So earlier today I was lamenting that an anonymous type can't be shared between functions with Wes Dyer, when he said "Well actually they can..."

Cue me learning something cool.

The first step is to create a seemingly innocent method:

public static T CastByExample<T>(this object o, T example)
    return (T) o;

Seems innocent enough right?

Well it is until you start using it with anonymous types. Imagine you had this function, that returns an anonymous type as object, because that is your only choice:

static object GetAnonymousType()
    return new { FullName = "Cosmo Kramer" };

Normally if you called this function anywhere you wouldn't be able to get at the anonymous type without using reflection... This is where CastByExample<T> comes to the rescue.

If you know the shape of the anonymous type, you use that to do a CastByExample...

object o = GetAnonymousType();

//get the original anonymous type back again
var v = o.CastByExample(new { FullName = "" });

//Use the properties of the anonymous type initialized in another
//function directly !!

This works because when an anonymous type is used the compiler first checks that one with the same signature (i.e. all fields are the same name and type) hasn't already been used. If one has the same CLR type is used.

Hence if you pass in an example that is the same shape as the original anonymous type to the CastByExample<T>(..) method will get you back to the original anonymous type... and var magic does the rest.

Nifty huh?