any instance of that class is set equal to New in COM (even though these classes are visible in the Office Visual Basic Object Browser). For example, we could never code something like Dim objWS As New Excel.Worksheet(Caption:="Sheet1") in Office VBA because the code (Caption:="Sheet1") does not follow COM coding syntax.
  • Non-public and static (shared) members: Any .NET Framework class library properties, methods, or events marked as Private, Protected, or Static (Shared) are not accessible to COM and will not appear in the Office Visual Basic Object Browser. Even if a .NET Framework class library member is marked as Public Instance, it will not appear in the Office Visual Basic Object Browser if there are any parameterized constructors for the member's class.

If you study the .NET Framework class library documentation, you will notice very few COM-callable classes and members. This is because many of the .NET Framework class library classes take parameterized constructors, and those that do not have parameterized constructors have very few public instance members.

Because of these limitations, I will move on to a more interesting and useful option, which is calling the Office VBA object libraries and ActiveX controls from .NET. 

Calling the Office VBA Object Libraries from Visual Basic .NET

A new and exciting opportunity in Office solutions development is to integrate the functionality of the Office VBA object libraries from .NET applications. I will show you how to do this by using Microsoft Visual Basic .NET, a member of Visual Studio .NET and a revolution to the Visual Basic language. Visual Basic .NET is somewhat similar to Office VBA, but there are several key differences that I will explain after I present the code.

Calling the Outlook Object Library Using a Console Application

Console applications are useful for solutions that do not need the overhead of a complex user interface. Let's demonstrate this by creating a new console application in Visual Studio .NET that reports on some Microsoft Outlook® data:

  1. On the File menu, point to New, and click Project.
  2. In the Project Types pane, click Visual Basic Projects.
  3. In the Templates pane, click Console Application.
  4. In the Name box, type olquery.
  5. In the Location box, type a location that is easy for you to remember, and click OK.
  6. On the Project menu, click Add Reference.
  7. On the COM tab, click Microsoft Outlook 10.0 Object Library (assuming you have Microsoft Outlook 2002 installed on your computer), click Select, and click OK.
  8. When the message "...Would you like to have a wrapper generated for you?" appears, click Yes. Because the Outlook object library references additional object libraries, Visual Studio .NET will create CCW assemblies for these object libraries as well. So, in the \bin folder in your application's solution folder using Microsoft Windows® Explorer, you should see two CCW assemblies—Interop.Outlook.dll (representing the Outlook object library), and Interop.Office.dll (representing the Microsoft Office 10.0 Object Library). You can now call the functionality of these libraries just as you would for any .NET class libraries.
  9. Edit the code in the Module1.vb file to look like the following code:
    Imports Outlook.OlDefaultFolders
    Imports Microsoft.VisualBasic.ControlChars
    Module Module1
        Sub Main()
            Dim objApp As Outlook.Application
            Dim objNameSpace As Outlook.NameSpace
            Dim objMAPIFolder As Outlook.MAPIFolder
            Dim objSubFolder As Outlook.MAPIFolder
            Dim strFolder As String
            Dim strAgain As String
            Dim intFolderEnum As Integer
            Dim lngItemCount As Long
            Dim lngAppointmentCount As Long
            Dim lngContactCount As Long
            Dim lngDistListCount As Long
            Dim lngJournalCount As Long
            Dim lngMailCount As Long
            Dim lngNoteCount As Long
            Dim lngPostCount As Long
            Dim lngTaskCount As Long
            Dim lngOtherCount As Long
            objApp = New Outlook.Application()
            objNameSpace = objApp.GetNamespace(Type:="MAPI")
                ' Reset counters from previous loops.
                lngItemCount = 0
                lngAppointmentCount = 0
                lngContactCount = 0
                lngDistListCount = 0
                lngJournalCount = 0
                lngMailCount = 0
                lngNoteCount = 0
                lngPostCount = 0
                lngTaskCount = 0
                lngOtherCount = 0
                Console.Write("Outlook folder name (case-sensitive): ")
                strFolder = Console.ReadLine()
                ' Map user's entry to an OlDefaultFolders constant.
                Select Case strFolder
                    Case "Deleted Items"
                        intFolderEnum = 3
                    Case "Outbox"
                        intFolderEnum = 4
                    Case "Sent Mail"
                        intFolderEnum = 5
                    Case "Inbox"
                        intFolderEnum = 6
                    Case "Calendar"
                        intFolderEnum = 9
                    Case "Contacts"
                        intFolderEnum = 10
                    Case "Journal"
                        intFolderEnum = 11
                    Case "Notes"
                        intFolderEnum = 12
                    Case "Tasks"
                        intFolderEnum = 13
                    Case "Drafts"
                        intFolderEnum = 16
                    Case Else
                        Console.Write("'" & strFolder & "'" & _
                            "is not a valid folder. " & _
                            "Valid folders are Calendar, " & _
                            "Contacts, Deleted Items, " & _
                            "Drafts, Inbox, Journal, Notes, " & _
                            "Outbox, Sent Mail, " & _
                            "and Tasks. Press any key to exit...")
                        Exit Sub
                End Select
                objMAPIFolder = _
                Console.Write("Calculating folder contents..." & CrLf)
                ' Examine each item in the folder. Increment
                ' appropriate counter and continue.
                For lngItemCount = 1 To objMAPIFolder.Items.Count
                    Select Case _
                        Case "AppointmentItem"
                            lngAppointmentCount = lngAppointmentCount + 1
                        Case "ContactItem"
                            lngContactCount = lngContactCount + 1
                        Case "DistributionListItem"
                            lngDistListCount = lngDistListCount + 1
                        Case "JournalItem"
                            lngJournalCount = lngJournalCount + 1
                        Case "MailItem"
                            lngMailCount = lngMailCount + 1
                        Case "NoteItem"
                            lngNoteCount = lngNoteCount + 1
                        Case "PostItem"
                            lngPostCount = lngPostCount + 1
                        Case "TaskItem"
                            lngTaskCount = lngTaskCount + 1
                        Case Else
                            lngOtherCount = lngOtherCount + 1
                    End Select
                Next lngItemCount
                ' Report results of tally.
                Console.Write("Total items in this folder: " & _
                    objMAPIFolder.Items.Count & CrLf & _
                    "Appointments: " & lngAppointmentCount & CrLf & _
                    "Contacts: " & lngContactCount & CrLf & _
                    "Distribution lists: " & lngDistListCount & CrLf & _
                    "Journal entries : " & lngJournalCount & CrLf & _
                    "E-Mail messages: " & lngMailCount & CrLf & _
                    "Notes: " & lngNoteCount & CrLf & _
                    "Posts: " & lngPostCount & CrLf & _
                    "Tasks: " & lngTaskCount & CrLf & _
                    "Other items: " & lngOtherCount & CrLf)
                Console.Write("Try again? (y/n): ")
                strAgain = Console.ReadLine().ToString
            Loop While strAgain = "y"
        End Sub
    End Module
  10. To run this code sample, click Run (or press F5) on the Debug menu. Figure 4 shows the finished console application.

