Accessing the index operator of an auto-property struct

Question

I've got a "Vector" struct which stores a x, y, and z values. In an effort to make them easier to access I added an index operator, like this:

public struct Vector
{
    public double x;
    public double y;
    public double z;

    public double this[int i]
    {
        get
        {
            if(i == 0) return x;
            else if(i == 1) return y;
            else if(i == 2) return z;
            else throw new ArgumentOutOfRangeException("i", i, "Index must be 0, 1, or 2");
        }
        set
        {
            if (i == 0) x = value;
            else if (i == 1) y = value;
            else if (i == 2) z = value;
            else throw new ArgumentOutOfRangeException("i", i, "Index must be 0, 1, or 2");
        }
    }
}

Note: Normally I would use an array instead of x, y, z but I'm working with pre-existing code and it'll be difficult to refactor at this point.

Now if I use a struct as an auto-property in a class and then try to modify its xyz values, I get this

public Vector v{ get; set; }
void funct(){
    for(int i = 0; i < 3; i++)
    {
        v[i] = 3.0;
    }
}

I get a compiler error:

"Cannot modify the return value of "v" because it is not a variable"

I've tried googling for the error, but I don't think anyone has done this the way I did (it's a pretty interesting case I guess, and probably really ugly). What would be the right way to solve this problem? Is it wrong to use the index operator like this?

Note: yes, I know this is a mutable struct. Again, pre-existing code

Answer 1

You got some options:

1. Make it a class instead of a struct

2. Obtain a copy of the struct value first, modify it, then put it back:

   Vector temp = v;
   for(int i = 0; i < 3; i++)
   {
       temp[i] = 3.0;
   }
   v = temp;
   

3. Make it immutable and add helper/factory methods:

   public struct Vector
   {
       ....
   
       public Vector WithX(double x)
       {
           return new Vector(x, this.y, this.z);
       }
   

I would personally go for nbr. 3 as mutable structs (as you've seen) is a pain in the a**.