即使我定义了方法，该方法也为空（Unity）

Question

我正在为Unity游戏制作一个库存类。但是，我遇到了一个问题。每当我在使用构造函数后调用方法并在其上使用+运算符时，由于某种原因，它无法在对象内部找到任何方法。
这是课程：

public class Inventory {
    public bool init = false;
    public string _1 = "";
    public string _2 = "";
    public string _3 = "";
    public string _4 = "";
    public string[] In {
        get {
            return new string[4] { _1, _2, _3, _4 };
        }
        set {
            _1 = value[0];
            _2 = value[1];
            _2 = value[2];
            _3 = value[3];
        }
    }
    public Inventory(string A, string B, string C, string D) {
        _1 = A;
        _2 = B;
        _3 = C;
        _4 = D;

        init = true;
    }
    public Inventory(string A, string B, string C) {
        _1 = A;
        _2 = B;
        _3 = C;

        init = true;
    }
    public Inventory(string A, string B) {
        _1 = A;
        _2 = B;

        init = true;
    }
    public string Index(int i) {
        switch(i) {
            case 1:
                return this._1;
            case 2:
                return this._2;
            case 3:
                return this._3;
            case 4:
                return this._4;
            default:
                return "";
        }
    }
    public void SetIndex(int i,string set) {
        switch (i) {
            case 1:
                _1 = set;
                return;
            case 2:
                _2 = set;
                return;
            case 3:
                _3 = set;
                return;
            case 4:
                _4 = set;
                return;
            default:
                return;
        }
    }
    public Inventory(string A) {
        _1 = A;

        init = true;
    }
    public int Length() {
        int length = 0;
        for (int i = 0; i < 4; i++) {
            if(Index(i+1)!="") length++;
        }
        return length;
    }

    public Inventory Format() {
        Inventory outt = this;
        bool Formatted = true;
        for(int i = 0; i< 4; i++) {
            if(Index(i+2)==""&&Index(i+3)=="") {
                Formatted = false;
                break;
            } else {
                Formatted = true;
            }
        }
        for(int i = 0; i < 4; i++) {
            if(Index(i+1)=="") {
                if (i != 3) {
                    outt.SetIndex(i + 1, outt.Index(i + 2));
                    outt.SetIndex(i + 2, "");
                }
            }
        }
        return !Formatted ? outt.Format() : outt;
    }
    public int FullLength = 4;
    public static Inventory operator +(Inventory a,string b) {
        //Get length
        a = a.Format(); //Error here
        Debug.Log(a.Length()); //If I comment out first line, this gets error too
        int l = a.Length();
        if(l==4) {
            return a;
        }
        switch(l) {
            case 0:
                return new Inventory(b);
            case 1:
                return new Inventory(a._1, b);
            case 2:
                return new Inventory(a._1, a._2, b);
            case 3:
                return new Inventory(a._1, a._2, a._3, b);
        }
        return a;
    }
    public static Inventory operator -(Inventory a,string b) {
        a = a.Format();
        Inventory A = a;
        for(var i=0;i<4;i++) {
            if(A.Index(i+1)==b) {
                A.SetIndex(i + 1,"");
                break;
            }
        }
        A = A.Format();
        return A;
    }
}

这是我称之为+运算符的区域：

public class Inevntory : MonoBehaviour {
    private Inventory Inv;
    public GameObject Crosshair;
    public float Radius;
    // Start is called before the first frame update
    void Start() {
        Crosshair.SetActive(false);
    }

    // Update is called once per frame
    void Update() {
        Transform transform1 = gameObject.transform;

        //Take items
        TakeItems(transform1);
    }
    void TakeItems(Transform transform1) {
        GameObject[] objs = GameObject.FindGameObjectsWithTag("Item");
        bool Rad = false;
        GameObject obj = gameObject; //Give object dummy value to pass compiler
        foreach (GameObject i in objs) {
            if ((transform1.position - i.transform.position).magnitude < Radius) {
                Rad = true;
                obj = i;
            }
        }

        Crosshair.SetActive(Rad);

        if (Rad) {
            //If there is an item within radius
            if (Input.GetButtonDown("Fire2")) {
                Inv = Inv + obj.name;
                obj.SetActive(false);
            }
        }
    }
}

  

NullReferenceException：对象引用未设置为对象的实例   Inventory.op_Addition（库存a，系统字符串b）（在Assets / Scripts / Inevntory.cs：112）   Inevntory.TakeItems（UnityEngine.Transform transform1）（位于Assets / Scripts / Inevntory.cs：176）   Inevntory.Update（）（位于Assets / Scripts / Inevntory.cs：158）