How to: Define Validation Constraints for UML Models

In Visual Studio Ultimate, you can define validation constraints that test whether the model meets a condition you specify. For example, you could define a constraint to make sure that a user does not create a loop of inheritance relationships. The constraint is invoked when the user tries to open or save the model, and can also be invoked manually. If the constraint fails, an error message that you define is added to the error window. You can package these constraints into a Visual Studio Integration Extension (VSIX) and distribute it to other Visual Studio Ultimate users.

You can also define constraints that validate the model against external resources such as databases.


If you want to validate program code against a layer diagram, see Adding Custom Architecture Validation to Layer Diagrams.


Applying Validation Constraints

Validation constraints are applied in three cases: when you save a model; when you open a model; and when you click Validate UML Model on the Architecture menu. In each case, only those constraints that have been defined for that case will be applied, although typically you would define each constraint to apply in more than one case.

Validation errors are reported in the Visual Studio errors window, and you can double-click the error to select the model elements that are in error.

For more information about applying validation, see How to: Validate a UML Model.

Defining a Validation Extension

To create a validation extension for a UML designer, you must create a class that defines the validation constraints, and embed the class in a Visual Studio Integration Extension (VSIX). The VSIX acts as a container that can install the constraint. There are two alternative methods of defining a validation extension:

  • Create a validation extension in its own VSIX using a project template. This is the quicker method. Use it if you do not want to combine your validation constraints with other types of extension such as menu commands, custom toolbox items, or gesture handlers. You can define several constraints in one class.

  • Create separate validation class and VSIX projects. Use this method if you want to combine several types of extension into the same VSIX. For example, if your menu command expects the model to observe specific constraints, you could embed it into the same VSIX as a validation method.

To create a validation extension in its own VSIX

  1. In the New Project dialog box, under Modeling Projects, select Validation Extension.

  2. Open the .cs file in the new project and modify the class to implement your validation constraint.

    For more information, see Implementing the Validation Constraint.


    Make sure that your .cs files contain the following using statement:

    using Microsoft.VisualStudio.ArchitectureTools.Extensibility.Uml;

  3. You can add additional constraints by defining new methods. To identify a method as a validation method, it must be tagged with the attributes in the same way as the initial validation method.

  4. Test your constraints by pressing F5. For more information, see Executing Validation.

  5. Install the menu command on another computer by copying the file bin\*\*.vsix that is built by your project. For more information, see Installing the Validation Constraints.

When you add other .cs files, you will typically require the following using statements:

using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.Linq;
using Microsoft.VisualStudio.ArchitectureTools.Extensibility.Uml;
using Microsoft.VisualStudio.Modeling.Validation;
using Microsoft.VisualStudio.Uml.Classes;

To create a separate validation constraint in a class library project

  1. Create a class library project, either adding it to an existing VSIX solution, or creating a new solution.

    1. On the File menu, point to New and then click Project.

    2. Under Installed Templates, click Visual C# or Visual Basic, then in the middle column click Class Library.

    3. Set Solution to indicate whether you want to create a new solution or to add a component to a VSIX solution that you have already opened.

    4. Set the project Name and Location and click OK.

  2. Unless your solution already contains one, create a VSIX project.

    1. In Solution Explorer, right-click the solution, point to Add, and then click New Project.

    2. Under Installed Templates, expand Visual C# or Visual Basic, then click Extensibility. In the middle column, click VSIX Project.

  3. Set the VSIX project as the startup project of the solution.

    • In Solution Explorer, right-click the VSIX project and then click Set as StartUp project.
  4. In source.extension.vsixmanifest, under Content, add the class library project as a MEF Component.

    1. Open source.extension.vsixmanifest

    2. Click Add Content.

    3. At Select a content type, select MEF Component.

    4. At Select a source, click Project and select the name of your class library project.

  5. Click Select Editions and select the Visual Studio editions you want your extension to run on.

  6. Set the name and descriptive fields of the VSIX. Save the file.

To define the Validation Class

  1. You do not need this procedure if you have created a validation class with its own VSIX from the validation project template.

