Why is the Fragment of a Uri ignored in the Equals method?

Question

I'm trying to maintain a collection of objects based on their URI:

public class ConceptCollection : KeyedCollection<Uri, Concept> {
    protected override Uri GetKeyForItem(Concept item) {
        return item.Uri;
    } 
}

However, the URI regularly only differs based on the Fragment of the Uri. So, the following causes an error:

ConceptCollection wines = new ConceptCollection();
Concept red = new Concept("http://www.w3.org/2002/07/owl#RedWine");
Concept white = new Concept("http://www.w3.org/2002/07/owl#WhiteWine");
wines.Add(red);
wines.Add(white); // Error: An item with the same key has already been added.

Per http://msdn.microsoft.com/en-us/library/f83xtf15.aspx:

The Equals method compares the two instances without regard to user information ( UserInfo) and fragment ( Fragment) parts that they might contain. For example, given the URIs http://www.contoso.com/index.htm#search and http://user%[email protected]/index.htm, the Equals method would return true.

I'm resigned to having to hack around this. But why does it behave this way? I can see the logic for user-info, but not for fragment.

Answer 1

I guess because 2 URIs that are identical apart from the fragment still refer to the same resource, just a different location within the resource.

So if you're asking the question 'are these the same resource?' then you could argue that it's correct to ignore the fragment.

Answer 2

From RFC 2396:

4.1. Fragment Identifier

When a URI reference is used to perform a retrieval action on the identified resource, the optional fragment identifier, separated from the URI by a crosshatch ("#") character, consists of additional reference information to be interpreted by the user agent after the retrieval action has been successfully completed. As such, it is not part of a URI, but is often used in conjunction with a URI.

The emphasis added is mine and is the reason the fragment is not considered in the Uri.Equals implementation.

In your example, the URI for the resource you are retrieving is: http://www.w3.org/2002/07/owl

The fragments are processed by the user agent and have no meaning to or influence on the actual retrieval of the resource.