readonly struct field

ReadonlyStructField.cs

struct Point
{
    public int X { get; set; }
    public int Y { get; set; }
}

class B
{
    public readonly Point P = new Point();
}

class Program
{
    static void Main()
    {
        var b = new B();
        b.P.X = 10; // ちゃんとエラー。P が構造体の時には有効。
    }
}