Validation in a Domain-Specific Language

As the author of a domain-specific language (DSL), you can define validation constraints to verify that the model created by the user is meaningful. For example, if your DSL allows users to draw a family tree of people and their ancestors, you could write a constraint that ensures that children have birth dates after their parents.

You can have the validation constraints execute when the model is saved, when it is opened, and when the user explicitly runs the Validate menu command. You can also execute validation under program control. For example, you could execute validation in response to a change in a property value or relationship.

Validation is particularly important if you are writing text templates or other tools that process your users’ models. Validation ensures that the models fulfill the preconditions assumed by those tools.

Running Validation

When a user is editing a model, that is, an instance of your domain-specific language, the following actions can run validation:

• Right-click the diagram and select Validate All.

• Right-click the top node in the Explorer of your DSL and select Validate All

• Save the model.

• Open the model.

• In addition, you can write program code that runs validation, for example, as part of a menu command or in response to a change.

  Any validation errors will appear in the Error List window. The user can double-click an error message to select the model elements that are the cause of the error.

Defining Validation Constraints

You define validation constraints by adding validation methods to the domain classes or relationships of your DSL. When validation is run, either by the user or under program control, some or all of the validation methods are executed. Each method is applied to each instance of its class, and there can be several validation methods in each class.

Each validation method reports any errors that it finds.

To define a validation constraint

1. Enable validation in the Editor\Validation node:

   1. Open Dsl\DslDefinition.dsl.

   2. In DSL Explorer, expand the Editor node and select Validation.

   3. In the Properties window, set the Uses properties to true. It is most convenient to set all these properties.

   4. Click Transform All Templates in the Solution Explorer toolbar.

2. Write partial class definitions for one or more of your domain classes or domain relationships. Write these definitions in a new code file in the Dsl project.

3. Prefix each class with this attribute:

   [ValidationState(ValidationState.Enabled)]
   

   • By default, this attribute will also enable validation for derived classes. If you want to disable validation for a specific derived class, you can use ValidationState.Disabled.

4. Add validation methods to the classes. Each validation method can have any name, but have one parameter of type ValidationContext.

   It must be prefixed with one or more ValidationMethod attributes:

   [ValidationMethod (ValidationCategories.Open | ValidationCategories.Save | ValidationCategories.Menu ) ]
   

   The ValidationCategories specify when the method is executed.

   For example:

using Microsoft.VisualStudio.Modeling;
using Microsoft.VisualStudio.Modeling.Validation;

// Allow validation methods in this class:
[ValidationState(ValidationState.Enabled)]
// In this DSL, ParentsHaveChildren is a domain relationship
// from Person to Person:
public partial class ParentsHaveChildren
{
  // Identify the method as a validation method:
  [ValidationMethod
  ( // Specify which events cause the method to be invoked:
    ValidationCategories.Open // On file load.
  | ValidationCategories.Save // On save to file.
  | ValidationCategories.Menu // On user menu command.
  )]
  // This method is applied to each instance of the
  // type (and its subtypes) in a model:
  private void ValidateParentBirth(ValidationContext context)
  {
    // In this DSL, the role names of this relationship
    // are "Child" and "Parent":
     if (this.Child.BirthYear < this.Parent.BirthYear
        // Allow user to leave the year unset:
        && this.Child.BirthYear != 0)
      {
        context.LogError(
             // Description:
                       "Child must be born after Parent",
             // Unique code for this error:
                       "FAB001ParentBirthError",
              // Objects to select when user double-clicks error:
                       this.Child,
                       this.Parent);
    }
  }

Notice the following points about this code:

• You can add validation methods to domain classes or domain relationships. The code for these types is in Dsl\Generated Code\Domain*.cs.

• Each validation method is applied to every instance of its class and its subclasses. In the case of a domain relationship, each instance is a link between two model elements.

• Validation methods are not applied in any specified order, and each method is not applied to the instances of its class in any predictable order.

• It is usually bad practice for a validation method to update the store content, because this would lead to inconsistent results. Instead, the method should report any error by calling context.LogError, LogWarning or LogInfo.

