How to change an array object without a reference to the array?

Question

Let's say that I have an object of type A at index 5 of array items and I want to change it to be a new object of type B.

If I had a reference to the array, I could just do items[5] = new B(); to change the item.

However, let's suppose that I only had the object at items[5] and not the reference to the array, and it's stored in a variable item. If I just do item = new B();, the change will not be reflected in the array, because only that variable will be changed.

Is it possible to change that item in such a way that the change will be reflected in the array, such that items[5] will contain that new item of type B?

Preferably something simple without the use of delegates or lambdas or the like because it is somewhat performance-critical.

Jenish Rabadiya · Accepted Answer · 2015-03-23 03:34:23Z

1

I believe in this case you should use ref.

Demonstrating example:

public class Program
{
    public static void Main()
    {
        var list = new  List<Sample>();
        list.Add(new SampleDerived(){ Age=2, DAge = 5 });
        list.Add(new SampleDerived(){ Age=3, DAge = 5 });
        list.Add(new SampleDerived(){ Age=4, DAge = 5 });
        list.Add(new SampleDerived(){ Age=5, DAge = 5 });
        list.Add(new SampleDerived(){ Age=6, DAge = 5 });
        var list2 = list.ToArray();
        Process(ref list2[2]);
        Console.WriteLine(list2[2].Age); // will print 10 not 4
    }

    public static void Process(ref Sample s)
    {
        s = new Sample(){Age=10};
    }
}
public class Sample
{
    public int Age {get; set;}
}

public class SampleDerived : Sample
{
    public int DAge {get; set;}
}

Dot net fiddler is here.

answered Mar 23, 2015 at 3:34

Jenish Rabadiya

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8 Comments

Nathanaël Radomír Over a year ago

Process uses list2[2] as an argument. One of the requirements is that I only have access to the object and not the array or the item's index in the array.

Veselin Vasilev Over a year ago

How exactly did you get that object? Is it a reference to the object in the array or a copy of the object in the array?

Jenish Rabadiya Over a year ago

@VeselinVasilev is is called pass by reference look at MSDN link for more detail.

Nathanaël Radomír Over a year ago

It is a reference to the object in the array. That is, it's a different variable but points to the object in the array.

Jenish Rabadiya Over a year ago

@NathanaëlRadomír I think it is not possible then. you will have to maintain some reference to the object before method call. You may go with pointer (unsafe code) but I would not recommend it.

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