LazyMetadataWrapper not super performant

[znakeeye]

When querying attribute data - multiple times - for lazy imports, I see that a significant amount of time is spent In LazyMetadataWrapper.

Why the need to copy the arrays like that? What would be your recommendation here? Should the consumer cache the metadata?

[AArnott]

It's been a long time since I looked at this code, but it looks like TypeRef.Get already has something like instance caching:

vs-mef/src/Microsoft.VisualStudio.Composition/Reflection/TypeRef.cs

Line 256 in c3eef68

public static TypeRef? Get(Type? type, Resolver resolver)

Have you confirmed that in fact the cost here is paid multiple times for the same type?

I don't see us copying arrays as much as filtering and transforming them:

vs-mef/src/Microsoft.VisualStudio.Composition/Reflection/TypeRef.cs

Lines 104 to 106 in c3eef68

	this.GenericTypeArguments = arrayElementType.GenericTypeArguments != null && arrayElementType.GenericTypeArguments.Length > 0
	? arrayElementType.GenericTypeArguments.Where(t => !(shallow && t.IsGenericParameter)).Select(t => new TypeRef(resolver, t, shallow: true)).ToImmutableArray()
	: ImmutableArray<TypeRef>.Empty;

But as this includes a few linq extension methods in a row, I suspect this could be optimized.

[znakeeye]

The performance problem is seen only when loading the MEF graph from a serialized file.

Also, I was performing a lookup multiple times. Hence, I was reading the ISomeMetadata metadata from a number of imported Lazy<IThing, ISomeMetadata>. Something like this:

lazyImports.Where(t => t.Metadata.MyTypeProperty == inputType);

Putting the SomeMetadataProperty value in a dictionary improves the performance, but you'd have to agree that this is a bit cumbersome:

// The metadata interface...
public interface ISomeMetadata
{
    Type MyTypeProperty { get; }
    Type OtherProperty { get; }
}

// And later I cache it:
private Dictionary<ISomeMetadata, Type> metadataCache;

More details

For 179 imported Lazy<IThing, ISomeMetadata> I got the following numbers when profiling:

• Time spent in SubstituteValueIfRequired: 4005 ms
• Number of calls: 6265
• Number of MoveNext() enumerator calls: 216 912