Damien Flury Damien Flury - 5 months ago 27
C# Question

C# - Object passed as method parameter changes state

I have a class Matrix with a 2d-array of values as a public property:

public class Matrix
{
public double[,] Values { get; set; }

public Matrix(double[,] values)
{
Values = values;
}
...
}


And I overloaded the *-Operator as a static method inside of Matrix:

public static Matrix operator *(Matrix m, double operand)
{
var resMatrix = new Matrix(m.Values);
for (var i = 0; i < resMatrix.Values.GetLength(0); i++)
{
for (var j = 0; j < resMatrix.Values.GetLength(1); j++)
{
resMatrix[i, j] *= operand;
}
}
return resMatrix;
}


And I did following in my main-class:

internal class Program
{
public static void Main(string[] args)
{
var m1 = new Matrix(new double[,]
{
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
});
Console.WriteLine(m1);

var m2 = m1 * 2;

Console.WriteLine(m2);
Console.WriteLine(m1);
}
}


The operator itself works fine, m2 is changed as intended. But m1 gets the same values as m2 after multiplication. I know that reference-types are passed as reference into a method, but I allocated a new object on the heap with the call "var resMatrix = new Matrix(m.Values);" right? Or does the compiler merge those two objects into one for performance-optimization? Is there a possibility to keep m1 the same just like value types, and if so, is it good and common practice?

Answer Source

As you know, Matrix is a reference type, so you made a new variable:

var resMatrix = new Matrix(m.Values);

However, double[,] is also a reference type! When you do this:

Values = values;

You are still making Values and values dependent on each other i.e. changing one's value will affect the other.

To do this, you need to create a copy. One way to do this is Clone:

Values = values.Clone() as double[,];