Mathias Brandewinder on .NET, F#, VSTO and Excel development, and quantitative analysis / machine learning.
by Mathias 27. November 2011 07:59

Earlier this month, on Simon Murphy’s blog, the project to organize a UK Excel Developer conference 2012 in London quickly took shape. As of now, the planned date is in the January 24 to 26 range, with a pretty cool agenda, covering most of the recent developments Excel developers should know about. I am looking forward to it!

I’ll be talking for about an hour on VSTO (Visual Studio Tools for Office). VSTO can be both awesome and painful: I plan on demoing building an Excel add-in from the ground up, illustrating some of the benefits and drawbacks/pitfalls of that approach, so that you know when it’s the right time to go that route.

If there are any topics or questions you are specifically interested in, please let me know in the comments – I’ll be happy to take requests.

As an aside, I usually talk at .NET developers events, where I need to convince the audience that developing for Excel (or Office) isn’t a terrible idea. This will be my first time with an Excel developers audience, and I expect the opposite challenge, namely why bother with C#/VB.NET and Visual Studio when VBA is free and works just fine?

Check out the Excel Conference page and Simon’s blog for updates.


Picture from the amazing series “The Kitten Covers

by Mathias 13. November 2011 11:03

We were doing some pair-programming with Petar recently, Test-Driven Development style, and started talking about how figuring out where to begin with the tests is often the hardest part. Petar noticed that when writing a test, I was typically starting at the end, first writing an Assert, and then coding my way backwards in the test – and that it helped getting things started.

I hadn’t realized I was doing it, and suspected it was coming from Kent Beck’s “Test-Driven Development, by Example”. Sure enough, the Patterns section of the book lists the following:

Assert First. When should you write the asserts? Try writing them first.

So why would this be a good idea?

I think the reason it works well, is that it helps focus the effort on one single goal at a time, and requires clarifying what that goal is. Starting with the Assert forces you to imagine one single fact that should be true once you have implemented the feature, and to think about how you are going to verify that the feature is indeed working.

Once the Assert is in place, you can now write the story backwards: what is the method that was called to get the result being checked, and  where does it belong? What classes and setup is required to make that method call? And, now that the story is written, what is it really saying, and what should the test method name be?

In other words, begin with the Assert, figure out the Act part, Arrange the actors, and (re)name the test method.

I think what trips some people is that while a good test will look like a little story, progressing from a beginning to a logical end, the process leading to it follows a completely opposite direction. Kent Beck points the Self-Similarity in the entire process: write stories which describe what the application will do once done, write tests which describe what the feature does once the code is implemented, and write asserts which will pass once the test is complete. Always start with the end in mind, and do exactly what it takes to achieve your goal.


by Mathias 6. November 2011 15:20

I am somewhat tests-obsessed, and as a result, often find Excel frustrating to work with, because writing automated tests against it isn’t trivial. So recently, while perusing the chapter on Scripting in Programming F#, I came across an Office automation example, and started wondering whether this would be a practical way to write automated tests against Excel.

The use case I have in mind is an existing Excel Workbook, which contains a model (say, your typical Financial model), with a fixed structure, and maybe a sprinkle of VBA, and no .NET.

For illustration purposes, let’s work with the following: our workbook, Model.xlsx, contains one worksheet, “Finances”, with a Profit cell in B3, computed as the difference between the revenue and cost named cells. Pretty impressive stuff.


What I want is a way to automatically set the Revenue and Cost to some arbitrary value, and check that the result in Profit is what it should be – so that I don’t have to do it myself by hand, and don’t have to remember how this Workbook was supposed to work later on.

Here is how this could look like in a F# script – create a Script file, say WorkbookTest.fsx, with the following code inside:

#r "Microsoft.Office.Interop.Excel"

open System
open Microsoft.Office.Interop.Excel
Console.WriteLine("Press [Enter] to start")

let excel = new ApplicationClass(Visible=false)
let workbooks = excel.Workbooks

let workbookPath = @"C:\Users\Mathias\Desktop\Model.xlsx"

let workbook = workbooks.Open(workbookPath)
let worksheets = workbook.Worksheets
let sheet = worksheets.["Finances"]
let worksheet = sheet :?> Worksheet

let revenueCell = worksheet.Range "Revenue"
revenueCell.Value2 <- 100

let costCell = worksheet.Range "Cost"
costCell.Value2 <- 10

let profitCell = worksheet.Range "Profit"
let profit = profitCell.Value2

Console.WriteLine("Check profit calculations")
Console.WriteLine("Expected: {0}, Actual {1}", 90, profit)

workbook.Close(false, false, Type.Missing)

Console.WriteLine("Done, press [Enter] to close")

The script launches Excel in Invisible mode, opens the workbook, sets the Revenue and Cost to 100 and 10, retrieves the value from Profit, printouts the value it found as well as the expected value – and closes back the Workbook without saving any of the changes.

The nice thing here is that I can now drop that file on my desktop, and simply right-click and select “Run with F# Interactive” to execute it, without building anything, and I’ll see something like this happen:


Nothing earth shattering, but still pretty nice: now I got a script which I can run anytime I want, to check whether the Workbook is behaving properly. Furthermore, what’s nice is that I don’t need to open Visual Studio to work with it: I can simply open WorkbookTest.fsx with Notepad, edit my code, and run it again.

There are some clear issues with the code in its current form. For instance, if anything goes wrong in the code (say, for instance, that I mis-typed a name which doesn’t exist with the workbook), the script will crash miserably, and let the hidden Excel instance hang in the background, waiting for someone to kill it manually. This would require some work to make sure that if exceptions are raised, everything is properly disposed, and no matter what, the file gets closed without saving any modification.

In any case, I thought it was worth sharing, even in its rough state – if only because it was fun, and also because the F# code looks surprisingly more appealing than the usual C# Interop code. Now the fun part would be to turn this into a decent testing framework for Excel…


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