CODEWITHSUNDEEP
c#stringobjectreference
Petr
Petr

Reputation: 14505

Is equal sign passing a reference or object copy?

When I have this code:

class A 
{
    public int X = 0;
 ...
}

public void Function()
{
    // here I create a new instance of class
    A a = new A();
    a.X = 10;
    // here I create a pointer to null
    A b = null;
    // here I assign a to b
    b = a;
    b.X = 20;
}

did I pass the reference to instance of class A now? or I cloned the instance of A to new instance and created a reference to it in b?

is changing X in b also changing X in a? Why? If not, what is a proper way to create a copy of a and insert that to b?

Why the same with strings would always create a copy? Is equal operator overridden in strings?

string a = "hello";
string b = a;
b = "world";
// "hello world"
Console.WriteLine( a + " " + b );

Upvotes: 2

Views: 3827

Answers (4)

Soner Gönül
Soner Gönül

Reputation: 98848

C# uses references not pointers. Classes are reference types.

On your example, b has the same reference with a. They referencing the same location on memory.

changing X in b also changing X in a? Why?

Yes, because they reference to the same objects and changing one reference will affect the other one.

string a = "hello";
string b = a;
b = "world";
// "hello world"
Console.WriteLine( a + " " + b );

Strings are reference types also. But they are also immutable type. Which means you can't change them. Even if you think you change them, you actually create new strings object.

  1. line you create an object contains "hello" with a reference called a.
  2. line you create a new reference called b referencing to the same object. ("hello")
  3. line you assign your b reference new object called "world". Your b referance is not referencing "hello" object anymore.

Upvotes: 6

Theodoros Chatzigiannakis
Theodoros Chatzigiannakis

Reputation: 29233

When you assign, you pass a copy of the return value of the assigned expression.

  • For value types, this is the value you usually get to see when you use them (like the numerical value of an integer).
  • For reference types, the actual value is something like an address pointing to the referenced object (but, what it really is, is an implementation detail). So, even though you pass a copy of that address, that copy points to the same object.

Upvotes: 1

Habib
Habib

Reputation: 223392

did I pass the pointer to instance of class A now? or I cloned the instance of A to new instance and created a pointer to it in b?

b is holding the same reference as a, both of them pointing to the same location.

changing X in b also changing X in a? Why?

Because both of them are pointing to the same reference.

what is a proper way to create a copy of a and insert that to b?

Implement IClonable interface

Supports cloning, which creates a new instance of a class with the same value as an existing instance

EDIT

Since you edited the question with string, although strings are reference types but they are immutable as well

string (C# Reference)

Strings are immutable--the contents of a string object cannot be changed after the object is created, although the syntax makes it appear as if you can do this.

Upvotes: 4

Rajesh Subramanian
Rajesh Subramanian

Reputation: 6490

Object b is pointing to the object a, you have to do the deep clone to make a copy using IClonable Interface.

Upvotes: 1

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