Mathias Brandewinder on .NET, F#, VSTO and Excel development, and quantitative analysis / machine learning.
by Mathias 26. April 2010 08:32

Today we will write code that reads the contents (values & formulas) of the two selected worksheets, generates a list of their differences, and feeds it to the view that displays the comparison. The code isn’t particularly complicated, and uses mostly ideas which have been seen in the previous posts, so rather than break this into multiple small posts, I chose to bite the bullet and get done with a large chunk, all at once. I hope you survive it and bear with me – promise, we are almost there!

You can find the complete list of episodes, and a link to the source code of theExcel VSTO add-in tutorial here.

Extracting the comparison from the comparison ViewModel

In the previous installment, I created a stub to provide a “canned” list of differences to the ComparisonViewModel, so that we would have something to display. It’s time to replace that stub, and generate a real comparison between 2 worksheets. We could add the required code inside the existing classes; however, we will likely have a good amount of logic, so to avoid clutter, and to keep our design tight, we will extract that responsibility into its own class, the WorksheetsComparer.

First, let’s create a stub for the class, in the Comparison folder; its key method will be FindDifferences, and we will temporarily move the “fake” comparison that was in the ComparisonViewModel into that method:

public class WorksheetsComparer
   public static List<Difference> FindDifferences(Excel.Worksheet firstSheet, Excel.Worksheet secondSheet)
      // Temporary code
      var difference1 = new Difference() { Row = 3, Column = 3 };
      var difference2 = new Difference() { Row = 3, Column = 5 };
      var difference3 = new Difference() { Row = 5, Column = 8 };
      var differences = new List<Difference>();

      return differences;

Now we need to hook it up to the add-in. We will add a button to the AnakinView, which, when clicked, will call WorksheetsComparer.FindDifferences, and pass the result to the ComparisonViewModel. Let’s first plug a button in the AnakinView, and bind it to a Command on the ViewModel, GenerateComparison:

<StackPanel Margin="5">
  <Comparison:ComparisonView x:Name="ComparisonView" />
  <Button Command="{Binding GenerateComparison}" 
          Content="Compare" Width="75" Height="25" Margin="5"/>

Following the same approach as in the previous post, we implement the command in the View Model (only added code is displayed):

public class AnakinViewModel
   private ICommand generateComparison;

   public ICommand GenerateComparison
         if (this.generateComparison == null)
            this.generateComparison = new RelayCommand(GenerateComparisonExecute);
         return this.generateComparison;

   private void GenerateComparisonExecute(object target)
      var differences = WorksheetsComparer.FindDifferences(null, null);


by Mathias 14. April 2010 12:02

You can find the complete list of episodes, and a link to the source code of the Excel VSTO add-in tutorial here.

In the second part of this series, we will generate a comparison between two open spreadsheets, and create a user interface to navigate between the differences, and reconcile them if need be.

For today, let’s assume that the add-in has already figured out what the differences are, and build first a user interface that allows the user to navigate to the cells that have differences. In later posts, we will focus on actually generating these differences.

To achieve this, I will leverage the Goto method of Excel. Application.Goto(object reference, object scroll) will navigate to the range defined by reference. If scroll is set to true, it will force the window to scroll so that the range is the upper-left corner, otherwise it will scroll only if necessary.

This is a good starting point for design. A spreadsheet comparison will be a collection of differences, and each difference should map to a cell. One approach is to store the row and column of each difference, so that from a Difference object, we can retrieve the corresponding range, and go to it:

private void NavigateToDifference(Difference difference)
   var row = difference.Row;
   var column = difference.Column;
   var activeSheet = (Excel.Worksheet)this.excel.ActiveSheet;
   var differenceLocation = activeSheet.Cells[row, column];
   this.excel.Goto(differenceLocation, Type.Missing);



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