Framework Fundamentals(Chapter 6 of C# 4.0 in a nutshell)

　　Here are the other rules for overriding object.GetHashCode:

　　• It must return the same value on two objects for which Equals returns true(hence, GetHashCode and Equals are overridden together).

　　• It must not throw exceptions.

　　• It must return the same value if called repeatedly on the same object (unless the　object has changed).

 1 public struct Area : IEquatable <Area>
 2 {
 3 public readonly int Measure1;
 4 public readonly int Measure2;
 5 public Area (int m1, int m2)
 6 {
 7 Measure1 = Math.Min (m1, m2);
 8 Measure2 = Math.Max (m1, m2);
 9 }
10 public override bool Equals (object other)
11 {
12 if (!(other is Area)) return false;
13 return Equals ((Area) other); // Calls method below
14 }
15 public bool Equals (Area other) // Implements IEquatable<Area>
16 {
17 return Measure1 == other.Measure1 && Measure2 == other.Measure2;
18 }
19 public override int GetHashCode()
20 {
21 return Measure2 * 31 + Measure1; // 31 = some prime number
22 }
23 public static bool operator == (Area a1, Area a2)
24 {
25 return a1.Equals (a2);
26 }
27 public static bool operator != (Area a1, Area a2)
28 {
29 return !a1.Equals (a2);
30 }
31 }