Generic List reference issue

Question

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????

Alan · Answer

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 Hamidi · Answer

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());

Alan · Answer

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 Hamidi · Answer

Casting is also something I was hoping to avoid.

Sherwin Hamidi · Answer

So for performance issues I was hoping not to have to use any sort of for or foreach loop. Any way around this?

Alan · Answer

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