取消装箱结构类型C＃

Question

我是C＃学习者。我遇到了德尔马。当我取消装箱时（将对象变量转换为int变量并遇到错误）。我的代码：

namespace test1
{
    public struct Tst 
    {

        public int x;
        public int y;
    }

    class Program
    {
        static void Main(string[] args)
        {
            var v1 = new Tst();
            v1.x = 5;
            Object o1 = v1;
            Tst ll = (Tst)o1;
            ((Tst)o1).x = 10;  // here I have an error. I cant understand why I cant do it.
            Console.WriteLine(ll.x);
            Console.ReadLine();

        }
    }
}