[Test]
        public async Task TestDocumentsDifferentFormats(
            [ValueSource(nameof(Formats))] string format,
            [ValueSource(nameof(Documents))] IDocument document)
        {

public class DocumentFactory
    {
        public static readonly List<IDocument> Documents=
            new List<IDocument>
            {
              // Init documents
            };
    }

The sourceName specified on a ValueSourceAttribute must refer to a non null static field, property or method.