I was working on an implementation of a REST service recently, making it respond to the HTTP Accept headers on the incoming request. As you can see from the HTTP/1.1 specification, Accept request-headers should be honored in the order that they’re listed. So it was absolutely awesome to learn that

using System.ServiceModel.Web;

var acceptHeaders = WebOperationContext.Current.IncomingRequest.GetAcceptHeaderElements();
foreach ( var acceptHeader in acceptHeaders )
{
  // process acceptHeader...
}

processes the headers in the opposite order in which they are specified in the request. And that’s not awesome in a good sense.

The IncomingWebRequestContext.GetAcceptHeaderElements method returns a System.Collections.ObjectModel.Collection<T> and that is not a class I own or can change. But still I’d like an iterator for Collection<T> that yields up its elements in reverse order so that I can still use the foreach syntax. Since Collection<T> is not a class I can modify, extension methods come to mind. Here’s an extension method for an iterator on Collection<T> that does the trick.

using System.Collections.Generic;
using System.Collections.ObjectModel;

public static class Extensions
{
  public static IEnumerable<T> Reverse<T>( this Collection<T> collection )
  {
    for ( int i = collection.Count - 1; i >= 0; --i )
      yield return collection[ i ];
  }
}

Now I can write the foreach loop to use the extension method.

foreach ( var acceptHeader in acceptHeaders.Reverse() )
{
  // process acceptHeader in the correct order...
}

I am still dumbfounded that GetAcceptHeaderElements reverses the order of the elements (why would that ever be good?) but a few lines sends me on my way without cluttering up the code.