Table of Contents

VSMEF012 Disallow MEF attribute version

This analyzer allows enforcing that only a specific version of MEF attributes (v1 or v2) is used in a project. This is useful for projects that want to standardize on a single MEF version.

This analyzer is disabled by default.

Cause

A MEF attribute from a disallowed version is used when the project has been configured to only allow a specific MEF version.

Rule description

MEF has two attribute libraries:

While VS-MEF supports both attribute libraries, mixing them can lead to confusion and subtle bugs. Some projects may want to standardize on a single version for consistency.

This analyzer can be configured to:

  • Only allow MEFv1 attributes (allowed_mef_version = System.ComponentModel.Composition)
  • Only allow MEFv2 attributes (allowed_mef_version = System.Composition)

When configured, using an attribute from the disallowed version will produce a diagnostic.

Configuration

Add the following to your .editorconfig file:

# Enable the analyzer and specify allowed version
[*.cs]
dotnet_diagnostic.VSMEF012.severity = warning

# Allow only MEFv1 attributes
dotnet_diagnostic.VSMEF012.allowed_mef_version = System.ComponentModel.Composition

# Or allow only MEFv2 attributes
dotnet_diagnostic.VSMEF012.allowed_mef_version = System.Composition

Examples of violations

When configured for System.ComponentModel.Composition only

// .editorconfig: dotnet_diagnostic.VSMEF012.allowed_mef_version = System.ComponentModel.Composition

using System.Composition;  // MEFv2 namespace

[Export]  // ❌ VSMEF012: MEFv2 attribute not allowed
class Service
{
    [Import]  // ❌ VSMEF012: MEFv2 attribute not allowed
    public ILogger Logger { get; set; }
}

When configured for System.Composition only

// .editorconfig: dotnet_diagnostic.VSMEF012.allowed_mef_version = System.Composition

using System.ComponentModel.Composition;  // MEFv1 namespace

[Export]  // ❌ VSMEF012: MEFv1 attribute not allowed
class Service
{
    [Import]  // ❌ VSMEF012: MEFv1 attribute not allowed
    public ILogger Logger { get; set; }
}

Valid scenarios (no diagnostic)

Using allowed version

// .editorconfig: dotnet_diagnostic.VSMEF012.allowed_mef_version = System.ComponentModel.Composition

using System.ComponentModel.Composition;  // MEFv1 namespace

[Export]  // ✅ OK - MEFv1 is allowed
class Service
{
    [Import]  // ✅ OK
    public ILogger Logger { get; set; }
}

Analyzer not configured

// No .editorconfig setting for VSMEF012

using System.Composition;

[Export]  // ✅ OK - analyzer is disabled by default
class Service { }

How to fix violations

Replace the disallowed MEF attributes with the equivalent attributes from the allowed version.

MEFv1 to MEFv2

MEFv1 Attribute MEFv2 Attribute
System.ComponentModel.Composition.ExportAttribute System.Composition.ExportAttribute
System.ComponentModel.Composition.ImportAttribute System.Composition.ImportAttribute
System.ComponentModel.Composition.ImportManyAttribute System.Composition.ImportManyAttribute
System.ComponentModel.Composition.ImportingConstructorAttribute System.Composition.ImportingConstructorAttribute
System.ComponentModel.Composition.PartCreationPolicyAttribute System.Composition.SharedAttribute / System.Composition.PartNotDiscoverableAttribute
System.ComponentModel.Composition.ExportMetadataAttribute Custom metadata attribute

MEFv2 to MEFv1

Use the reverse mapping of the table above.

When to suppress warnings

Suppress this warning if:

  • You intentionally need to use both MEF versions in a transitional codebase
  • You're implementing adapters or bridges between MEFv1 and MEFv2 components

Notes

  • This analyzer is disabled by default. You must explicitly enable it and configure the allowed version.
  • This analyzer is distinct from VSMEF002, which warns about mixing MEF versions on the same type. VSMEF012 disallows a version entirely.
  • This analyzer walks the inheritance hierarchy, so custom attributes that derive from MEF attributes (e.g., a custom export attribute that derives from ExportAttribute) are correctly detected.
  • Some MEFv1-only features (like InheritedExportAttribute, PartCreationPolicyAttribute, ExportMetadataAttribute) have no direct MEFv2 equivalents.
Edit this page