Figure 4. The olquery.exe console application

Here is how the code works:

  • The Imports statement is shorthand for writing out a fully qualified reference to a .NET namespace. Just as in Office VBA, we code references like Office.Outlook.Application to distinguish the use of the Application object, in Visual Basic .NET, we can code Imports Outlook.OlDefaultFolders to keep us from having to type Outlook.OlDefaultFolders.olFolderInbox later, using just olFolderInbox instead.
  • The Module...End Module statement defines an instance of a module—in this case, Module1. The interesting thing about Visual Basic .NET is that you can cram an entire console application into one .vb file, including class modules, code modules, and so on. The Module...End Module statement isolates the code for the class from other code components in the file.
  • Sub Main is the main subroutine for this application. In a console application, program execution must start beginning with the Main subroutine.
  • The Console.Write method is similar to the Debug.Print method in Office VBA in that it prints information—in this case, to the command prompt. The Console.Read method is similar to the Office VBA InputBox function in that it can get information from users at the command prompt.
  • The rest of the code is very straightforward for Outlook developers. One item to note is that the MAPIFolder object's Items property returns an Items object, not an Items collection (there is no Items collection in Outlook). Therefore, because the Items object does not support the .NET IEnumerable interface to be treated like a collection, we must iterate through the Outlook items by index number, not by using the familiar For Each objItem In objMAPIFolder.Items.
  • In the interest of space, I am not using any error-handling code in this solution. See the next section for details on how to create error-handling code.

Although this is a simple example, it is easy to see how console applications can add power and flexibility to Office solutions. For instance, you could create an Office automation application that runs on a recurring schedule through the Scheduled Tasks program in Control Panel. You can also write cross-application Office macros, applications that do not require a sophisticated user interface, and so on.

