C#中如何初始化结构体中类变量

[读万卷书不如行万里路呀]

如题：

public struct cylinder
    {
        public
            int height;
        public
            Circle underside;
        public cylinder(int n)
        {
            height = n;
            underside = new Circle();
            underside.diameter = 10;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            cylinder myCylinder;
            myCylinder.height = 10;

            Console.WriteLine("The Cylinder's height={0},usderside={1}",myCylinder.height,myCylinder.underside.diameter*3.1415926);
//使用了可能未赋值的字段“underside
            Console.ReadKey();
        }
    }

[bdmh]

myCylinder.underside = new Circle();

[读万卷书不如行万里路呀]

引用 1 楼 bdmh 的回复:

myCylinder.underside = new Circle();

嗯，这样确实可以。。。 想问下，为什么在构造函数中初始化不行呢？