C# Object reference not set to an instance of an object

Question

I am getting this NullReferenceException in the second execution of the while loop of the changeColors function.

public class myClass {
    Tuple<int,int>[] theArray = new Tuple<int, int>[100];
}

public myClass() {
    theArray = null;
}

public Tuple<int,int>[] findRedPixels(){
    Tuple<int,int>[] myArray = new Tuple<int, int>[100];
    int k = 0;
    for (int i=0; i<n; i++) {   
        for (int j=0; j<n; j++) {
            if (graph.pixelMap [i, j].color == "red") {
                myArray[k]= new Tuple<int,int>(i,j);
                k++;
            }
        }
    }

    return myArray;
}

public void changeColors(){  
    while (true) {
        theArray = findRedPixels();
        foreach (var item in theArray) {

            //error appears here in the second time it executes the while loop
            Console.WriteLine (item.Item1 ); 
        }
    }                
}

Answer 1

You should not return array from function findRedPixels as you have done because that array will already be initialized with 100 elements, try using List as it provide you flexibility and you can increase decrease size on fly may be something like this

public  Tuple<int,int>[]  findRedPixels(){
            List<Tuple<int,int>> myArray = new List<Tuple<int, int>>();
            int k = 0;
            for (int i=0; i<n; i++) {   
                for (int j=0; j<n; j++) {
                    if( graph.pixelMap [i, j].color=="red"){
                         myArray.Add( new Tuple<int,int>(i,j));
                        k++;
                    }
                }
            }
            return myArray.ToArray();
        }