Question

I need to use a class that I have written as the type of the key for a Dictionary

I read the documentation on MSDN about the default constructor of Dictionary

Dictionary<TKey, TValue> requires an equality implementation to
determine whether keys are equal. This constructor uses the default
generic equality comparer, EqualityComparer<T>.Default. If type
TKey implements the System.IEquatable<T> generic interface, the
default equality comparer uses that implementation. Alternatively, you
can specify an implementation of the IEqualityComparer<T> generic
interface by using a constructor that accepts a comparer parameter.

This makes think that the only thing I have to do is to have my class for the key implement System.IEquatable<T>

However i’m very surprised that System.IEquatable<T> doesn’t have a HashCode() method.

So will the Dictionary created this way use a hash code? If yes where does it come from ? Otherwise, will my Dictionary have constant cost access operations (I don’t think it’s achievable without a hash code)

Answer 1

It still uses the overridden object.GetHashCode() method to obtain the hash code. The reason that there is a separate IEquatable<T> interface (i.e. why the default EqualityComparer<T> doesn’t always just call the overridden object.Equals() method to compare two objects) is for performance reasons – object.Equals() takes an object argument and therefore the implementation has to cast it to the target type before it can perform a meaningful comparison (value types also have to be boxed and unboxed); whereas the argument to IEquatable<T>.Equals() is already of type T. This performance consideration doesn’t apply to the GetHashCode() method since it takes no argument, hence there is no reason for it to exist on the IEquatable<T> interface.