.Net Tips – Working with anonymous types

Yan Cui

I help clients go faster for less using serverless technologies.

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: }

   5:

   6: ...

   7:

   8: object o = GetAnonymousType();

   9:

  10: // get the original anonymous type back again 

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

  12:

  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();

   3:

   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!