Are MEF exports cached or discovering every time on request?

Question

If I have one type MyClass, register with

[Export(typeof(Myclass))] attribute, and

[PartCreationPolicy(CreationPolicy.Shared)]

or

[PartCreationPolicy(CreationPolicy.NonShared)]

and later trying to call

compositionContainer.GetExportedValue<Myclass>() multiple times.

Question: with the first call, I will get my registered class via MEF - llokup all registered assemblies, then trying to find one registered contract. Question is about second time and so on - will MEF do global lookup again or it caches somewhere internally?

Answer 1

will MEF do global lookup again or it caches somewhere internally

Yes, MEF perfoms some caching and widely uses lazy initialization, if you question is about MEF performance:

1) metadata (composable parts, export definitions and import definitions) is cached. Example:

public override IEnumerable<ExportDefinition> ExportDefinitions
{
    get
    {
        if (this._exports == null)
        {
            ExportDefinition[] exports = this._creationInfo.GetExports().ToArray<ExportDefinition>();
            lock (this._lock)
            {
                if (this._exports == null)
                {
                    this._exports = exports;
                }
            }
        }
        return this._exports;
    }
}

2) exported values are cached too:

public object Value
{
    get
    {
        if (this._exportedValue == Export._EmptyValue)
        {
            object exportedValueCore = this.GetExportedValueCore();
            Interlocked.CompareExchange(ref this._exportedValue, exportedValueCore, Export._EmptyValue);
        }
        return this._exportedValue;
    }
}

Of course, when using CreationPolicy.NonShared, exported value becomes created again and again, when you requesting it. But even in this case "global lookup" isn't performed, because metadata is cached anyway.