• In the LogError call, you can provide a list of model elements or relationship links that will be selected when the user double-clicks the error message.

• For information about how to read the model in program code, see Navigating and Updating a Model in Program Code.

  The example applies to the following domain model. The ParentsHaveChildren relationship has roles that are named Child and Parent.

  

Validation Categories

In the ValidationMethodAttribute attribute, you specify when the validation method should be executed.

Where to Place Validation Methods

You can often achieve the same effect by placing a validation method on a different type. For example, you could add a method to the Person class instead of the ParentsHaveChildren relationship, and have it iterate through the links:

[ValidationState(ValidationState.Enabled)]
public partial class Person
{[ValidationMethod
 ( ValidationCategories.Open
 | ValidationCategories.Save
 | ValidationCategories.Menu
 )
]
  private void ValidateParentBirth(ValidationContext context)
  {
    // Iterate through ParentHasChildren links:
    foreach (Person parent in this.Parents)
    {
        if (this.BirthYear <= parent.BirthYear)
        { ...

Aggregating validation constraints. To apply validation in a predictable order, define a single validation method on an owner class, such the root element of your model. This technique also lets you aggregate multiple error reports into a single message.

Drawbacks are that the combined method is less easy to manage, and that the constraints must all have the same ValidationCategories. We therefore recommend that you keep each constraint in a separate method if possible.

Passing values in the context cache. The context parameter has a dictionary into which you can place arbitrary values. The dictionary persists for the life of the validation run. A particular validation method could, for example, keep an error count in the context, and use it to avoid flooding the error window with repeated messages. For example:

List<ParentsHaveChildren> erroneousLinks;
if (!context.TryGetCacheValue("erroneousLinks", out erroneousLinks))
erroneousLinks = new List<ParentsHaveChildren>();
erroneousLinks.Add(this);
context.SetCacheValue("erroneousLinks", erroneousLinks);
if (erroneousLinks.Count < 5) { context.LogError( ... ); }

Validation of Multiplicities

Validation methods for checking minimum multiplicity are automatically generated for your DSL. The code is written to Dsl\Generated Code\MultiplicityValidation.cs. These methods take effect when you enable validation in the Editor\Validation node in DSL Explorer.

If you set the multiplicity of a role of a domain relationship to be 1..* or 1..1, but the user does not create a link of this relationship, a validation error message will appear.

For example, if your DSL has classes Person and Town, and a relationship PersonLivesInTown with a relationship 1..\* at the Town role, then for each Person that has no Town, an error message will appear.

Running Validation from Program Code

You can run validation by accessing or creating a ValidationController. If you want the errors to be displayed to the user in the error window, use the ValidationController that is attached to your diagram’s DocData. For example, if you are writing a menu command, CurrentDocData.ValidationController is available in the command set class:

using Microsoft.VisualStudio.Modeling;
using Microsoft.VisualStudio.Modeling.Validation;
using Microsoft.VisualStudio.Modeling.Shell;
...
partial class MyLanguageCommandSet
{
  private void OnMenuMyContextMenuCommand(object sender, EventArgs e)
  {
   ValidationController controller = this.CurrentDocData.ValidationController;
...

For more information, see How to: Add a Command to the Shortcut Menu.

You can also create a separate validation controller, and manage the errors yourself. For example:

using Microsoft.VisualStudio.Modeling;
using Microsoft.VisualStudio.Modeling.Validation;
using Microsoft.VisualStudio.Modeling.Shell;
...
Store store = ...;
VsValidationController validator = new VsValidationController(s);
// Validate all elements in the Store:
if (!validator.Validate(store, ValidationCategories.Save))
{
  // Deal with errors:
  foreach (ValidationMessage message in validator.ValidationMessages) { ... }
}

Running validation when a change occurs

If you want to make sure that the user is warned immediately if the model becomes invalid, you can define a store event that runs validation. For more information about store events, see Event Handlers Propagate Changes Outside the Model.

