immutable and readonly in C#

ReadonlyField.cs

class A
{
    public int X { get; set; }
}

class B
{
    public readonly A A = new A();
}

class Program
{
    static void Main()
    {
        var b = new B();
        b.A = new A();   // エラー: A は readonly
        b.A.X = 10;      // OK    : X は別に readonly じゃない
    }
}