Question

Consider the Foo struct as follows:

struct Foo
{
  public float X;
  public float Y;

  public Foo(float x, float y)
  {
    this.X = x;
    this.Y = y;
  }

  public void Change(float x)
  {
    this.X = x;
  }
}

I understand modifying the field in the constructor, that’s perfectly logical to me and my understanding of structs as value, number-like immutable types.

However, since one can‘t do:

Foo bar = new Foo(1, 2);
bar.X = 5;

Why can one use:

Foo bar = new Foo(1, 2);
bar.Change(5);

EDIT: If structs are mutable, then why can’t they be modified when in a list or returned from a property?

Cannot modify expression because it is not a variable

Answer 1

You’ve made a key mistaken assumption.

.NET structs are mutable. You can absolutely perform bar.X = 5;.

You should design structs to be immutable, but by the code you have provided, they are mutable.

Have a look at this question for a description of where mutable structs can get your into trouble.
Immutability of structs