private int num;
    //只读属性
    public int Num
    {
        get { return this.num; }
    }

    private string str;
    //只写属性
    public string Str
    {
        set { this.str = value; }
    }

    private double doub;
    //读写属性
    public double Doub
    {
        get { return this.doub; }
        set
        {
            //有条件的写，如果大于0，返回value，否则返回0
            if (value >= 0)
            {
                this.doub = value;
            }
            else
            {
                this.doub = 0;
            }
        }

    }