Collections

Example "Comparer"

The following example shows the implementation of an IComparer object that performs a string comparison on the basis of length. Note here that a null reference is considered to be the smallest element (in conformance with Microsoft documentation):
/book/samples/4/Collections/Comparer.cs

using System; 
using System.Collections;

public class StringLengthComparer : IComparer {
  public int Compare(object x, object y) {
    if (x == null && y == null) return 0;  // x == y
    if (x == null) return -1;  // x < y
    if (y == null) return 1;  // x > y
    if (x.ToString().Length == y.ToString().Length) return 0;  // x == y
    if (x.ToString().Length < y.ToString().Length) return -1;  // x < y
    return 1;  // x > y
  }
}
public class TestComparer {
  public static void Main() {
    string[] a = { "Birngruber", "Woess", "Moessenboeck", "Beer" };
    Array.Sort(a, new StringLengthComparer());
    foreach (string s in a) Console.WriteLine("{0}", s);
  }
}

Example "Hashtable"

In the following example persons are inserted into a hash table and the social security number is used as key.

/book/samples/4/Collections/Hashtable.cs

public class Person {
  string lastName;
  string firstName;
  
  public Person(string firstName, string lastName) {
    this.firstName = firstName; this.lastName = lastName;
  }
  public override string ToString() {
    return firstName + " " + lastName;
  }
  
}
public class HashtableExample {    
  public static void Main() {
    Hashtable tab = new Hashtable();
    tab.Add(3181030750, new Person("Mike", "Miller"));
    tab.Add(1245010770, new Person("Susanne", "Parker"));
    tab.Add(2345020588, new Person("Roland", "Howard"));
    tab.Add(1245300881, new Person("Douglas", "Adams"));
    foreach (DictionaryEntry e in tab)
      Console.WriteLine(e.Value + ": " + e.Key);
    if (tab.Contains(1245010770))
      Console.WriteLine("Person with SSN 1245010770: " + tab[1245010770]);
  }
}