Readonly not really readonly. From http://www.slideshare.net/SergeyTeplyakov/c-sharp-deep-dive — by @steplyakov

ReadonlyNotReallyReadonly.cs

// from http://www.slideshare.net/SergeyTeplyakov/c-sharp-deep-dive 
// by @STeplyakov
void Main()
{
  var s3 = new S3();
	s3.S2.X = 42;
	s3.Dump();
}

struct S1
{
	// this is readonly, really? :)
	private readonly int X;
	public int GetX() { return X; }
}

struct S2
{
	public int X;
}

[StructLayout(LayoutKind.Explicit)]
struct S3
{
	[FieldOffset(0)]
	public S1 S1;
	
	[FieldOffset(0)]
	public S2 S2;
}