Object equality behaves different in .NET

Question

I have these statements and their\' results are near them.

string a = \"abc\";
string b = \"abc\";

Console.Writeline(a == b); //true

object x = a;
object y         


        

Answer 1

string a = "abc"; 
string b = "abc"; 

Console.Writeline(a == b); //true 

String references are the same for the same string due to String Interning

object x = a; 
object y = b; 

Console.Writeline(x == y); // true 

Because the references are the same, two objects created from the same reference are also the same.

string c = new string(new char[] {'a','b','c'}); 
string d = new string(new char[] {'a','b','c'}); 

Here you create two NEW arrays of characters, these references are different.

Console.Writeline(c == d); // true 

Strings have overloaded == to compare by value.

object k = c; 
object m = d; 

Since the previous references are different, these objects are different.

Console.Writeline(k.Equals(m)) //true 

.Equals uses the overloaded String equals method, which again compare by value

Console.Writeline(k == m); // false 

Here we check to see if the two references are the same... they are not

The key is figuring out when an equality is comparing references or values.

Objects, unless otherwise overloaded, compare references.

Structs, unless otherwise overloaded, compare values.

Answer 2

The == operator compares references (the memory addresses) while .Equals compares the actual object values. In the case of string, you get lucky and two identical strings can refer to the same address frequently. One of the joys of managed languages.

Answer 3

   string c = new string(new char[] {'a','b','c'});
   string d = new string(new char[] {'a','b','c'});

   Console.WriteLine(c == d); // true

   object k = c;
   object m = d;

   Console.WriteLine(k.Equals(m)); //true

   Console.WriteLine(k == m); // false

Generates IL code, like this:

IL_0001:  ldc.i4.3    
IL_0002:  newarr      System.Char
IL_0007:  dup         
IL_0008:  ldtoken     <PrivateImplementationDetails>{61BB33F4-0CA5-4733-B259-764124AD1A79}.$$method0x6000002-1
IL_000D:  call        System.Runtime.CompilerServices.RuntimeHelpers.InitializeArray
IL_0012:  newobj      System.String..ctor
IL_0017:  stloc.0     
IL_0018:  ldc.i4.3    
IL_0019:  newarr      System.Char
IL_001E:  dup         
IL_001F:  ldtoken     <PrivateImplementationDetails>{61BB33F4-0CA5-4733-B259-764124AD1A79}.$$method0x6000002-2
IL_0024:  call        System.Runtime.CompilerServices.RuntimeHelpers.InitializeArray
IL_0029:  newobj      System.String..ctor
IL_002E:  stloc.1     
IL_002F:  ldloc.0     
IL_0030:  ldloc.1     
IL_0031:  call        System.String.op_Equality   //STRING EQUALITY 
IL_0036:  call        System.Console.WriteLine
IL_003B:  nop         
IL_003C:  ldloc.0     
IL_003D:  stloc.2     
IL_003E:  ldloc.1     
IL_003F:  stloc.3     
IL_0040:  ldloc.2     
IL_0041:  ldloc.3     
IL_0042:  callvirt    System.Object.Equals
IL_0047:  call        System.Console.WriteLine
IL_004C:  nop         
IL_004D:  ldloc.2     
IL_004E:  ldloc.3     
IL_004F:  ceq         //CEQ INSTRUCTION: **VALUES** EQUALITY !
IL_0051:  call        System.Console.WriteLine

As you can see the last instruction call CEQ instruction that makes comparison of values equality pushed on stack. The values pushed on stack are references of both boxed string, which are not equal.