Well,for any object (obj1 == obj2) is true if

ObjectType obj1 = ~SomeValue~;

ObjectType obj2 = obj1;

OR

ObjectType obj1 = Obj#;

ObjectType obj2 = Obj#; WHERE Obj# is of type ObjectType

Coz, all are same references.

And (obj1==obj2) is false otherwise.

Because each object has different hashcode.

i.e. each object has different hashcode, if two object are same that means they have same hashcode

refer : http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx

and  http://msdn.microsoft.com/en-us/library/system.object.gethashcode%28VS.71%29.aspx

Well, when you do obj1.equal(obj2) you are using it for value comparision and not for

reference identity.

But now look at following issue:


    string s1 = "Elias";
    string s2 = "Elias";


    Here “==” and Equals() is as:
    In above case the results will be,
    s1 == s2 >> True
    s1.Equals(s2) >> True

But here, A String object is called immutable (read-only) because its value cannot be modified once it has been created. Methods that appear to modify a String object actually return a new String object that contains the modification.

There is something called "String literal pool".

When you make a string object say Str1 = "123"

it goes in this pool.

Now you prepare second object as Str2 = "123"

new object might not get prepared as it already exist in pool

Instead it points to same reference.

So,

    str1.Equals(str2) >> True 

Another thing StringBuilder is Mutable object i.e

if you do

StringBuilder StBlObj = new StringBuilder();

StBlObj = "123";

StBlObj = "123245";

Same object gets updated , no new object is created.

Please refer to this link : http://en.csharp-online.net/CSharp_String_Theory%E2%80%94String_intern_pool

For more info on String intern/Literal Pool.

Last one in this case :

  

Now another interesting case:
    int i = 0;
    byte b = 0;

Here “==” and Equals() is as:
    i == b >> True
    i.Equals(b) >> False
So, it means Equals method compare not only value but it compares TYPE also in Value Types.

Here i and b  are 2 different types.

when i.equals(b) is called, it cannot be determined which equals is to be called

one for i type's or b type's so, default equals methods is called

which cannot recognise values in i and b, hence returns false, but that does

not make "equals()" method suitable for reference identity.