Passing Objects by Reference or Value in C#

Passing Objects By Reference or Value in C#

Objects aren't passed at all. By default, the argument is evaluated and its value is passed, by value, as the initial value of the parameter of the method you're calling. Now the important point is that the value is a reference for reference types - a way of getting to an object (or null). Changes to that object will be visible from the caller. However, changing the value of the parameter to refer to a different object will not be visible when you're using pass by value, which is the default for all types.

If you want to use pass-by-reference, you must use out or ref, whether the parameter type is a value type or a reference type. In that case, effectively the variable itself is passed by reference, so the parameter uses the same storage location as the argument - and changes to the parameter itself are seen by the caller.

So:

public void Foo(Image image)
{
    // This change won't be seen by the caller: it's changing the value
    // of the parameter.
    image = Image.FromStream(...);
}

public void Foo(ref Image image)
{
    // This change *will* be seen by the caller: it's changing the value
    // of the parameter, but we're using pass by reference
    image = Image.FromStream(...);
}

public void Foo(Image image)
{
    // This change *will* be seen by the caller: it's changing the data
    // within the object that the parameter value refers to.
    image.RotateFlip(...);
}

I have an article which goes into a lot more detail in this. Basically, "pass by reference" doesn't mean what you think it means.

Why are objects automatically passed by reference?

Why are objects automatically passed by reference?

They're not.

Is there any particular benefit from forcing the cloning process for them instead of treating objects more like int, double, boolean, etc. in these cases?

There's no "cloning process" for reference types, only for value types.

I think you're confusing different concepts:

value types vs. reference types
For value types (such as primitive numeric types, enums, and structures like DateTime), the value of the variable is the object itself. Assigning the variable to another (or passing it as a parameter by value) creates a copy of the object.
For reference types (such as object, string, classes (not structs) etc), the value of the variable is a reference to the object. Assigning the variable to another (or passing it as a parameter by value) creates a copy of the reference, so it still refers to the same object instance.
passing parameters by value vs. by reference
Passing parameters by value means that you pass a copy of the value. Depending on whether it's a value type or reference types, that means a copy of the object itself, or a copy of the reference. If the callee modifies members of a value type passed as a parameter, the caller won't see the changes, since the callee is working on a copy. On the other hand, if the callee modifies members of a reference type passed as a parameter, the caller will see the changes, because the callee and caller both have a reference to the same object instance.
Passing parameters by reference means that you pass a reference to a variable (which may be a variable of value type or reference type). The value is not copied: it is shared between the caller and the callee. So any change made by the callee (including assignment of a new value to the parameter) will be seen by the caller.
Unless specified otherwise (with the ref or out keywords), all parameters are passed by value, including reference types. It's just that for reference types, the value that is passed is a reference, but it's still passed by value.

I suggest you read Jon Skeet's article Parameter passing in C# for a better explanation.

C# pass by value vs. pass by reference

Re: OP's Assertion

It is universally acknowledged (in C# at least) that when you pass by reference, the method contains a reference to the object being manipulated, whereas when you pass by value, the method copies the value being manipulated ...

TL;DR

There's more to it than that. Unless you pass variables with the ref or out keywords, C# passes variables to methods by value, irrespective of whether the variable is a value type or a reference type.

If passed by reference, then the called function may change the variable's address at the call-site (i.e. change the original calling function's variable's assignment).
If a variable is passed by value:
- if the called function re-assigns the variable, this change is local to the called function only, and will not affect the original variable in the calling function
- however, if changes are made to the variable's fields or properties by the called function, it will depend on whether the variable is a value type or a reference type in order to determine whether the calling function will observe the changes made to this variable.

Since this is all rather complicated, I would recommend avoiding passing by reference if possible (instead, if you need to return multiple values from a function, use a composite class, struct, or Tuples as a return type instead of using the ref or out keywords on parameters)

Also, when passing reference types around, a lot of bugs can be avoided by not changing (mutating) fields and properties of an object passed into a method (for example, use C#'s immutable properties to prevent changes to properties, and strive to assign properties only once, during construction).

In Detail

The problem is that there are two distinct concepts:

Value Types (e.g. int) vs Reference Types (e.g. string, or custom classes)
Passing by Value (default behaviour) vs Passing by Reference(ref, out)

Unless you explicitly pass (any) variable by reference, by using the out or ref keywords, parameters are passed by value in C#, irrespective of whether the variable is a value type or reference type.

When passing value types (such as int, float or structs like DateTime) by value (i.e. without out or ref), the called function gets a copy of the entire value type (via the stack).

Any change to the value type, and any changes to any properties / fields of the copy will be lost when the called function is exited.

However, when passing reference types (e.g. custom classes like your MyPoint class) by value, it is the reference to the same, shared object instance which is copied and passed on the stack.

