c# arrays in nested structs

Question

I am trying to build a simulation in C#. The model itself I want to adress as a struct. Inside the model are a lot of elements (I refer to them as cells, considering that elements also refer to the chemical elements in my program and that is confusing) Each cel is a struct. One of the data types inside is an array of floats.

Below is the code I wrote:

struct Model
{
    //model consists of an array of cells
    public Cel[] cel = new Cel[100];

    public Model() { }
}
struct Cel
{
    public float[] n = new float[3];
    public Cel() { }
}


class Programs
{
    static int Main()
    {
        Model model = new Model();

        model.cel[1].n[1] = 0.2f;
        Console.WriteLine(model.cel[1].n[1]);

        //prevent console from closing
        Console.ReadLine();

        return 0;
    }
}

When I run this, it compiles, but gives a message: System.NullReferenceException: 'Object reference not set to an instance of an object.' model.cel[].n was null. at line 24 model.cel[1].n[1] = 0.2f;

I think this is a logical way to do this and would like a solution to make it work. If this is not the way to do it, could someone point me in the right direction?

Thank you for your help.

This looks like a logical model to a human but unfortunately it will be a nightmare for a computer due to lots of heap allocations and cache misses. You're better of creating a new float[100 * 3] to store all values in memory consecutively. This initializes all values with 0.0f and you can iterate over the 3 float values of each cell. The downside is that you'll have to do index calculations inside Model. — Good Night Nerd Pride
– Good Night Nerd Pride, Commented Aug 8, 2022 at 12:53

Marc Gravell · Accepted Answer · 2022-08-08 12:53:57Z

4

Array creation does not initialize each element inside the array - they are all defaults; you could do something like:

    public Cel[] cel;

    public Model() {
        cel = new Cel[100];
        for (int i = 0; i < cel.Length; i++)
        {
            cel[i] = new Cel();
        }
    }

to initialize each manually, but... I'm not sure this is a great outcome, honestly. In particular, instead of a float[3], having 3 discreet float fields in the Cel struct would be a more typical implementation, perhaps making Model a class. You should also usually be very cautious of public fields and mutable structs.

Perhaps something like:

sealed class Model
{
    //model consists of an array of cells
    private readonly Cel[] cel = new Cel[100];

    public ref Cel this[int index] => ref cel[index];
}
readonly struct Cel
{
    public float X { get; }
    public float Y { get; }
    public float Z { get; }
    public Cel(float x, float y, float z)
    {
        X = x;
        Y = y;
        Z = z;
    }

    public Cel WithX(float x) => new Cel(x, Y, Z);
    public Cel WithY(float y) => new Cel(X, y, Z);
    public Cel WithZ(float z) => new Cel(X, Y, z);
}
class Programs
{
    static int Main()
    {
        Model model = new Model();

        ref var cel = ref model[1];
        cel = cel.WithY(0.2f);

        Console.WriteLine(model[1].Y);

        //prevent console from closing
        Console.ReadLine();

        return 0;
    }
}

or with a fixed buffer and span hack:

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

sealed class Model
{
    //model consists of an array of cells
    private readonly Cel[] cel = new Cel[100];

    public ref Cel this[int index] => ref cel[index];
}
unsafe struct Cel
{
    private fixed float n[3];
    public float X => n[0];
    public float Y => n[1];
    public float Z => n[2];
    public Span<float> N
    {
        get
        {
            fixed (float* ptr = n)
            {
                return MemoryMarshal.CreateSpan(ref Unsafe.AsRef<float>(ptr), 3);
            }
        }
    }
    public Cel(float x, float y, float z)
    {
        fixed (float* ptr = n)
        {
            ptr[0] = x;
            ptr[1] = y;
            ptr[2] = z;
        }
    }

    private Cel(in Cel from, int index, float value)
    {
        this = from;
        n[index] = value;
    }

    public Cel WithX(float x) => new Cel(this, 0, x);
    public Cel WithY(float y) => new Cel(this, 1, y);
    public Cel WithZ(float z) => new Cel(this, 2, z);
}
class Programs
{
    static int Main()
    {
        Model model = new Model();

        ref var cel = ref model[1];
        cel = cel.WithY(0.2f);

        Console.WriteLine(model[1].Y);

        Console.WriteLine();
        foreach (var val in model[1].N) // and iterate via span
        {
            Console.WriteLine(val);
        }

        //prevent console from closing
        Console.ReadLine();

        return 0;
    }
}

edited Aug 8, 2022 at 12:53

answered Aug 8, 2022 at 12:41

Marc Gravell

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3 Comments

Kvaestr Over a year ago

This works. For my application it makes sense to have float[3], as I will be performing loop actions over the values in the array. Values inside the arrays will change a couple of million times over the course of the simulation. So I would like to work with a mutable structs. In the past I used pointers in C to keep it working quickly, but I was assured that modern compilers can get it to run quickly in C#.

Marc Gravell Over a year ago

@Kvaestr that's still incredibly expensive, though; and there are other ways to do that... perhaps with a span hack

Marc Gravell Over a year ago

@Kvaestr (added that as an edit, for illustration)

Joost00719 · Accepted Answer · 2022-08-08 12:40:31Z

2

You haven't initialized a Cel yet. You have only reserved space in the array, but it is still empty.

Try this:

model.cel[1] = new Cel();
model.cel[1].n[1] = 02f;

answered Aug 8, 2022 at 12:40

Joost00719

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Comments

John Alexiou · Accepted Answer · 2022-08-08 13:41:46Z

The code

struct Cel
{
    public float[] n = new float[3];
    public Cel() { }
}

is not valid c# syntax. Structs cannot have parameterless constructors Cel() and cannot have field initializers.

To make a struct with an array inside you have some options

Wrap the array if you want the size to be variable. The contents for the struct is just a reference to the array in memory and not the data itself.

    struct Cel
    {
        public readonly float[] n;
        public Cel(int size)
        {
            n = new float[size];
        }
    }

This is pretty useless as it is as it offers no extra functionality than the array itself. You might as well create a jagged array

float[][] model = new float[100][];
for(int i=0; i<100; i++)
{
    model[i] = new float[3];
}
// assign last value
model[99][2] = 0.123f;

Declare the fields one by one to have an array of structures
```
struct Cel
{
    public readonly float a, b, c;

    public Cel(float a, float b, float c)
    {
        this.a = a;
        this.b = b;
        this.c = c;
    }
}
```
and then create an array of structures
```
Cel[] model = new Cel[100];
model[99] = new Cel(0.1f, 0.2f, 0.3f);
```
Note that the fields a,b,c go together and cannot (or should not be) individually assigned. This is assured with the readonly keyword making the fields read-only, assignable only by the constructor Cel(float a, float b, float c)

There are more advanced options with fixed-sized buffers (see the fixed keyword) and unsafe code.

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