Create an Automation Add-In for Excel using .NET

Introduction

Excel versions 2002 (XP) and 2003 introduce the concept of an Automation Add-In. Automation Add-Ins allow public functions in COM libraries to act as User-Defined Functions (UDFs) in Excel, thus to be referenced directly from cell formulae.

This article provides a detailed walk-through of how to create an Automation Add-In for Excel using Visual Studio .NET.

Background

Some relevant knowledge base articles that discuss Automation Add-Ins are:

• Q291392 - INFO: Excel COM Add-ins and Automation Add-ins.
• Q285337 - How To Create a Visual Basic Automation Add-in for Excel Worksheet Functions.
• Q278328 - XL2002: How to Mark an Automation Add-In Function as Volatile.

A commercial library that presents equivalent (and more) functionality, and makes this all very easy is ManagedXLL. However, this library is quite expensive and requires run-time licenses to distribute user code. It uses the native XLL API for creating an add-in to Excel, and thus also supports older versions of Excel.

The exact requirements for creating a COM server that can be used as an Automation Add-In are poorly documented, complicated by the fact that the default options in Visual Basic 6.0 seem to work perfectly.

When creating a COM library using .NET, an attempt to add the library as an Automation Add-In in Excel causes the error: “The file you selected does not contain a new Automation Server, or you do not have sufficient privileges to register the Automation Server”.

Here, I describe how to create an Automation Add-In in .NET, using C#. The techniques should apply to any .NET language.

There seem to be three tricks for implementing an Automation Add-In for Excel using .NET:

1. The library needs to be registered for use through COM. This can be done by marking the project to ‘Register for COM Interop’ or by manual registration using RegAsm.exe.
2. The ‘Programmable’ registry key needs to be added in the registry, under HKCR\CLSID\{xxx}\. This can be automated by adding appropriate ComRegisterFunction methods to the class.
3. The class needs to be marked with the ClassInterface attribute, with value ClassInterfaceType.AutoDual (explicit interface implementations can work too). The default class interface that is generated is a dispatch interface - Excel seems to ignore the dispatch interface for Automation Add-Ins.

Walk-through

1. Create the library:
   1. Create a new C# Class Library Project, called NAddIn.
   2. Select the project's properties; under ‘Configuration Properties’, ‘Build’, set ‘Register for COM Interop’ to True.
   3. Rename the class, add a namespace declaration, set the ClassInterface attribute of the class, and add a function to be used from Excel:

      using System;
      using System.Runtime.InteropServices;
      
      namespace NAddIn
      {
          [ClassInterface(ClassInterfaceType.AutoDual)]
          public class Functions
          {
              public Functions()
              {
              }
      
              public double Add2(double v1, double v2)
              {
                  return v1 + v2;
              }
      
              [ComRegisterFunctionAttribute]
              public static void RegisterFunction(Type t)
              {
                  Microsoft.Win32.Registry.ClassesRoot.CreateSubKey(
                      "CLSID\\{" + t.GUID.ToString().ToUpper() + 
                         "}\\Programmable");
              }
      
              [ComUnregisterFunctionAttribute]
              public static void UnregisterFunction(Type t)
              {
                  Microsoft.Win32.Registry.ClassesRoot.DeleteSubKey(
                      "CLSID\\{" + t.GUID.ToString().ToUpper() + 
                        "}\\Programmable");
              }
          }
      }

   4. Build the NAddIn project to create bin\debug\NAddIn.dll.
2. Test the Add-In in Excel:
   1. Open a new workbook in Excel.
   2. Select Tools, Add-Ins, Automation.
   3. NAddIn.Functions should be listed - select it. OK.
   4. In a cell, type =Add2(3,4)
   5. The cell should display 7.

History

1. Initial version - 19 July 2004.
2. Removed type library embedding instructions - 19 July 2004.
3. Added ComRegisterFunction bits to automate registry changes - 30 July 2004.

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