Azure Functions 2.0 extensibility - extending extensions

This is my fourth post about Azure Functions extensibility. So far I've written about triggers, inputs, and outputs (maybe I should devote some more time to outputs, but that's something for another time). In this post, I want to focus on something I haven't mentioned yet - extending existing extensions. As usual, I intend to do it in a practical context.

Common problem with the Azure Cosmos DB Trigger

There is a common problem with the Azure Cosmos DB Trigger for Azure Functions. Let's consider the sample from the documentation.

public class ToDoItem
{
    public string Id { get; set; }
    public string Description { get; set; }
}

public static class CosmosTrigger
{
    [FunctionName("CosmosTrigger")]
    public static void Run([CosmosDBTrigger(
        databaseName: "ToDoItems",
        collectionName: "Items",
        ConnectionStringSetting = "CosmosDBConnection",
        LeaseCollectionName = "leases",
        CreateLeaseCollectionIfNotExists = true)]IReadOnlyList<Document> documents, 
        ILogger log)
    {
        if (documents != null && documents.Count > 0)
        {
            log.LogInformation($"Documents modified: {documents.Count}");
            log.LogInformation($"First document Id: {documents[0].Id}");
        }
    }
}

It looks ok, but it avoids the problem by not being interested in the content of the documents. The trigger has a limitation, it works only with IReadOnlyList. This means that accessing values may not be as easy as one could expect. There is GetPropertyValue m