Generic List reference issue

Sherwin Hamidi

This seems backwards to me, according to the logic that I've learned to code with. I have a large amount of data that has to be used for a performance intensive windows form application. The Data is stored - for a multitude of reasons -in an object that is basically a 5-layered generic list

The exact instantiation is

List> xList = new List>();

Now, if I want to create a shallow copy of this Generic List, I would have to use the GetRange method...I've tried this....I've done a lot of different things to achieve this shallow copy, but what ends up happening is any edits I make to the shallow copy are reflected in the original List as well.

here's an example of what I'm talking about:

       int[] x = new int[5] { 3, 5, 7, 9, 11 };

            List tList = new List();
            tList.Add(x);
            List yList = new List(tList.GetRange(0, tList.Count));

            yList[0][0] = 12;

In this case, xList[0][0] Also gets changed to the value of 12. Can anyone help me with this short of looping through the entire data list????

6 Replies

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Alan18y ago
I didn't follow your first example as yList is a list of int arrays and you can't therefore set the first element to 12.

However, when I did this, the results were the same as I'd have expected because the same int[] object is being referenced:

       List> xList = new List>();
       List iList = new List();
       int[] x = new int[5] { 3, 5, 7, 9, 11 };
       iList.Add(x);
       xList.Add(iList);
       List yList = new List(xList[0]);
       yList[0][0] = 12;
       Console.WriteLine(yList[0][0]); // 12
       Console.WriteLine(xList[0][0][0]); // 12
Sherwin Hamidi18y ago
I was playing around and noticed that this issue does not exist when I do:

List> xList = new List();
//Populate xList with data here
List yList = new List(xList[0]);

yList[0] = 12;

Console.WriteLine(yList[0].ToString());
Console.WriteLine(xList[0][0].ToString());

results in different values.

The issue does exist, however, when I try to do:

List>> xList = new List>>();
//Populate xList
List> yList = new List>(xList[0]);

yList[0][0] = 12;

Console.WriteLine(yList[0][0].ToString());
Console.WriteLine(xList[0][0][0].ToString());
Alan18y ago
Casting from object to int[] won't have any performance implications but there's no way that you can avoid a for/foreach loop except perhaps if you use unsafe code and pointers to copy the arrays.
Sherwin Hamidi18y ago
Casting is also something I was hoping to avoid.
Sherwin Hamidi18y ago
So for performance issues I was hoping not to have to use any sort of for or foreach loop.

Any way around this?
Alan18y ago
As the List contains int[], which is a reference type, creating a shallow copy of the List just copies the references, not the array elements.

You therefore need to create shallow copies of the int[]'s themselves:

       int[] x = new int[5] { 3, 5, 7, 9, 11 };

       List tList = new List();
       tList.Add(x);
       List yList = new List();
       foreach(int[] ia in tList)
       {
          yList.Add((int[])ia.Clone());
       }
       yList[0][0] = 12;
       Console.WriteLine(yList[0][0]); // 12
       Console.WriteLine(tList[0][0]); // 3