c# object pass by reference or pass by value

Question

The output of the following code surprised me. I think "a" should hold a reference to the newly created object. Can someone explain why the result is not 2?

class Program
{
    static void Main(string[] args)
    {
        aclass a = new aclass();
        Process(a);
        Console.WriteLine(a.number);

        Console.ReadLine();
    }

    static void Process(aclass a)
    {
        aclass temp = new aclass();
        temp.number++;
        //Console.WriteLine(temp.number);

        a = temp;
        a.number++;
        //Console.WriteLine(a.number);
    }

}

class aclass
{
    public int number = 0;
}

Edit: This is an interview question. I just realized I had misunderstood the concept for long time. The argument a is different to the original a although they reference to the same address.

Because when you do `a = temp` you cause a disconnect between the argument that was passed and what you are going to be modifying from that point onwards. If you didn't do that you 'd get `1` in `Main`. — Jon, Oct 01 '13 at 10:37
In c# args are passed by value not by reference, take a look at [ref](http://msdn.microsoft.com/en-us/library/14akc2c7.aspx) keyword. — Alessandro D'Andria, Oct 01 '13 at 10:40
You're incrementing `number` on `temp` twice, but you're not doing any work on the `a` defined in `Main`. — Sam, Oct 01 '13 at 10:40
@AlessandroD'Andria I'm not sure if your comment is correct here. — mortdale, Oct 01 '13 at 11:18

Lloyd · Accepted Answer · 2013-10-01T10:57:00.673

You're not changing the actual original reference you're just changing the reference held in the parameter which subtly isn't the same, the changes aren't persisted back to the caller. You can change this behaviour by using out or ref.

In this case specifically you'd want to use ref as you're also passing a reference in.

Try:

class Program
{
    static void Main(string[] args)
    {
        aclass a = new aclass();
        Process(ref a);
        Console.WriteLine(a.number);

        Console.ReadLine();
    }

    static void Process(ref aclass a)
    {
        aclass temp = new aclass();
        temp.number++;
        //Console.WriteLine(temp.number);

        a = temp;
        a.number++;
        //Console.WriteLine(a.number);
    }

}

Remember you're assigning a whole new reference with a = temp. If you just wanted to update the existing class you originally passed in then you could do:

a.number = temp.number;
a.number++;

This would negate the need for ref.

You can read more on MSDN:

Passing Reference Type Parameters

ref Keyword

out Keyword

This clarified my misunderstanding about the argument and the original object. Argument a is a different object than the original a, although they reference to the same value initially. — mortdale, Oct 01 '13 at 11:16

Yaqub Ahmad · Answer 2 · 2013-10-01T11:07:37.770

0

This line aclass a = new aclass(); creates a variable (Space where we can store data) in memory. Consider that its address in memory is *(0x12DF) and the value which is stored at that location is object a

This line Process(a) passes the VALUE of object a NOT THE ADDRESS to function Process, so anything happens in Process() has nothing to do with the contents of location *(0x12DF), so the contents of location *(0x12DF) will remain the same as it was before calling Process().

CONTENTS OF *(0x12DF) = a

I hope its helpful instead of creating more confusion!!

edited Oct 01 '13 at 11:07

answered Oct 01 '13 at 11:02

Yaqub Ahmad

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I think _Process(a)_ passes the address of the content. The argument in _Process(aclass a)_ is a new object pointing to the same content 0x12DF. The content in 0x12DF has never been duplicated. It would be more efficient to pass the address than passing the whole Value. If _aclass_ is a value type, the argument _a_ will be a new object with value duplicated at a different address. – mortdale Oct 02 '13 at 04:29

score 0 · Answer 3 · answered Oct 01 '13 at 11:15

It is basically the difference between

Passing Reference Types by Value // Process(a);
Passing Reference Types by References // Process(ref a);

In the example, Process(a) - 'a' which is a reference type, is passed to the method without the ref parameter. In such a case, a copy of the reference, which points to a, is passed to the method.

Allocating a new portion of memory by using the new operator inside the Process method makes the variable 'a' reference a new object of aclass. Thus, any changes after that will not affect the original object 'a'.

Refer MSDN : http://msdn.microsoft.com/en-us/library/s6938f28.aspx

c# object pass by reference or pass by value

3 Answers3