C# Copy 2D matrix issue

Question

I have a class A:

public class A {
    private Point[,] matrix = null;
    private int r, c;

    public Point[,] Matrix
    {
        get
        {
            if (this.matrix == null || this.matrix.Length == 0)
            {
                // Matrix is null or it has no elements on it.
                return null;
            }
            else
            {
                return this.matrix;
            }
        }
    }

    public A(int r, int c)
    {
        this.r = r;
        this.c = c;
        this.matrix = new Point[r, c];
    }

    public void InitializeMatrix
    {
        for (int i = 0; i < r; i++)
        {
            for (int j = 0; j < c; j++)
            {
                this.matrix[i, j] = new Point(i, j);
            }
        }
    }

    public void CopyMatrix(out Point[,] copyMatrix)
    {
        if (this.Matrix == null)
        {
            throw new ArgumentException("Matrix is null.", "Matrix");
        }

        Point[,] m = new Point[r, c];

        for (int i = 0; i < r; i++)
        {
            for (int j = 0; j < c; j++)
            {
                m[i, j] = this.Matrix[i, j];
            }
        }

        copyMatrix = m;
    }
}

Point is a class:

public class Point 
{
    public int x, y;

    public Point (int x, int y)
    {
        this.x = x;
        this.y = y;
    }
}

So from another class, let's say, B:

public class B
{
     private A instanceA;

     public B (A a)
     {
         // imagine a valid not null instance of class A is passed.
         this.instanceA = a;
     }

     private void SomeMethod()
     {
         Point[,] m;
         this.instanceA.CopyMatrix(out m);

         m[2, 4].x = 9;    <--- here is the problem    
         Console.WriteLine(String.Format("{0}",this.instanceA.Matrix[2, 4].x));
     }
}

The problem is:

In SomeMethod method of class B, when I update my local matrix Point[,] m also is updated matrix 'matrix' in class A. If I do:

Console.WriteLine(this.instanceA.Matrix[2, 4]);

it outputs 9 as well. Why? Its content has been altered as well...

Answer 1

Your issue here is that you don't properly understand the difference between a 'reference' type and a 'primitive' type.

Reference Types vs. Primitive Types

For a primitive type (numbers, like an int or a long ); the variable itself is allocated on the stack and contains the actual numerical value itself. If I do

int foo = 1;
int bar;
bar = foo;

This allocates space on the stack for a variable named foo , assigns it the value 1 , allocates more space on the stack for bar , then copies the number 1 into bar . Both variables remain independent of each other, so if I now do

bar = 4

foo remains unchanged.

For an object, something a little different is happening. Take the following:

class MyObject {
    public int x;
}


// Inside some method somewhere
MyObject foo = new MyObject();
MyObject bar;
foo = bar;

In this case what is happening is memory is allocated on the heap for an instance of MyObject . A reference variable named foo is then allocated on the stack and initialised with the memory address of the MyObject that was just created.

This is a crucial point; in this instance foo does not contain the object , simply a reference to it. This means when on the next line we allocate a new reference type bar , the line foo = bar is now assigned the memory address of the same allocated object .

This means we can do the following

foo.x = 1;
bar.x = 10;

Since foo and bar now both point to the same memory address , if we were to print the value of either foo.x or bar.x to the console, the outcome would be 10.

The Solution

SO, how does this relate to your problem? Easy; your matrix is an array of references to Point objects, not Point objects themselves. So the line in your code

m[i, j] = this.Matrix[i, j];

Is simply copying these references to the same Point objects to the new matrix. To solve your problem; change the above line to the following:

m[i, j] = new Point(this.Matrix[i, j].x, this.Matrix[i, j].y);

What this does instead is to allocate memory on the heap for entirely new Point objects, and initialise them with the values from the old ones.