GetComponent: NullReferenceException

Question

我得到了以下错误：

NullReferenceException:未设置为对象实例的对象引用

当试图在这一行选择对象时：

Ray ray = mainCamera.GetComponent<Camera>().ScreenPointToRay(new Vector3(x, y));

完整代码：

using UnityEngine;
using System.Collections;
public class pickupobject : 

MonoBehaviour {
    GameObject mainCamera;
    public float distance;
    GameObject carryObject;
    bool carrying;
    void start() {
        mainCamera = GameObject.FindWithTag("MainCamera");
    }

    void pickup()
    {
        if (Input.GetKeyDown(KeyCode.F))
        {
            int x = Screen.width / 2;
            int y = Screen.height / 2;
            Ray ray = mainCamera.GetComponent<Camera>().ScreenPointToRay(new Vector3(x, y));
            // Debug.DrawRay(ray.origin, ray.direction * 10, Color.yellow);
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit))
            {
                pickupcube p = hit.collider.GetComponent<pickupcube>();
                if (p != null)
                {
                    carrying = true;
                    carryObject = p.gameObject;
                }
            }
        }
    }

    void carry(GameObject o)
    {
        o.GetComponent<Rigidbody>().isKinematic = true;
        o.transform.position = mainCamera.transform.position + mainCamera.transform.forward * distance;
    }

    // Update is called once per frame
    void Update()
    {
        if (carrying)
        {
            carry(carryObject);
        }
        else
        {
            pickup();
        }
    }

}

Answer 1

试着像下面这样写同一行：

Ray ray = Camera.main.gameObject.GetComponent<Camera>().ScreenPointToRay(new Vector3(x, y, 0));

首先，FindWithTag和任何查找都是相当广泛的(在您的情况下并不重要，因为您只调用了它一次，但只是为了记录一下)，您应该检查拼写，并使用Debug.Log()；打印到控制台值的mainCamera，雷等……看看你什么时候能得到意想不到的结果。现在，只要尝试一下我的代码，它几乎是一样的，它只使用不同的引用相机

http://docs.unity3d.com/ScriptReference/Camera-main.html