Question

the ‘Equal’ and ‘GetHashcode’ method are exist in the object class, and our type inherit the object base class.
what’s the different between implement the two methods of the object directly and using the IComparer interface?

if we overriding object’s Equal and GetHashCode , and push to a hashtable , it will use the overring ‘s equal method?

what’ the differents of new a hashtable with the IEqualityComparer constructor?

Answer 1

The IComparable interface is used when you need to be able to “sort” objects, and it gives you a method (CompareTo) that tells you if two objects are <, = or > . The constructor that uses IEqualityComparer let you give a specific Equals/GetHashCode that can be different than the ones defined by your object. Normally the Hashtable will use your object overridden Equals and GetHashCode (or the base object Equals and GetHashCode).

To make an example, the standard string compares in case sensitive way ("A" != "a"), but you could make an IEqualityComparer helper class to be able to hash your strings in a case insensitive way. (technically this class is already present in multiple variants: they are called StringComparer.InvariantCultureIgnoreCase and all the other static methods of StringComparer that return a StringComparer object that implements the IComparer, IEqualityComparer, IComparer<string>, IEqualityComparer<string>)

As a note, the Hashtable uses a IEqualityComparer optional parameter, not the generic version IEqualityComparer<T>, because Hashtable is pre-generics.