unity-test: UnityのC#でインスタンスを取得しよう https://dkrevel.com/makegame-beginner/program-beginner-instance/

get-instance-1.cs

// https://dkrevel.com/makegame-beginner/program-beginner-instance/
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class NewBehaviourScript : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        BoxCollider box = null;
        Debug.Log(box);
        box = GetComponent<BoxCollider>();
        Debug.Log(box);
    }

    // Update is called once per frame
    void Update()
    {
    }
}

get-instance-2.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class NewBehaviourScript : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        GameObject g = null;
        Debug.Log(g);
        g = GameObject.Find("Directional Light");
        Debug.Log(g);
    }

    // Update is called once per frame
    void Update()
    {
    }
}

get-instance-3.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class NewBehaviourScript : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        GameObject g = null;
        Debug.Log(g);
        g = GameObject.Find("Directional Light");

        Light l = null;
        l = g.GetComponent<Light>();
        Debug.Log(l);
    }

    // Update is called once per frame
    void Update()
    {
    }
}

new-1.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class NewBehaviourScript : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        Vector3 v = new Vector3();  // Vector3はclassではあるが、コンポーネントではない
                                    // コンポーネントではないclassをインスタンス化するには new してやる必要あり 
        Debug.Log(v);
    }

    // Update is called once per frame
    void Update()
    {
    }
}