Very odd, but it works

gistfile1.cs

void Main()
{
	var f = new Foo ();
	f.Inc ();
	f.Inc ();
	f.X.Dump ();
}

struct Foo {
	public readonly int X;
	
	public Foo (int x) { X = x; }
	
	public void Inc () {
		this = new Foo (X + 1);
	}
}

gistfile2.cs

readonly Foo f = new Foo ();

void Main()
{
	// Inc cannot reassign f since its readonly
	// the assignment is silently ignored. The same
	// behaviour can be observed when manually trying 
	// to change a readonly field using IL.
	f.Inc ();
	f.Inc ();
	f.X.Dump ();
}

struct Foo {
	public readonly int X;

	public Foo (int x) { X = x; }

	public void Inc () {
		this = new Foo (X + 1);
	}
}