C# Language Reference
== Operator (C# Reference)

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.

Remarks

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.
Example
C# 
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
*/
See Also

Concepts

C# Programming Guide

Reference

C# Operators

Other Resources

C# Reference
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Community Content

Joren Vermijs
Guidelines for Overloading Equals() and Operator == (C# Programming Guide)

Another useful place to look is: http://msdn.microsoft.com/en-us/library/ms173147.aspx

Here is made clear with some examples how to override correctly and what the difference is between == and the Equals method.
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