Question

Say I define a variable like this:

var o = new { RBI = 108, Name = "Roberto Alomar" };

I can do something like:

Console.WriteLine("{0}", o);

But if I try:

foreach (var i in o) {
    Console.WriteLine("{0}", o[i]);
}

I get an error:

foreach statement cannot operate on variables of type 'AnonymousType#1' because 'AnonymousType#1' does not contain a public definition for 'GetEnumerator'

So how does it work under the hood? I’d think that a method for turning an object into a string would have to loop through all the properties to accomplish the task. Is there some special method that allows this to happen, or am I misunderstanding how this works?

Answer 1

How does it work under the hood? I’d think that a method for turning an object into a string would have to loop through all the properties to accomplish the task.

Your assumption is that the implementation of ToString is shared between all instances of all anonymous types; that, for example, there is some helper that is logically something like you would do in JavaScript:

var s = "";
for (property in this)
   s += property + ":" + this[property];

This assumption is wrong; there is no single one-size-fits-all implementation of ToString for anonymous types. Rather, the compiler knows what all the properties of the anonymous method are and so it generates a brand-new custom implementation of ToString for every distinct anonymous type.

In C#, the foreach loop does not do what the for-in loop does in JavaScript. The C# loop enumerates the members of a collection. The JS loop enumerates the properties of an object.

If you want to enumerate the properties of an object in C# you can do that, it just takes a bit more work:

var s = "";
foreach (PropertyInfo propertyInfo in this.GetType().GetProperties())
   s += propertyInfo.Name + ":" + propertyInfo.GetValue(this).ToString();