unsafeaccess.cs

using System;
using System.Runtime.CompilerServices;

Bar b = CreateBar(42); // invoke a private .ctor
ref int x = ref GetBarX(b); // read field *ref*
Console.WriteLine(x); // deref field value
x = 17; // rewrite a private readonly field
Console.WriteLine(b.A); // 17

[UnsafeAccessor(UnsafeAccessorKind.Constructor)]
static extern Bar CreateBar(int a);

[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "x")]
static extern ref int GetBarX(Bar bar);

file class Bar
{
    private readonly int x;
    public int A => x;
    private Bar(int a) => x = a;
}