.Net Tips – Working with anonymous types

Check out my new course Learn you some Lambda best practice for great good! and learn the best practices for performance, cost, security, resilience, observability and scalability.

Anonymous types are useful, especially in LINQ queries where you often want to iterate through an array of items and project them into some arbitrarily shaped objects that are little more than simple data containers, using anonymous types save you the hassle of having to first declare those types which you will simply throw away afterwards.

One thing you can’t easily do with anonymous types however, is to return them from the method it’s declared in and use them elsewhere. Sure, you can return an instance of an anonymous type as object, but that doesn’t make it any easier to work with them as you don’t have access to any of the properties on the anonymous type. So what can you do in those cases?

Cast By Example

This is a nifty technique which can deals with the above problem, and all you need is a simple method like this:

   1: public static T CastByExample<T>(this object o, T example)

   2: {

   3:     return (T) o;

   4: }

You can then take an instance of an anonymous type and cast it back to its original shape:

   1: static object GetAnonymousType()

   2: {

   3:     return new { FirstName = "Yan", Surname = "Cui", Age = 29 };

   4: }


   6: ...


   8: object o = GetAnonymousType();


  10: // get the original anonymous type back again 

  11: var o2 = o.CastByExample(new { FirstName = "", Surname = "", Age = 0 });


  13: // you can now use the properties on the original anonymous type freely

  14: Console.WriteLine(o2.FirstName);

This works, but only if the example is the exact shape as the original, otherwise you’ll get an InvalidCastException:

   1: // throws InvalidCastException

   2: // Unable to cast object of type '<>f__AnonymousType0`3[System.String,System.String,System.Int32]'

   3: // to type '<>f__AnonymousType1`2[System.String,System.String]'.

   4: var o2 = o.CastByExample(new { FirstName = "", Surname = "" });

Dynamic Programming

An alternative to using the Cast By Example technique is to take advantage of the dynamic keyword introduced in C# 4, whilst this approach makes everything soooo easy, the trade off is that you lose intellisense support.

   1: // simply cast the returned object as a dynamic and off you go!

   2: var o = (dynamic)GetAnonymousType();


   4: Console.WriteLine(o.FirstName);

Of course, if you try to use a property that isn’t defined on the anonymous type then you’ll get an exception at runtime, specifically the RuntimeBinderException.

Parting Thoughts..

Both techniques can help make life easier if on the rare occasion you need to return an anonymous type from your method, but if you find yourself doing that frequently in your code then it’s probably best to take a step back and rethink your overall architecture. Anonymous types are confined to methods by design, and for a good reason! If you’ve structured your code correctly following good OOP design principles then there should be very little need to ‘hack’ your way through using these techniques! So as a parting shot, please use these techniques with caution and only when absolutely necessary!

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter

Hi, I’m Yan. I’m an AWS Serverless Hero and the author of Production-Ready Serverless.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.

Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.

Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!

Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!

Are you working with Serverless and looking for expert training to level-up your skills? Or are you looking for a solid foundation to start from? Look no further, register for my Production-Ready Serverless workshop to learn how to build production-grade Serverless applications!

Find a workshop near you