private static ImmutableHashSet <INamedTypeSymbol> GetWellKnownCollectionTypes(Compilation compilation) { var builder = ImmutableHashSet.CreateBuilder <INamedTypeSymbol>(); var iCollection = WellKnownTypes.ICollection(compilation); if (iCollection != null) { builder.Add(iCollection); } var genericICollection = WellKnownTypes.GenericICollection(compilation); if (genericICollection != null) { builder.Add(genericICollection); } var genericIReadOnlyCollection = WellKnownTypes.GenericIReadOnlyCollection(compilation); if (genericIReadOnlyCollection != null) { builder.Add(genericIReadOnlyCollection); } return(builder.ToImmutable()); }