Question

Nostalgic for Collections.unmodifiableMap(), I’ve been implementing a read-only IDictionary wrapper based on this discussion, and my unit test quickly ran into a problem:

Assert.AreEqual (backingDictionary, readOnlyDictionary);

fails, even though the key-value pairs match. I played around a little more, and it looks like at least (thank Simonyi)

Assert.AreEquals (backingDictionary, new Dictionary<..> { /* same contents */ });

does pass.

I took a quick look through the Dictionary and IDictionary documentation, and to my surprise I couldn’t find any equivalent of the Java Map contract that two Maps with equal entrySet()s must be equal. (The docs say that Dictionary — not IDictionary — overrides Equals(), but don’t say what that override does.)

So it looks like key-value equality in C# is a property of the Dictionary concrete class, not of the IDictionary interface. Is this right? Is it generally true of the whole System.Collections framework?

If so, I’d be interested to read some discussion of why MS chose that approach — and also of what the preferred way would be to check for equality of collection contents in C#.

And finally, I wouldn’t mind a pointer to a well-tested ReadOnlyDictionary implementation. 🙂

---

ETA: To be clear, I’m not looking for suggestions on how to test my implementation — that’s relatively trivial. I’m looking for guidance on what contract those tests should enforce. And why.

---

ETA: Folks, I know IDictionary is an interface, and I know interfaces can’t implement methods. It’s the same in Java. Nevertheless, the Java Map interface documents an expectation of certain behavior from the equals() method. Surely there must be .NET interfaces that do things like this, even if the collection interfaces aren’t among them.

Answer 1

For later readers, here’s what I’ve been told / been able to figure out:

1. The contract for .NET collections,
   unlike Java collections, doesn’t
   include any specific behavior for
   Equals() or GetHashCode().
2. LINQ Enumerable.SequenceEqual()
   extension method will work for
   ordered collections, including
   dictionaries — which present as
   IEnumerable<KeyValuePair>;
   KeyValuePair is a struct, and its
   Equals method uses reflection
   to compare the contents.
3. Enumerable provides other extension
   methods that can be used to cobble
   together a content equality check, such
   as Union() and Intersect().

I’m coming around to the idea that, convenient as the Java methods are, they might not be the best idea if we’re talking about mutable collections, and about the typical implicit equals() semantics — that two equal objects are interchangeable. .NET doesn’t provide very good support for immutable collections, but the open-source PowerCollections library does.