Why is this an issue?

To customize the default behavior for an export in the Managed Extensibility Framework (MEF), applying the PartCreationPolicyAttribute is necessary. For the PartCreationPolicyAttribute to be meaningful in the context of an export, it is required to also annotate the class with the ExportAttribute.

This rule raises an issue when a class is annotated with the PartCreationPolicyAttribute but not with the ExportAttribute.

How to fix it

Code examples

Noncompliant code example

Imports System.ComponentModel.Composition

<PartCreationPolicy(CreationPolicy.Any)> ' Noncompliant
Public Class FooBar
    Inherits IFooBar
End Class

Compliant solution

Imports System.ComponentModel.Composition

<Export(GetType(IFooBar))>
<PartCreationPolicy(CreationPolicy.Any)>
Public Class FooBar
    Inherits IFooBar
End Class

Resources

Documentation

• Microsoft Learn - Managed Extensibility Framework (MEF)
• Microsoft Learn - Attributed programming model overview (MEF)
• Microsoft Learn - PartCreationPolicyAttribute Class
• Microsoft Learn - ExportAttribute Class
• Microsoft Learn - CreationPolicy Enum

Articles & blog posts

• Stefan Henneken - MEF Part 1 – Fundamentals, Imports and Exports
• Stefan Henneken - MEF Part 2 – Metadata and creation policies
• Stefan Henneken - MEF Part 3 – Life cycle management and monitoring