How to: Link Model Updates using Transactions

When you define an extension to the UML designers in Visual Studio Ultimate, you can group several changes into a single transaction called a linked undo context.

By default, each modification that your code makes to a model can be separately undone by the user. For example, if you define a menu command that swaps the names of two UML classes, a user could invoke the command, and then perform a single undo. This would undo the change to one name, but not the other, leaving your model in an unintended state.

To avoid this, your code can perform a series of changes within a transaction. This makes the changes look like a single change to the user. A subsequent undo command will undo the whole series.

An additional benefit is that your code can undo a partially-complete set of changes by throwing an exception or by aborting the transaction.

Ensure your project References include this .NET assembly:

Microsoft.VisualStudio.Modeling.Sdk.12.0.dll

Inside your class, declare an imported property that has type ILinkedUndoContext:

using Microsoft.VisualStudio.Modeling.ExtensionEnablement;

...

class … {

[Import]

public ILinkedUndoContext LinkedUndoContext { get; set; }

In a method that modifies the model, enclose your changes in a transaction:

using (ILinkedUndoTransaction transaction =

LinkedUndoContext.BeginTransaction("my updates"))

{

// code to update model elements or shapes goes here

transaction.Commit();

}

Notice the following:

  • You must always include Commit() at the end of the transaction. If a transaction is disposed without being committed, the transaction will be rolled back. That is, the model will be restored to its state at the start of the transaction.

  • If an exception occurs that is not caught inside the transaction, the transaction will be rolled back. It is a frequent pattern to enclose the using block of the transaction inside a try…catch block.

  • You can nest transactions.

  • You can provide any non-blank name to BeginTransaction().

  • Only the UML Model Store is affected by these transactions. Modeling transactions do not affect: variables, external stores such as files and databases, layer diagrams, sequence diagrams generated from code, and code models.

    using Microsoft.VisualStudio.Modeling.ExtensionEnablement;
    using Microsoft.VisualStudio.Uml.Interfaces;
    using Microsoft.VisualStudio.Uml.Classes;
    using Microsoft.VisualStudio.Uml.Extensions;
    using System.Linq;
    using System.ComponentModel.Composition;
 ...
  [Import]
  public ILinkedUndoContext LinkedUndoContext { get; set; }

  /// <summary>
  /// Swap the names of the currently selected elements.
  /// </summary>
  public void Execute(IMenuCommand command)
  {
    var selectedShapes =
      Context.CurrentDiagram.GetSelectedShapes<IClassifier>();
    if (selectedShapes.Count() < 2) return;
    IClassifier firstElement = selectedShapes.First().Element;
    IClassifier lastElement = selectedShapes.Last().Element;
    string firstName = firstElement.Name;
    // Perform changes inside a transaction so that undo
    // works as a single change.
    using (ILinkedUndoTransaction transaction = 
      LinkedUndoContext.BeginTransaction("Swap names"))
    {
        firstElement.Name = lastElement.Name;
        lastElement.Name = firstName;
        transaction.Commit();
    }
 }
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