Question about string and object equality in C#

Question

As we know, in C#,

1. string is a reference type
2. for string == operator is overloaded so that comparison is performed by value

There is the following variables defined:

string s1 = "foo";
object s2 = "foo";
string s3 = new String(new char[] {'f','o','o'});

bool b1 = (s1==s2); //true
bool b2 = (s2==s3); //false
bool b3 = (s1==s3); //true
bool b4 = (s1.equals(s3)); //true

gettype() for all variables s1, s2, and s3, returned the same val String.

Why is s1==s3 true and s2 == s3 is false? s1 and s2 have the same type - so the == operator should behave the same. As I understand, s1 and s2 also interned. (object.ReferenceEquals(s1, s2) returned True.)

What do I miss?

Answer 1

Why object == string does not match string.operator == (string, string) ? Because one of the parameter is not a string... So the next matching one is object.operator == which does not behave as a string comaprison.

Why interned version of the string (s1 and s2) point to the same object? Because it is what interning do - makes sure that same value present only once as an object. In particular it is done for all string constants in same module (at compiler discretion) so both s1 and s2 point to the same and only object that represents "foo" in that assembly.

Why non interned string (s3) is not the same object as (s1/s2)? Because it just not the same - creating strings at run-time does not intern strings.

Answer 2

The == operator (and any operator overloads in C#) are static methods and therefore the invoked operator is choosen by the static types of the arguments. In (s2==s3) the static type of the left hand operand is object and therefore it cannot match the signature of the operator ==(string, string) method.

Try s2.Equals(s3) instead. Equals is a virtual function and therefore operates on the dynamic type of s2, which is string . (That's also the reason why s2.GetType() returns string ).