Question

I’ve been reading up extensively on structs, and I have a decent understanding of where you would use them. One thing though bothers me is, no matter how much i read about it, I don’t understand the immutability of structs.

I understand that, like strings, if you change them you essentially create an entirely new object in memory, but does this also hold for the values inside a struct. For instance:

public struct SomeValues
{
    public int AnInt;
    public float AFloat;
    public string AString;
}
public SomeValues[] ArrayOfValues;
private void SomeFunctionThatRunsContinuously()
{
    /*so for every value I change, do I essentially create a whole new instance
      of SomeValues?*/

    for (int i = 0; i < ArrayOfValues.Length; i++)
    {
        //Is this another struct?
        ArrayOfValues[i].AnInt++;

        //And then another again?
        ArrayOfValues[i].AFloat += 0.33f;

        /*I realise adding letters to a string is a horrible idea 
                       -- but is it WORSE to do it in a struct?*/

        ArrayOfValues[i].AString += "s";
    }
}

So for instance, if I had a struct which for instance was holding the coordinates of every person inside a 3D/2D space (coordinates were given as an example of when to use structs), and the positions of the people change and are updated inside a two/three int struct, from maybe an array, is that creating new structs because they’re immutable?

Answer 1

A struct is not immutable by default, it is just a very good idea to design the struct so that it acts immutable.

So for immutable structs/classes:

• don’t use public fields, use properties instead without a public setter. A private setter is fine, setting the backing-field (from the constructor) is also fine.
• only allow values to be set through the constructor.
• methods that appear to modify your struct should not modify it, they should return a new instance instead (think DateTime.AddYears).