Reference type

Question

Data type string is a reference type. Therefore it will not create any copy in program, changes alter the result. In the following program o/p expected was:

X
Copy X
Copy A

But o/p was:

X
Copy X
Copy X

using System;

class Program
{
static void Main()
{
string s = "X";

string copy = s;
Console.WriteLine(s);
Console.WriteLine("Copy " + copy);

s = "A"; //Now "s" has been changed

Console.WriteLine("Copy " + copy);

Console.ReadKey();
}
}
/*
X
Copy X
Copy X
*/

Maha · Answer

Thank you for your explanation.

Vulpes · Answer

To illustrate the basic difference between reference types and value types you need to change something so you can see whether it's the same instance or a copy. Unfortunately all the structs I can think of in the .NET framework are immutable - they can't be changed - and so too is the String class. So in my example I thought I'd create a simple 'mutable' class and struct to illustrate the difference.

Maha · Answer

So class and struct should be there and object should be instantiated in order to demonstrate difference in behavior between value type and reference type. Only along with a client program (program with Main() method) it is not possible to demonstrate difference.

Vulpes · Answer

OK try this example: using System; class C { public int I; public C(int i) { I = i; } } struct S { public int I; public S(int i) { I = i; } } class Program { static void Main() { C c = new C(3); C d = c; d.I = 4; Console.WriteLine(c.I = {0} c.I); // 4 S s = new S(3); S t = s; t.I = 4; Console.WriteLine(s.I = {0} s.I); // still 3 Console.ReadKey(); } } The difference between a reference type and a value type is that: 1. A variable of a reference type contains a reference (or pointer) to where an instance of that type is stored on the heap. 2. A variable of a value type contains an actual instance of the value type not a pointer to it. So the variables 'c' and 'd' both refer to the same instance of C. When C's field I is changed to 4 via 'd' it also changes c.I because it's the same instance. In contrast 't' contains a copy of the S instance contained in 's'. When S's field I is changed to 4 using 't' it doesn't change s.I because it's a different instance.

Imtiyaz · Answer

crap example!!

Maha · Answer

Please support with a single program to demonstrate difference behavior between value type and reference type.

Vulpes · Answer

As a result of this line: string copy = s; the variable 'copy' now refers to the same string instance as the variable 's' namely X. As a result of this line: s = A; the variable 's' now refers to the string instance A. This doesn't affect what 'copy' refers to - namely X. So this line: Console.WriteLine(Copy  + copy); prints out: Copy X