C# List.Add() keeps updating all my entries instead of adding new ones

Question

I have read through all the answers to similar questions and have tried a bunch of different permutations but nothing seams to work! I've spent the last 4-5h trying to get around I keep getting the same result. Please help!

here is my problematic code: I need to copy the contents of the cruMatrix into cruMatric_tm1 on each iteration. I've left some of the things I've tried commented out

while (true)
        {
            //cruMatrix = new List<double[]>();
            //cruMatrix = cruMatrix_tm1.ToList();
            T1 = new Thread(updateCaConcentration); T1.Start(); T1.Join();
            T2 = new Thread(updateSystemState); T2.Start(); T2.Join();


            //cruMatrix_tm1.Clear();
            systemUpdate();
            plotOpenVsTime(ref time); 

            //cruMatrix_tm1 = new List<double[]>(cruMatrix);
            cruMatrix_tm1 = new List<double[]>();
            foreach(double[] arr in cruMatrix){
                cruMatrix_tm1.Add(arr);
            }

            run++;
            if (run > runUntil) break;
            time++;
        }

Answer 1

double[] is a reference type. When you add the array from one list to another you adding a reference to the same array. Any modifications made to the array will be reflected in both lists. What you want to do is create a new array and use something like Array.Copy like this:

cruMatrix_tm1 = new List<double[]>();
foreach(double[] arr in cruMatrix){
    double[] copy = new double[arr.Length];
    Array.Copy(arr,copy,arr.Length);
    cruMatrix_tm1.Add(copy);
}

Answer 2

You keep creating a new List for cruMatrix_tm1 inside the looping, that's why cruMatrix_tm1 always have the same element.

//put it outside the loop
cruMatrix_tm1 = new List<double[]>();

while (true)
        {
            T1 = new Thread(updateCaConcentration); T1.Start(); T1.Join();
            T2 = new Thread(updateSystemState); T2.Start(); T2.Join();

            systemUpdate();
            plotOpenVsTime(ref time); 

            foreach(double[] arr in cruMatrix){
                cruMatrix_tm1.Add(arr);
            }

            run++;
            if (run > runUntil) break;
            time++;
        }