Checking an array for duplicate before inserting a new value

Here is code for using array List

ArrayList email = new ArrayList();

  if (!email.Contains(your email string read from file))

           {

                email.Add(your email string read from file);

            }

If you are using dictionary

Dictionary<string, string> email2 = new Dictionary<string, string>();

if (!email2.ContainsKey(your email string read from file))

            {

                email2.Add(your email string read from file, your email string read from file);

            }

Dictionary is a collection of key, value pair. Key must be unique. Retrieving the value is very fast as it is implemented as a hash table.

ArrayList EmailAddresses=new ArrayList();

Dictionary dictionary = new Dictionary();

foreach (string field in fields)

{

  if (dictionary.ContainsKey(field))

  {

    //duplicate found!

  }

  else

  {

    int value = 0; //not used but required by Dictionary

    dictionary.Add(field,value);

  }

}

But what I suggest you to use hash table where you can use the key-value pair to eliminate the duplicate e-mail address.