Excel – VSTO Excel Workbook add-in not loading via Visual Studio debugger on 1 computer

VSTO has no support for creating Excel UDFs. Automation Add-Ins can be created in .Net, and seem to be the Microsoft approved way of doing it.

You should have a look at ExcelDna - http://www.codeplex.com/exceldna. ExcelDna allows managed assemblies to expose user-defined functions (UDFs) and macros to Excel through the native .xll interface. The project is open-source and freely allows commercial use. And you'll find that the performance of your .Net-based UDF is similar to native .xll add-ins for Excel. Excel 2007 features like the large sheet, long Unicode strings and multi-threaded recalculation are supported.

With ExcelDna your function as posted above will be exposed to Excel with no VSTO - you can put to code into an xml-based .dna file or compile it to a .dll.

The .dna file exposing your UDF would look like this:

<DnaLibrary Language="C#">
   using System;
   using ExcelDna.Integration;

   public class MyFunctions
   {
      [ExcelFunction(Description="Calculate Stuff", Category="Cool Functions")]
      public static double HeronicCal(int a, int b, int c)
      {
         //first compute S = (a+b+c)/2
         double S = (a + b + c) / 2;
         double area = Math.Sqrt(S * (S - a) * (S - b) * (S - c));
         return area;        
      }
   }
</DnaLibrary>

Update: These days, the easiest way to get started with Excel-DNA is to make a new Class Library project in Visual Studio, then add the 'ExcelDna.AddIn' package from NuGet. That makes a starter add-in - just paste your code and press F5 to run.

I got a similar error - in order to fix it, while in Excel I went to Options -> Add-ins -> Manage Excel Add-ins -> Automation -> and Selected "Visual Studio Tools for Office Design-Time Adaptor for Excel". Then restart Excel and (if you had the same issue) you should be good to go.