C# Passing struct as parameter

Question

I have the following code in C#:

public class ELL
{
    public struct RVector
    {
        private int ndim;
        private double[] vector;

        public RVector(double[] vector) => (ndim, this.vector) = (vector.Length, vector);
        public double this[int i] { get => vector[i];  set => vector[i] = value; }

        public override string ToString()
        {
            string str = "(";

            for (int i = 0; i < ndim - 1; i++)
                str += vector[i].ToString() + ", ";

            str += vector[ndim - 1].ToString() + ")";
            return str;
        }
    }
    private static void SwapVectorEntries(RVector b, int m, int n)
    {
        double temp = b[m];
        b[m] = b[n];
        b[n] = temp;
    }
    public static void M(string[] args)
    {
        var a = new double[4] { 1, 2, 3, 4 };
        var b = new RVector(a);

        Console.WriteLine(b);
        SwapVectorEntries(b, 1, 2); // Why after this command, b will be changed?
        Console.WriteLine(b);
    }
}

In this program, i creates a struct RVector. After that, i use a method SwapVectorEntries which have a struct parameter. Because, Struct is a value type, so i think the method SwapVectorEntries will not change the struct parameter. But, in the program, after the command SwapVectorEntries(b, 1, 2);, b has changed. Please explain me about this. Thank you !

You are not modifying the structure, you are modifying the array. Arrays are not value types. Declaring a structure with a mutable reference type as a field is not a great idea. — Hans Passant
– Hans Passant, Commented Nov 5, 2015 at 10:22
Also, you can skip vector values initialization (in public RVector(int ndim) constructor): in c#, every item of the new created array will have it's default value (0.0, in your case using double). That's different from C/C++ — Gian Paolo
– Gian Paolo, Commented Nov 5, 2015 at 10:27
You can also avoid having a ndim member variable: you can replace it with vector.length every time you need it. — Gian Paolo
– Gian Paolo, Commented Nov 5, 2015 at 10:32

Suren Srapyan · Accepted Answer · 2015-11-05 10:56:06Z

4

Problem is in this.You have an array wich is reference type.When you create your

double[] a = new double[4] { 1, 2, 3, 4 };
RVector b = new RVector(a);

you have two references to that array.After when you pass your object into the method,

SwapVectorEntries(b, 1, 2);

your object is copied,BUT your new object have the same reference to that array.Here your have only one array and many references to it.

edited Nov 5, 2015 at 10:56

answered Nov 5, 2015 at 10:24

Suren Srapyan

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3 Comments

Le Chi Hiep Over a year ago

But, b is an instance of structure RVector. It 's created by using a constructor with an array-parameter. So, i think b is a value type.

Suren Srapyan Over a year ago

Yes b is a value type.But when it copies as parameter,all it's fields also copies,and beacuse "double[] vector" is a reference,it's value(the address of the array) also copies .So you are having 2 different objects of RVector,but they have to vectors which refer to the same object.

Le Chi Hiep Over a year ago

Your answer is very clear. Now I has understood it. Thank you very much !

AntiHeadshot · Accepted Answer · 2015-11-05 10:22:26Z

2

B itself is not passed as a reference, but the copy of b has a Reference to the same double[].

answered Nov 5, 2015 at 10:22

AntiHeadshot

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