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C# reference types are passed by value!

, 19 Aug 2010
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C# reference types are passed by value!

How many times have I read: "In C# reference types are passed by reference, primitive types by value"? The essential point is that reference types have nothing to do with passing by reference!

But let us start with some basics. There are two types of objects, the value types (structs) where the variable is the object and the reference types which only points to the object. When you pass in a variable to a method, its value gets copied to the method by default. For value types, that means the object itself gets copied. For reference types, that means only the thing that points at the object gets copied!

It is a little bit confusing, because only the reference to the object is copied, not the entire structure. It is a way to save performance, otherwise larger the reference type would be, more performance it would cost. This strategy is also known as call by sharing (which makes it a less confusing than calling it also call by value with reference types). So in a call by value scenario, if the data of a reference type is changed inside the method, the caller variable is also affected. If you change the value of a value type, which is passed to a method, it will not affect the caller variable.

public void Sample()
{
    Point point = new Point(20, 30);
    Person person = new Person() { Name = "Meier" };

    CallByValue(point, person); 

    Console.WriteLine("Point X: {0}", point.X);
    Console.WriteLine("Name: {0}", person.Name); 
}

public void CallByValue(Point point, Person person)
{
    point.X = 10;
    person.Name = "Müller"; 
}

The ref keyword indicates in C# a call by reference. When passing a value type by reference to a method, the changes to the value which are done in the method scope will also affect the variable in the code of the caller. But what happens if you pass reference type to a method by ref?

public void Sample()
{
    Point point = new Point(20, 30);
    Person person = new Person() { Name = "Meier" };

    CallByValue(ref point, ref person); 

    Console.WriteLine("Point X: {0}", point.X);
    Console.WriteLine("Name: {0}", person.Name); 
}

public void CallByValue(ref Point point, ref Person person)
{
    point.X = 10;
    person = new Person() { Name = "Müller" };
}

It allows to change the instance which the variable points to. So you can assign a new object to the variable and also the caller variable will point to the new object. By the way, the behaviour in VB.NET is similar to the behaviour of C#. The only difference is that in VB.NET, you always have to define if you want to pass a value by value or reference.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Mattia Baldinger
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Switzerland Switzerland
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Comments and Discussions

 
GeneralMy vote of 1 Pinmembergrmrppr10-Sep-12 11:12 
GeneralMy vote of 2 Pinmembergbb2115-Feb-11 17:46 
Question[My vote of 2] maybe... handle? PinmemberDaniele Rota Nodari24-Aug-10 3:31 
GeneralPoint Isn't the Object Pinmembertf_ics24-Aug-10 3:20 
GeneralMy vote of 1 Pinmemberaganyadevi23-Aug-10 23:48 
GeneralNot sure I agree either PinmvpRichard MacCutchan23-Aug-10 10:47 
GeneralI don't think you're right PinmemberMichal Blazejczyk23-Aug-10 10:37 
GeneralExactly Pinmembersupercat920-Aug-10 6:14 
My dander is raised whenever I read that objects are passed by reference.
 
I think it might be useful to use a different term to refer to the data on the heap, since the term "object" has two meanings. If one describes the data on the heap as "Things", one can say that an object is a "Thing identifier"(*). A request to operate on an object is really a request to operate on the thing which the object identifies.
 
If I say ThisObject.BackColor = Blue, what that's saying is to find the thing whose ID is stored in ThisObject and color it blue. ThisObject still continues to hold the same ID, and it still refers to the same Thing.
 
When an "object" is passed by value, it's possible to make changes to the Thing it identifies, but not change what Thing it identifies.
 
(*) Note that a thing identifier may or may not be related to an address. Incidentally, if I had my druthers, even 64-bit .Net would have a 32-bit "Object" type which would index into an object table. I see no realistic need for over two billion objects in a single application. Two billion objects of 5Kbytes each would require ten terabytes of memory, and that's quite a ways off; an application which would use two billion objects smaller than 5K each should probably aggregate information into larger objects.
GeneralRe: Exactly PinmemberSilic0re0920-Aug-10 7:55 
GeneralConfusing title... [modified] PinmemberSilic0re0920-Aug-10 3:53 
GeneralRe: Confusing title... PinmemberNinja-the-Nerd23-Aug-10 16:11 
GeneralRe: Confusing title... Pinmemberjohannesnestler26-Aug-10 2:33 
GeneralRe: Confusing title... PinmemberNinja-the-Nerd26-Aug-10 8:03 
Generalhave 5 PinmemberPranay Rana20-Aug-10 1:23 

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