This means that:

If the passed object has mutable (settable) fields and properties, any changes to those fields or properties of the shared object are permanent (i.e. any changes to x or y are seen by anyone observing the object)
However, during method calls, the reference itself is still copied (passed by value), so if the parameter variable is reassigned, this change is made only to the local copy of the reference, so the change will not be seen by the caller. This is why your code doesn't work as expected

What happens here:

void Replace<T>(T a, T b) // Both a and b are passed by value
{
    a = b;  // reassignment is localized to method `Replace`
}

for reference types T, means that the local variable (stack) reference to the object a is reassigned to the local stack reference b. This reassign is local to this function only - as soon as scope leaves this function, the re-assignment is lost.

If you really want to replace the caller's references, you'll need to change the signature like so:

void Replace<T>(ref T a, T b) // a is passed by reference
{
    a = b;   // a is reassigned, and is also visible to the calling function
}

This changes the call to call by reference - in effect we are passing the address of the caller's variable to the function, which then allows the called method to alter the calling method's variable.

However, nowadays:

Passing by reference is generally regarded as a bad idea - instead, we should either pass return data in the return value, and if there is more than one variable to be returned, then use a Tuple or a custom class or struct which contains all such return variables.
Changing ('mutating') a shared value (and even reference) variable in a called method is frowned upon, especially by the Functional Programming community, as this can lead to tricky bugs, especially when using multiple threads. Instead, give preference to immutable variables, or if mutation is required, then consider changing a (potentially deep) copy of the variable. You might find topics around 'pure functions' and 'const correctness' interesting further reading.

Edit

These two diagrams may help with the explanation.

Pass by value (reference types):

In your first instance (Replace<T>(T a,T b)), a and b are passed by value. For reference types, this means the references are copied onto the stack and passed to the called function.

Sample Image

Your initial code (I've called this main) allocates two MyPoint objects on the managed heap (I've called these point1 and point2), and then assigns two local variable references a and b, to reference the points, respectively (the light blue arrows):

MyPoint a = new MyPoint { x = 1, y = 2 }; // point1
MyPoint b = new MyPoint { x = 3, y = 4 }; // point2

The call to Replace<Point>(a, b) then pushes a copy of the two references onto the stack (the red arrows). Method Replace sees these as the two parameters also named a and b, which still point to point1 and point2, respectively (the orange arrows).
The assignment, a = b; then changes the Replace methods' a local variable such that a now points to the same object as referenced by b (i.e. point2). However, note that this change is only to Replace's local (stack) variables, and this change will only affect subsequent code in Replace (the dark blue line). It does NOT affect the calling function's variable references in any way, NOR does this change the point1 and point2 objects on the heap at all.

Pass by reference:

If however we we change the call to Replace<T>(ref T a, T b) and then change main to pass a by reference, i.e. Replace(ref a, b):

Sample Image

As before, two point objects allocated on the heap.
Now, when Replace(ref a, b) is called, while mains reference b (pointing to point2) is still copied during the call, a is now passed by reference, meaning that the "address" to main's a variable is passed to Replace.
Now when the assignment a = b is made ...
It is the the calling function, main's a variable reference which is now updated to reference point2. The change made by the re-assignment to a is now seen by both main and Replace. There are now no references to point1

Changes to (heap allocated) object instances are seen by all code referencing the object

In both scenarios above, no changes were actually made to the heap objects, point1 and point2, it was only local variable references which were passed and re-assigned.

However, if any changes were actually made to the heap objects point1 and point2, then all variable references to these objects would see these changes.

So, for example:

void main()
{
   MyPoint a = new MyPoint { x = 1, y = 2 }; // point1
   MyPoint b = new MyPoint { x = 3, y = 4 }; // point2

   // Passed by value, but the properties x and y are being changed
   DoSomething(a, b);

   // a and b have been changed!
   Assert.AreEqual(53, a.x);
   Assert.AreEqual(21, b.y);
}

public void DoSomething(MyPoint a, MyPoint b)
{
   a.x = 53;
   b.y = 21;
}

Now, when execution returns to main, all references to point1 and point2, including main's variables a and b, which will now 'see' the changes when they next read the values for x and y of the points. You will also note that the variables a and b were still passed by value to DoSomething.

Changes to value types affect the local copy only

Value types (primitives like System.Int32, System.Double) and structs (like System.DateTime, or your own structs) are allocated on the stack, not the heap, and are copied verbatim onto the stack when passed into a call. This leads to a major difference in behaviour, since changes made by the called function to a value type field or property will only be observed locally by the called function, because it only will be mutating the local copy of the value type.

e.g. Consider the following code with an instance of the mutable struct, System.Drawing.Rectangle

public void SomeFunc(System.Drawing.Rectangle aRectangle)
{
    // Only the local SomeFunc copy of aRectangle is changed:
    aRectangle.X = 99;
    // Passes - the changes last for the scope of the copied variable
    Assert.AreEqual(99, aRectangle.X);
}  // The copy aRectangle will be lost when the stack is popped.