  2. In the validation class project, add references to the following .NET assemblies:





  3. Add a file to the class library project containing code that is similar to the following example.

    • Each validation constraint is contained within a method that is marked with a specific attribute. The method accepts a parameter of a model element type. When validation is invoked, the validation framework will apply every validation method to every model element that conforms to its parameter type.

    • You can place these methods in any classes and namespaces. Change them to your preference.

    using System.Collections.Generic;
    using System.ComponentModel.Composition;
    using System.Linq;
    using Microsoft.VisualStudio.Modeling.Validation;
    using Microsoft.VisualStudio.ArchitectureTools.Extensibility.Uml;
    using Microsoft.VisualStudio.Uml.Classes;
    // You might also need the other Microsoft.VisualStudio.Uml namespaces.
    namespace Validation
      public class MyValidationExtensions
        // All validation methods have the following attributes.
        [Export(typeof(System.Action<ValidationContext, object>))]
         | ValidationCategories.Open
         | ValidationCategories.Menu)]
        public void ValidateClassNames
          (ValidationContext context, 
           // This type determines what elements 
           // will be validated by this method:
           IClass elementToValidate)
          // A validation method should not change the model.
          List<string> attributeNames = new List<string>();
          foreach (IProperty attribute in elementToValidate.OwnedAttributes)
            string name = attribute.Name;
            if (!string.IsNullOrEmpty(name) && attributeNames.Contains(name))
                string.Format("Duplicate attribute name '{0}' in class {1}", name, elementToValidate.Name),
                "001", elementToValidate);
        // Add more validation methods for different element types.

Executing a Validation Constraint

For test purposes, execute your validation methods in debug mode.

To test the validation constraint

  1. Press F5, or on the Debug menu, click Start Debugging.

    An experimental instance of Visual Studio starts.

    Troubleshooting: If a new Visual Studio does not start:

    • If you have more than one project, make sure that the VSIX project is set as the Startup project of the solution.

    • In Solution Explorer, right-click the startup or only project, and then click Properties. In the project properties editor, click the Debug tab. Make sure that the string in the Start external program field is the full pathname of Visual Studio, typically:

      C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\devenv.exe

  2. In the experimental Visual Studio, open or create a modeling project, and open or create a modeling diagram.

  3. To set up a test for the sample constraint given in the previous section:

    1. Open a class diagram.

    2. Create a class, and add two attributes that have the same name.

  4. Right-click anywhere on the diagram, and then click Validate.

  5. Any errors in the model will be reported in the errors window.

  6. Double-click the error report. If the elements mentioned in the report are visible on the screen, they will be highlighted.

    Troubleshooting: If the Validate command does not appear on the menu, make sure that:

    • The validation project is listed as a MEF component in the Content list in source.extensions.manifest in the VSIX project.

    • The correct Export and ValidationMethod attributes are attached to the validation methods.

    • ValidationCategories.Menu is included in the argument for the ValidationMethod attribute, and it is composed with other values using Logical OR (|).

    • The parameters of all the Import and Export attributes are valid.

Evaluating the Constraint

The validation method should determine whether the validation constraint that you want to apply is true or false. If true, it should do nothing. If false, it should report an error using the methods provided by the ValidationContext parameter.


Validation methods should not change the model. There is no guarantee when or in what order the constraints will be executed. If you have to pass information between successive executions of a validation method within a validation run, you can use the context cache described under Coordinating Multiple Validations.

For example, if you want to ensure that every type (class, interface, or enumerator) has a name that is at least three characters long, you could use this method:

public void ValidateTypeName(ValidationContext context, IType type)
  if (!string.IsNullOrEmpty(type.Name) && type.Name.Length < 3)
      string.Format("Type name {0} is too short", type.Name),
               "001", type);

See Programming with the UML API for information about the methods and types you can use to navigate and read the model.

