Is there a faster/cleaner way to copy structs to an array in C#?

Question

I have a float4x4 struct which simply containts 16 floats:

struct float4x4
{
        public float M11; public float M12; public float M13; public float M14;
        public float M21; public float M22; public float M23; public float M24;
        public float M31; public float M32; public float M33; public float M34;
        public float M41; public float M42; public float M43; public float M44;
}

I want to copy an array of these structs into a big array of floats. This is as far as I know a 1:1 copy of a chunk of memory

What I do know is rather ugly, and not that fast:

        int n = 0;
        for (int i = 0; i < Length; i++)
        {
            array[n++] = value[i].M11;
            array[n++] = value[i].M12;
            array[n++] = value[i].M13;
            array[n++] = value[i].M14;

            array[n++] = value[i].M21;
            array[n++] = value[i].M22;
            array[n++] = value[i].M23;
            array[n++] = value[i].M24;

            array[n++] = value[i].M31;
            array[n++] = value[i].M32;
            array[n++] = value[i].M33;
            array[n++] = value[i].M34;

            array[n++] = value[i].M41;
            array[n++] = value[i].M42;
            array[n++] = value[i].M43;
            array[n++] = value[i].M44;
        }

If I was using a lower level language, I would simply use memcpy, what can I use as an equivilant in C#?

Answer 1

You can't use a memory copy, as you can't blindly assume anything about how the members are stored inside the structure. The JIT compiler could decide to store them with a few bytes of padding between them, if that would make it faster.

Your structure is way too large for the recommended size of a structure anyway, so you should make it a class. Also, structures should not be mutable, which also talks for a class.

If you store the properties in an array internally, you can use that for copying the values:

class float4x4 {

  public float[] Values { get; private set; } 

  public float4x4() {
    Values = new float[16];
  }

  public float M11 { get { return Values[0]; } set { Values[0] = value; } }
  public float M12 { get { return Values[0]; } set { Values[0] = value; } }
  ...
  public float M43 { get { return Values[14]; } set { Values[14] = value; } }
  public float M44 { get { return Values[15]; } set { Values[15] = value; } }

}

Now you can get the Values array from the object and copy to the array using the Array.CopyTo method:

int n = 0;
foreach (float4x4 v in values) {
  v.Values.CopyTo(array, n);
  n += 16;
}