// Which when called:
var myRectangle = new System.Drawing.Rectangle(10, 10, 20, 20);
// A copy of `myRectangle` is passed on the stack
SomeFunc(myRectangle);
// Test passes - the caller's struct has NOT been modified
Assert.AreEqual(10, myRectangle.X);

The above can be quite confusing and highlights why it is good practice to create your own custom structs as immutable.

The ref keyword works similarly to allow value type variables to be passed by reference, viz that the 'address' of the caller's value type variable is passed onto the stack, and assignment of the caller's assigned variable is now directly possible.

Passing objects by reference vs value

What I don't understand is what happens when I invoke a method, what actually happens. Does new() get invoked? Does it just automagically copy the data? Or does it actually just point to the original object? And how does using ref and out affect this?

The short answer:

The empty constructor will not be called automatically, and it actually just points to the original object.

using ref and out does not affect this.

The long answer:

I think it would be easier to understand how C# handles passing arguments to a function.

Actually everything is being passed by value

Really?! Everything by value?

Yes! Everything!

Of course there must be some kind of a difference between passing classes and simple typed objects, such as an Integer, otherwise, it would be a huge step back performance wise.

Well the thing is, that behind the scenes when you pass a class instance of an object to a function, what is really being passed to the function is the pointer to the class. the pointer, of course, can be passed by value without causing performance issues.

Actually, everything is being passed by value; it's just that when
you're "passing an object", you're actually passing a reference to that
object (and you're passing that reference by value).

once we are in the function, given the argument pointer, we can relate to the object passed by reference.

You don't actually need to do anything for this, you can relate directly to the instance passed as the argument (as said before, this whole process is being done behind the scenes).

After understanding this, you probably understand that the empty constructor will not be called automatically, and it actually just points to the original object.

EDITED:

As to the out and ref, they allow functions to change the value of an arguments and have that change persist outside of the scope of the function.

In a nutshell, using the ref keyword for value types will act as follows:

int i = 42;
foo(ref i);

will translate in c++ to:

int i = 42;    
int* ptrI = &i;
foo(ptrI)

while omitting the ref will simply translate to:

int i = 42;
foo(i)

using those keywords for reference type objects, will allow you to reallocate memory to the passed argument, and make the reallocation persist outside of the scope of the function (for more details please refer to the MSDN page)

Side note:

The difference between ref and out is that out makes sure that the called function must assign a value to the out argument, while ref does not have this restriction, and then you should handle it by assigning some default value yourself, thus, ref Implies the the initial value of the argument is important to the function and might affect it's behaviour.

Pass by Value in C#

By default object types are passed by value in C#. But when you pass a object reference to a method, modifications in the object are persisted. If you want your object to be inmutable, you need to clone it.

In oder to do it, implement the ICloneable interface in your class. Here is a mock example of how to use ICloneable:

public class MyClass : ICloneable
{
  private int myValue;

  public MyClass(int val)
  {
     myValue = val;
  }

  public void object Clone()
  {
     return new MyClass(myValue);
  }
}

C# is passing objects by value?

In C#, instances of classes are passed as references, whereas instances of structs are passed by copy (by default).

The answer was just where it was supposed to be: http://msdn.microsoft.com/en-us/library/vstudio/ms173109.aspx

A class is a reference type. When an object of the class is created, the variable to which the object is assigned holds only a reference to that memory. When the object reference is assigned to a new variable, the new variable refers to the original object. Changes made through one variable are reflected in the other variable because they both refer to the same data.

A struct is a value type. When a struct is created, the variable to which the struct is assigned holds the struct's actual data. When the struct is assigned to a new variable, it is copied. The new variable and the original variable therefore contain two separate copies of the same data. Changes made to one copy do not affect the other copy.

Object passed as parameter to another class, by value or reference?

Objects will be passed by reference irrespective of within methods of same class or another class. Here is a modified version of same sample code to help you understand. The value will be changed to 'xyz.'

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public class Employee
    {
        public string Name { get; set; }
    }

    public class MyClass
    {
        public Employee EmpObj;

        public void SetObject(Employee obj)
        {
            EmpObj = obj;
        }
    }

    public class Program
    {
       static void Main(string[] args)
        {
            Employee someTestObj = new Employee();
            someTestObj.Name = "ABC";

            MyClass cls = new MyClass();
            cls.SetObject(someTestObj);

            Console.WriteLine("Changing Emp Name To xyz");
            someTestObj.Name = "xyz";

            Console.WriteLine("Accessing Assigned Emp Name");
            Console.WriteLine(cls.EmpObj.Name); 

           Console.ReadLine();
       }       
    }
 }

Passing Objects by Reference or Value in C#