For predefined value types, the equality operator (==) returns true if the values of its operands are equal, false otherwise. For reference types other than string, == returns true if its two operands refer to the same object. For the string type, == compares the values of the strings.
User-defined value types can overload the == operator (see operator). So can user-defined reference types, although by default == behaves as described above for both predefined and user-defined reference types. If == is overloaded, != must also be overloaded. Operations on integral types are generally allowed on enumeration.
class Equality { static void Main() { // Numeric equality: True Console.WriteLine((2 + 2) == 4); // Reference equality: different objects, // same boxed value: False. object s = 1; object t = 1; Console.WriteLine(s == t); // Define some strings: string a = "hello"; string b = String.Copy(a); string c = "hello"; // Compare string values of a constant and an instance: True Console.WriteLine(a == b); // Compare string references; // a is a constant but b is an instance: False. Console.WriteLine((object)a == (object)b); // Compare string references, both constants // have the same value, so string interning // points to same reference: True. Console.WriteLine((object)a == (object)c); } } /* Output: True False True False True */
Another useful place to look is: http://msdn.microsoft.com/en-us/library/ms173147.aspx