Calling the Word Object Library Using a Windows Forms Application

Now let me introduce you to Windows Forms in Visual Studio .NET, which are similar to VBA UserForms. Here is how to create a new Windows Forms application in Visual Studio .NET that reports on some document properties in Microsoft Word:

  1. On the File menu, point to New, and click Project.
  2. In the Project Types pane, click Visual Basic Projects.
  3. In the Templates pane, click Windows Application.
  4. Fill in the Name and Location boxes, and click OK.
  5. On the Project menu, click Add Reference.
  6. On the COM tab, click Microsoft Word 10.0 Object Library (assuming you have Word 2002 installed on your computer), click Select, and click OK.
  7. When the message "...Would you like to have a wrapper generated for you?" appears, click Yes.
  8. On the View menu, click Toolbox. On the Windows Forms tab, double-click the Button control and the OpenFileDialog control to add a button and Open file dialog box to Form1.
  9. Right-click the OpenFileDialog1 icon below Form1 and click Properties.
  10. In the Filter box, type Microsoft Word Documents (*.doc)|*.doc.
  11. In the Title box, type Select Word Document.
  12. Double-click Button1. The Code window for the form appears.
  13. Edit the code in the Code window to match the code listed below.
  14. To run this code sample, on the Debug menu, click Run.
  15. Click Button1 on Form1 and select a Word document from your local file system (to see the Output window in Visual Studio .NET, press CTRL+ALT+O). Figure 5 shows what the finished application looks like.

Figure 5. The Windows Forms application at work

Here is the code that opens a selected Word document and reports on some of its document properties:

Imports Microsoft.VisualBasic.ControlChars
Imports System.Runtime.InteropServices
Imports Word.WdBuiltInProperty

Public Class Form1
    Inherits System.Windows.Forms.Form

' Windows Form Designer generated code

    Private Sub Button1_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles Button1.Click

        Dim strFileName As String
        Dim wdApp As New Word.Application()
        Dim objDoc As Word.Document

        If OpenFileDialog1.ShowDialog() = DialogResult.OK Then
                strFileName = OpenFileDialog1.FileName
                objDoc = wdApp.Documents.Open(FileName:=strFileName)

                With objDoc
                    ' Print information in a message box.
                    MessageBox.Show(Text:="Title: " & _
                        .BuiltInDocumentProperties(wdPropertyTitle).value & _
                        CrLf & "Author: " & _
                        .BuiltInDocumentProperties(wdPropertyAuthor).value & _
                        CrLf & _
                        .BuiltInDocumentProperties(wdPropertyPages).value & _
                        " pages.")

                    ' Print information to the Output window.
                    Console.Write(Format:="Contents of " & _
                        strFileName & " are:" & CrLf)
                End With

            Catch ex As COMException
                ' "File not found" error.
                MessageBox.Show(Text:="Can't find the file '" & _
                    strFileName & "'. Please try again.")
            End Try

        End If

    End Sub

End Class

Here is how the code works:

  • The Imports statement allows us to code Imports Word.wdBuiltInProperty to keep us from having to type Word.wdBuiltInProperty.wdPropertyAuthor later, using just wdPropertyAuthor instead.
  • The Class...End Class statement defines an instance of a class—in this case, a Windows Form. The Class...End Class statement isolates the code for the class from other code components in the file.
  • The Inherits statement indicates that Form1 inherits, or takes on the behaviors and characteristics of, a generic Windows Form. This would include things like the form's caption at the top, the icon in the upper-left hand corner, minimize/restore/close buttons in the upper-right hand corner, and so on.
  • The Windows Form Designer-generated code in this example does not need to be viewed or modified.
  • The Sub Click...End Sub statement is familiar to VBA programmers as an event. However, this event is known in .NET terminology as a delegate, because it is delegated to handle the Button1.Click method. When Button1 on Form1 is clicked, the code in this event runs.
  • The Try...Catch statement is a new version of the familiar VBA On Error GoTo statement (you can still use On Error GoTo if you want to, but you should begin using the .NET equivalent). To use Try...Catch, you surround any code that might generate errors in the Try block, and you handle any errors in the Catch block. You can have multiple Catch blocks, which are similar to the Case statements in a Select Case Err.Number construct in Office VBA. Additionally, you can add a Finally block to perform any clean-up work (similar to a FunctionName_End convention in standard Office VBA error handling code).
  • The Dim statements introduce the concept of initializers. In Visual Basic .NET, you can declare and initialize variables all in one line of code. Notice also that you do not use the Set keyword anymore (it is no longer available).
  • The MessageBox.Show method replaces the Office VBA MsgBox function (you can still use the MsgBox function if you want to, but you should begin using the .NET equivalent). In this case, the MessageBox.Show method displays the document's title, author, and number of pages. The CrLf enumerated constant replaces the Office VBA vbCrLf enumerated constant (although you can still use the vbCrLf enumerated constant if you want to, but you should begin using the .NET equivalent).
  • The Console.Write method replaces the VBA Debug.Print method (the Debug.Print method is no longer available in Visual Basic .NET). In this case, the Console.Write method lists the document's contents in the Output window.

