Dictionaries try to pair key/value and work with it in order to store and retrieve information. This is the main difference between dictionaries and lists. One widely used dictionary is Hashtable. There are two ways to store information one is by using Add method and another is by the means of indexer. Indexer helps you set a key and then assigns value to this key.
Hashtable myHashedEemail = new Hashtable();
myHashedEemail.Add("test@test.com", "Joe, Doe");
myHashedEemail ["email@site.com"] = "James, Shmo";

Indexer also helps with the retrieval of the value
Console.WriteLine(myHashedEemail ["test@test.com "]); Hashtable stores objects so iteration is not as simple as with lists. Here is how it’s done. First we iterate via object by casting them at the same time since we know they are DictioanryEntry objects and then getting property of these objects.
foreach (DictionaryEntry myEntry in myHashedEemail)
{
    Console.WriteLine(myEntry.Value);
}

Or

foreach (object myName in myHashedEemail.Values)
{
    Console.WriteLine(myName);
}

Dictionary classes support IDictionary interface which is derived from ICollection interface and Hastable also supports IDictionary.

Properties of IDictionary

 

Name	Description
IsFixedSize	Deterines an indicator of whether this collection can be resized
IsReadOnly	Deterines an indicator of whether a collection can be changed
Item	Deterines or sets the item at a specific element in the collection
Keys	Deterines an ICollection object containing a list of the keys
Values	Deterines an ICollection object containing a list of the values in the collection

Methods

 

Name	Description
Add	Adds a key/value
Clear	Removes all items
Contains	Key is contained
GetEnumerator	Returns an IDictionaryEnumerator object
Remove	Removes the item in the collection

IDictionary does not have index like IList and allow access only by key.

In addition to IDictionary method Hashtable has additional two methods

 

Name	Description
ContainsKey	collection contains a specific key
ContainsValue	collection contains a specific value

When we store value in the Hashtable it creates unique value that identifies this value since it’s derive from Object class it has this method GetHash and uses GetHashCode to get this unique value from the object. However, we can reference value with the string and in this case it will return only one value even when we replace value with another value for the same key. However, by creating and saving Object we used Object GetHashValue which is unique so we get two results.
Hashtable myHashOne = new Hashtable();
myHashOne["First"] = "1st";
myHashOne["First"] = "the first";
Console.WriteLine(myHashOne.Count); // 1

Vs.

Hashtable myHashOne = new Hashtable();
Fish key1 = new Fish("Herring");
Fish key2 = new Fish("Herring");
myHashOne[key1] = "Hello";
myHashOne[key2] = "Hello";
Console.WriteLine(myHashOne.Count); // 2

To compare two objects we need to compare names.