About Validation Constraint Methods

Each validation constraint is defined by a method of the following form:

[Export(typeof(System.Action<ValidationContext, object>))]
  | ValidationCategories.Menu 
  | ValidationCategories.Open)]
public void ValidateSomething
  (ValidationContext context, IClassifier elementToValidate)

The attributes and parameters of every validation method are as follows:

[Export(typeof(System.Action <ValidationContext, object>))]

Defines the method as a validation constraint using Managed Extensibility Framework (MEF).

[ValidationMethod (ValidationCategories.Menu)]

Specifies when validation will be performed. Use bitwise OR (|) if you want to combine more than one option.

Menu = invoked by the Validate menu.

Save = invoked on saving the model.

Open = invoked on opening the model. Load = invoked on saving the model, but for a contravention warns the user that it may not be possible to re-open the model. Also called on loading, before the model is parsed.

public void ValidateSomething

(ValidationContext context,

IElement element)

Replace the second parameter IElement by the type of element to which you want the constraint to apply. The constraint method will be invoked on all elements in the specified type.

The name of the method is unimportant.

You can define as many validation methods as you want, with different types in the second parameter. When validation is invoked, each validation method will be called on each model element that conforms to the parameter type.

Reporting Validation Errors

To create an error report, use the methods provided by ValidationContext:

context.LogError("error string", errorCode, elementsWithError);

  • "error string" appears in the Visual Studio Error List

  • errorCode is a string that should be a unique identifier of the error

  • elementsWithError identifies elements in the model. When the user double-clicks the error report, the shape representing this element will be selected.

LogError(), LogWarning() and LogMessage() place messages in different sections of the error list.

How Validation Methods are Applied

Validation is applied to every element in the model, including relationships and the parts of larger elements, such as attributes of a class and parameters of an operation.

Each validation method is applied to each element that conforms to the type in its second parameter. This means that, for example, if you define a validation method with a second parameter of IUseCase and another with its supertype IElement, then both of these methods will be applied to each use case in the model.

The hierarchy of types is summarized in Model Element Types.

You can also access elements by following relationships. For example, if you were to define a validation method on IClass, you could loop through its owned properties:

public void ValidateTypeName(ValidationContext context, IClass c)
   foreach (IProperty property in c.OwnedAttributes)
       if (property.Name.Length < 3)
                        "Property name {0} is too short", 
                 "001", property);

Creating a validation method on the Model

If you want to ensure that a validation method is called exactly once during each validation run, you can validate the IModel:

using Microsoft.VisualStudio.Uml.AuxiliaryConstructs; ...
[Export(typeof(System.Action<ValidationContext, object>))]
public void ValidateModel(ValidationContext context, IModel model)
{  foreach (IElement element in model.OwnedElements)
   { ...

Validating shapes and diagrams

Validation methods are not invoked on display elements such as diagrams and shapes, because the primary purpose of validation methods is to validate the model. But you can access the current diagram using the diagram context.

In your validation class, declare DiagramContext as an imported property:

using Microsoft.VisualStudio.ArchitectureTools.Extensibility.Presentation; 
public IDiagramContext DiagramContext { get; set; }

In a validation method, you can use DiagramContext to access the current focus diagram, if there is one:

[Export(typeof(System.Action<ValidationContext, object>))]
public void ValidateModel(ValidationContext context, IModel model)
  IDiagram focusDiagram = DiagramContext.CurrentDiagram;
  if (focusDiagram != null)
    foreach (IShape<IUseCase> useCaseShape in
    { ...

To log an error, you must obtain the model element that the shape represents, because you cannot pass a shape to LogError:

       IUseCase useCase = useCaseShape.Element;
       context.LogError(... , usecase);

Coordinating Multiple Validations

When validation is invoked, for example by the user from a diagram menu, each validation method is applied to each model element. This means that, in a single invocation of the validation framework, the same method may be applied many times to different elements.

This presents a problem for validations that deal with the relationships between elements. For example, you might write a validation that starts from, say, a use case, and traverses the include relationships to verify that there are no loops. But when the method is applied to each use case in a model that has many include links, it is likely to repeatedly process the same areas of the model.

