asked on

Passing object by value

Hello everybody

I'm am still fairly new to C# (and OOP in general) and I've started to stumble when it comes to passing objects by value.
I've read a lot of documentation explaining what goes on when you pass an object by value, and I understand why I'm seeing the results I'm getting. But every single article I've found just falls short of offering advice on how to overcome the problems seen by people at my level of experience. So here I am....!

How can/should I get the following sample code to output "Dink Dank Do!", not "Bish Bash Bosh!" ?

Thanks in advance.

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{

myTestClass mainObject = new myTestClass();
myProcessClass processObject = new myProcessClass(mainObject);

Console.WriteLine(mainObject.strProperty_1);
Console.WriteLine(mainObject.strProperty_2);
Console.WriteLine(mainObject.strProperty_3);

Console.Read();
}
}

class myTestClass
{
public String strProperty_1;
public String strProperty_2;
public String strProperty_3;

public myTestClass()
{
strProperty_1 = "Dink ";
strProperty_2 = "Dank ";
strProperty_3 = "Do! ";
}
}

class myProcessClass
{
public myProcessClass(myTestClass objectPassed)
{
objectPassed.strProperty_1 = "Bish ";
objectPassed.strProperty_2 = "Bash ";
objectPassed.strProperty_3 = "Bosh! ";
}
}
}

andrewjb

Objects are passed by reference, not value.

You need a struct, not a class. Just change "class myTestClass" to "struct myTestClass" ...

Rusty_Adams

ASKER

Thanks Andrew

But can I perform all the same things with a struct that I can with a class, such as have methods, hold other objects or arraylists as properties, have constructors?

andrewjb

Yes etc., except.....

You can't inherit from a struct, or inherit a struct from an object. Though you can implement interfaces in a struct.
You can have constructors, but there's always a default (no parameter) constructor that just initialises the elements to zero/null/false. You can't provide your own implementation of this constructor.

Rusty_Adams

ASKER

Does anybody know of an alternative solution?
I have an application with a large amount of classes already, and I wouldn't like to have to go through and alter them all. And I'm not convinced that I wouldn't have to rewrite much of the app if I were to change the required classes to structs.

Is there no way of making a copy of an object and passing that, so that the original object remains intact?

AlexFM

Reference types passed to functions can be always changed by this functions, this is how .NET is developed, there is nothing to do with this. I don't recommend you to replace classes with structures - structures are not used for this purpose. Structure is usually small object which contains few members of primitive types (like int), which can be copied and allocated fast - like Point. They are restricted and used only in special cases.
What is exactly your purpose? To restrict access to class members inside of function? There are two ways to make copy: using Object.MemberwiseClone Method or implementing ICloneable interface. Object.MemberwiseClone creates shallow copy, this means, if class contains references, they are not cloned.
Other way to restrict access to class instance is passing interface to function. For example, class implements some interface which allows only to read some values. Pass interface to the function instead of instance reference. I don't recommend you to clone objects - this is not efficient.

Rusty_Adams

ASKER

Thanks Alex

This problem came up in the following situation:

I have a form called frmUser_Group. In this form a use an instance of another class, clUser_Group. clUser_Group has properties for holding some info about the group. It also holds an ArrayList containing instances of clUser (a group can have many users, you get the picture).
On this form I have a button labelled "Add/Remove" users. This button opens up a second form which I'm opening up as a dialog. When I open the dialog I pass through the arrayList property of the clUser_Group object in the constructor.

frmUser_Group_AddRemoveUsers frmAddRemove = new frmUser_Group_AddRemoveUsers(_oUser_Group.arrUser_Group_Xrefs);
frmAddRemove.StartPosition = FormStartPosition.CenterParent;
frmAddRemove.oParentForm_instanceRef = this;
frmAddRemove.ShowDialog(this);

In the constructor of the second form I then use the array passed through to fill another array, which I though meant I was working on a copy.

public frmUser_Group_AddRemoveUsers(ArrayList arrUser_Group_Xrefs_Passed)
{
InitializeComponent();

this.arrUser_Group_Xrefs = arrUser_Group_Xrefs_Passed;

PopulateForm();
}

When the user adds and removes users from the group in the dialog, it's also adding and removing from the parent form. This caused a problem if the user changes his/her mind and simply closes the dialog. The parent form then still reflects the changes they made but chose not to keep.

I wanted the array in the dialog to be seperate, and pass it back somehow when the user presses ok, or simply ignore it if the user presses cancel or closes the dialog.

I've never heard of interfaces.
Is this a suitable situaltion for interfaces?

SOLUTION

Gautham Janardhan

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ASKER CERTIFIED SOLUTION

AlexFM

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