红色的地方提示报错什么情况哪位大神解释一下

[待解决问题]

// 定义一个角色(Role)类, 成员有 hp血条, atk攻击, def防御, weapon武器实例

//定义一个武器(Weapon)类, 成员有 name atk

//在Main()实例化一个角色, 实例化 n 个武器

//在 Role 中写方法, 装备武器，更换武器，拆下武器

//当需要用角色的攻击力的时候：攻击 = 自身攻击力 + 武器攻击力

//封装Role的hp, hp不能小于0大于9999，

//武器的名字在构造函数中赋值，只读不能写
class Program { static void Main(string[] args) { var role = new Role() { Atk = 1000, Hp = 1000 }; var weapon1 = new Weapon("刀") { }; var weapon2 = new Weapon("枪") { }; var weapon3 = new Weapon("剑") { }; role.AddWeapon(weapon1); role.ChangeWeapon(weapon2); role.DeleteWeapon(weapon2); Console.ReadKey();//停止 } } public class Weapon { public Weapon(string name) { this.Name = name; } public string Name { get; } public int Atk { get; set; } } public class Role { private int _hp; public int Hp
{ get { return_hp; } set { if (value < 0 || value > 9999) throw new Exception("设置不在范围内"); _hp=value; } } public int Atk{ get; set; } public string Def { get; set; } public Weapon Weapon { get; set; } public int GetAtk() { int atk = (Weapon == null) ? 0 : Weapon.Atk; return Atk + atk; } public void AddWeapon(Weapon weapon) { this.Weapon = weapon; } public void ChangeWeapon(Weapon weapon) { this.Weapon = weapon; } public void DeleteWeapon(Weapon weapon) { this.Weapon = null; } } }