To avoid this situation, there is a context cache in which information is preserved during a validation run. You can use it to pass information between different executions of the validation methods. For example, you could store a list of the elements that have already been dealt with in this validation run. The cache is created at the start of each validation run, and cannot be used to pass information between different validation runs.

context.SetCacheValue<T> (name, value)

Store a value

context.TryGetCacheValue<T> (name, out value)

Get a value. Returns true if successful.


Get a value.


Get a value of the specified type.

Installing and uninstalling an extension

You can install a Visual Studio extension both on your own computer and on other computers.

To install an extension

  1. On your computer, find the .vsix file that was built by your VSIX project.

    1. In Solution Explorer, right-click the VSIX project, and then click Open Folder in Windows Explorer.

    2. Locate the file bin\*\YourProject.vsix

  2. Copy the .vsix file to the target computer on which you want to install the extension. This can be your own computer or another one.

    • The target computer must have one of the editions of Visual Studio that you specified in source.extension.vsixmanifest.
  3. On the target computer, double-click the .vsix file.

    Visual Studio Extension Installer opens and installs the extension.

  4. Start or restart Visual Studio.

To uninstall an extension

  1. On the Tools menu, click Extension Manager.

  2. Expand Installed Extensions.

  3. Select the extension, and then click Uninstall.

Rarely, a faulty extension fails to load and creates a report in the error window, but does not appear in Extension Manager. In that case, you can remove the extension by deleting the file from the following location where %LocalAppData% is typically DriveName:\Users\UserName\AppData\Local:



This example finds loops in the Dependency relationship between elements.

It will validate both on save and on the validate menu command.

/// <summary>
/// Verify that there are no loops in the dependency relationsips.
/// In our project, no element should be a dependent of itself.
/// </summary>
/// <param name="context">Validation context for logs.</param>
/// <param name="element">Element to start validation from.</param>
[Export(typeof(System.Action<ValidationContext, object>))]
     | ValidationCategories.Save | ValidationCategories.Open)]
public void NoDependencyLoops(ValidationContext context, INamedElement element)
    // The validation framework will call this method
    // for every element in the model. But when we follow
    // the dependencies from one element, we will validate others.
    // So we keep a list of the elements that we don't need to validate again. 
    // The list is kept in the context cache so that it is passed
    // from one execution of this method to another.
    List<INamedElement> alreadySeen = null;
    if (!context.TryGetCacheValue("No dependency loops", out alreadySeen))
       alreadySeen = new List<INamedElement>();
       context.SetCacheValue("No dependency loops", alreadySeen);

    NoDependencyLoops(context, element, 
                new INamedElement[0], alreadySeen);    

/// <summary>
/// Log an error if there is any loop in the dependency relationship.
/// </summary>
/// <param name="context">Validation context for logs.</param>
/// <param name="element">The element to be validated.</param>
/// <param name="dependants">Elements we've followed in this recursion.</param>
/// <param name="alreadySeen">Elements that have already been validated.</param>
/// <returns>true if no error was detected</returns>
private bool NoDependencyLoops(ValidationContext context, 
    INamedElement element, INamedElement[] dependants, 
    List<INamedElement> alreadySeen)
    if (dependants.Contains(element))
        context.LogError(string.Format("{0} should not depend on itself", element.Name), 
        "Fabrikam.UML.NoGenLoops", // unique code for this error
        dependants.SkipWhile(e => e != element).ToArray()); 
            // highlight elements that are in the loop
        return false;
    INamedElement[] dependantsPlusElement = 
        new INamedElement[dependants.Length + 1];
    dependants.CopyTo(dependantsPlusElement, 0);
    dependantsPlusElement[dependantsPlusElement.Length - 1] = element;

    if (alreadySeen.Contains(element))
        // We have already validated this when we started 
        // from another element during this validation run.
        return true;

    foreach (INamedElement supplier in element.GetDependencySuppliers())
        if (!NoDependencyLoops(context, supplier,
             dependantsPlusElement, alreadySeen))
        return false;
    return true;

See Also


How to: Define and Install a Modeling Extension

Programming with the UML API