c# - 如何访问另一个变量C#

Question

我必须从此类访问变量ammoMagazine

public class Pistol : MonoBehaviour {
    public int ammoMagazine = 7;
}

然后我尝试了这段代码：

public class AmmoCounter : MonoBehaviour {

    public int ammo;
    private Pistol _pistol;     

    void Start () {
        _pistol = GetComponentInChildren<Pistol>();
    }

    void Update () {
        ammo = _pistol.ammoMagazine;
        guiText.text = "Pistol: " + ammo + "/7";
    }
}

为什么会有NullReferenceException：对象引用未设置为对象的实例？
谢谢，我是C＃的新手

Answer 1

public class AmmoCounter : MonoBehaviour {

    public int ammo;
    private Pistol _pistol = GetComponentInChildren<Pistol>();     

    void Update () {
        ammo = _pistol.ammoMagazine;
        guiText.text = "Pistol: " + ammo + "/7";
    }
}

在调用更新函数之前初始化_pistol变量（检查GetComponentInChildren（）函数是否返回null）