In addition to the validation code, add a custom code file to your DslPackage project, with content similar to the following example. This code uses the ValidationController that is attached to the document. This controller displays the validation errors in the Visual Studio error list.

using System;
using System.Linq;
using Microsoft.VisualStudio.Modeling;
using Microsoft.VisualStudio.Modeling.Validation;
namespace Company.FamilyTree
{
  partial class FamilyTreeDocData // Change name to your DocData.
  {
    // Register the store event handler:
    protected override void OnDocumentLoaded()
    {
      base.OnDocumentLoaded();
      DomainClassInfo observedLinkInfo = this.Store.DomainDataDirectory
         .FindDomainClass(typeof(ParentsHaveChildren));
      DomainClassInfo observedClassInfo = this.Store.DomainDataDirectory
         .FindDomainClass(typeof(Person));
      EventManagerDirectory events = this.Store.EventManagerDirectory;
      events.ElementAdded
         .Add(observedLinkInfo, new EventHandler<ElementAddedEventArgs>(ParentLinkAddedHandler));
      events.ElementDeleted.Add(observedLinkInfo, new EventHandler<ElementDeletedEventArgs>(ParentLinkDeletedHandler));
      events.ElementPropertyChanged.Add(observedClassInfo, new EventHandler<ElementPropertyChangedEventArgs>(BirthDateChangedHandler));
    }
    // Handler will be called after transaction that creates a link:
    private void ParentLinkAddedHandler(object sender,
                                ElementAddedEventArgs e)
    {
      this.ValidationController.Validate(e.ModelElement,
           ValidationCategories.Save);
    }
    // Called when a link is deleted:
    private void ParentLinkDeletedHandler(object sender,
                                ElementDeletedEventArgs e)
    {
      // Don't apply validation to a deleted item!
      // - Validate store to refresh the error list.
      this.ValidationController.Validate(this.Store,
           ValidationCategories.Save);
    }
    // Called when any property of a Person element changes:
    private void BirthDateChangedHandler(object sender,
                      ElementPropertyChangedEventArgs e)
    {
      Person person = e.ModelElement as Person;
      // Not interested in changes in other properties:
      if (e.DomainProperty.Id != Person.BirthYearDomainPropertyId)
          return;

      // Validate all parent links to and from the person:
      this.ValidationController.Validate(
        ParentsHaveChildren.GetLinksToParents(person)
        .Concat(ParentsHaveChildren.GetLinksToChildren(person))
        , ValidationCategories.Save);
    }
  }
}

The handlers are also called after Undo or Redo operations that affect the links or elements.

Custom Validation Categories

In addition to the standard validation categories, such as Menu and Open, you can define your own categories. You can invoke these categories from program code. The user cannot invoke them directly.

A typical use for custom categories is to define a category that tests whether the model satisfies the preconditions of a particular tool.

To add a validation method to a particular category, prefix it with an attribute like this:

[ValidationMethod(CustomCategory = "PreconditionsForGeneratePartsList")]
[ValidationMethod(ValidationCategory.Menu)]
private void TestForCircularLinks(ValidationContext context)
{...}

To invoke a custom validation:

// Invoke all validation methods in a custom category:
validationController.ValidateCustom
  (store, // or a list of model elements
   "PreconditionsForGeneratePartsList");

Alternatives to Validation

Validation constraints report errors, but do not change the model. If, instead, you want to prevent the model becoming invalid, you can use other techniques.

However, these techniques are not recommended. It is usually better to let the user decide how to correct an invalid model.

Adjust the change to restore the model to validity. For example, if the user sets a property above the allowed maximum, you could reset the property to the maximum value. To do this, define a rule. For more information, see Rules Propagate Changes Within the Model.

Roll back the transaction if an invalid change is attempted. You could also define a rule for this purpose, but in some cases it is possible to override a property handler OnValueChanging(), or to override a method such as OnDeleted(). To roll back a transaction, use this.Store.TransactionManager.CurrentTransaction.Rollback(). For more information, see Domain Property Value Change Handlers.

See Also

Navigating and Updating a Model in Program Code Event Handlers Propagate Changes Outside the Model