Some changes you will notice in Visual Studio .NET include the following:

  • The default development environment behavior is for a general Visual Studio developer. For example, the default shortcut key combination for viewing the Object Browser is CTRL+ALT+J, and the default shortcut key combination for a Step Into action is F11. To associate the F2 key with the Object Browser and the F8 key with a Step Into action, click the Start Page tab, click My Profile, and click Visual Basic Developer in the Profile list.
  • There is no Immediate window. The Command window (on the View menu, point to Other Windows and click Command Window) provides an Immediate mode so that you can issue commands as you used to do in the Immediate window (type immed at the > prompt for Immediate mode; type >cmd in Immediate mode to switch back to Command mode). In Command mode, you can send commands directly to Visual Studio .NET instead of using menus. For example, type Help in the Command window to expose the various Help commands. Furthermore, the Output window (on the View menu, point to Other Windows and click Output) is used to print out information as you used to do in the Immediate window using the Debug.Print statement.
  • Solution (.sln) files are used instead of Visual Basic Project Workgroup (.vbg) files. A solution may contain one or more project (.proj) files, which are used instead of Visual Basic Project (.vbp) files. All Visual Basic code files are contained within .vb files.
  • Some Office objects are converted to interfaces, and some Office events are converted to delegates. This doesn't change your ability to use these objects and events, it's just that you might see them referred to differently within the Visual Studio .NET Object Browser.

Using ActiveX Controls in .NET Solutions

There will be plenty of occasions where you will want to embed Office ActiveX controls on .NET forms. Let's demonstrate this by adding an instance of the Microsoft Outlook View control (assuming you have the control installed on your computer) to our existing Windows application:

  1. With the Windows application from the previous section open, and the Form Designer visible, on the Toolbox, right-click and click Customize Toolbox.
  2. On the COM Components tab, select the check box next to Microsoft Outlook View Control, and click OK.
  3. Click the OVCtl icon in the Toolbox, and, using the mouse, draw an instance of the Outlook View Control on Form1.
  4. Run the application by clicking Start on the Debug menu.
    Note   Be aware that to call ActiveX control functionality in .NET, you need to know the control name that Visual Studio .NET generates for you (in our case, AxViewCtl1). You can, of course, rename this from the Properties window in the Form Designer.

Packaging and Deploying .NET Solutions

To package and deploy a simple client-based solution, you can copy the files in the \bin directory of the solution's application folder to a file share or source media, request that users copy and paste these files to a single local folder on their computer, and run the application's .exe file. For a simple Web application, you provide users with the path to the application's root .aspx file.

However, if you want to perform special setup options, such as installing files into specific folders, registering assemblies in the global assembly cache, placing entries on the user's Programs menu or desktop, registering COM DLLs, and so on, you should create a .NET setup and deployment project in Visual Studio .NET. To do so, follow these steps:

  1. With the target solution open, on the File menu, point to New, and click Project.
  2. In the Project Types pane, click Setup and Deployment Projects.
  3. In the Templates pane, click Setup Wizard.
  4. Fill in the Name box, select Add to Solution, and click OK.
  5. Complete the steps in the Setup Wizard. For example, for a simple Windows Forms application, you can leave the defaults, except for Create a setup for a Windows application (step 2 of 5), and Primary output from ProjectName and Content Files from ProjectName (step 3 of 5).
  6. On the Build menu, click Build SetupProjectName.
  7. When finished, there should be a file named SetupProjectName.msi in either the \Debug or \Release folder of the setup project's application folder.
  8. Distribute this single SetupProjectName.msi file to users, who simply open the .msi file on their local computer.

To add files to the user's file system, add registry entries, add file types, change the look and feel of the user's installation experience, or add custom actions or launch conditions during installation, you need to add information to the .msi file and then rebuild the .msi file. With the setup solution open, right-click the setup solution in the Solution Explorer window, point to View, click the appropriate menu commands, and follow the on-screen directions (see Figure 6). Then, on the Build menu, click Rebuild SetupProjectName to create a new SetupProjectName.msi file.

Figure 6. Modifying a Visual Studio .NET setup project to edit the files added to a user's file system on install

Where to Go for More Information

General .NET Topics

Office and .NET Interoperability

Visual Studio .NET

Visual Basic .NET

COM and .NET Interoperability

.NET Application Setup and Deployment

  • .NET Framework Deployment Basics (technical reference)
  • Deployment Scenarios (technical reference)

And, as always, check in regularly at the Office Developer Center for information and technical articles on Office solution development.


Paul Cornell works for the MSDN Online Office Developer Center and the Office developer documentation team. He spends his free time with his wife and 18-month-old daughter.