When do we do GetHashCode() for a Dictionary?

Question

I have used Dictionary(TKey, TValue) for many purposes. But I haven\'t encountered any scenario to implement GetHashCode() which I believe is because my keys were of primary

Answer 1

One example is when you need to create a composite key (that is a key comprised of more that one piece of data). That composite key would be a custom type that would need to override those methods.

For example, let's say that you had an in-memory cache of address records and you wanted to check to see if an address was in cache to save an expensive trip to the database to retrieve it. Let's also say that addresses are unique in terms of their street 1 and zip code fields. You would implement your cache with something like this:

class AddressCacheKey
{
    public String StreetOne { get; set; }
    public String ZipCode { get; set; }

    // overrides for Equals and GetHashCode
}

and

static Dictionary cache;

Since your AddressCacheKey type overrides the Equals and GetHashCode methods they would be a good candidate for a key in the dictionary and you would be able to determine whether or not you needed to take a trip to the database to retrieve a record based on more than one piece of data.