Unity 5 An object reference is required for the non-static field, method, or property error

Question

In Unity 5 using c# on line 35 of my PlayerMovement class I have this error:

Error CS0120 An object reference is required for the non-static field, method, or property 'GameManager.completeLevel()'

PlayerMovement Class:

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

public class PlayerMovement : MonoBehaviour {
    public GameObject deathParticals;
    public float moveSpeed;
    private Vector3 spawn;

    // Use this for initialization
    void Start () {
        spawn = transform.position;
        moveSpeed = 5f;
    }

    // Update is called once per frame
    void Update() {
        transform.Translate(moveSpeed * Input.GetAxis("Horizontal") * Time.deltaTime, 0f, moveSpeed * Input.GetAxis("Vertical") * Time.deltaTime);
        if (transform.position.y < -2)
        {
            Die();
        }
    }
    void OnCollisionStay(Collision other)
    {
        if (other.transform.tag == "Enemy")
        {
            Die();
        }
    }
    void OnTriggerEnter(Collider other)
    {
        if (other.transform.tag == "Goal")
        {
            GameManager.completeLevel();
        }
    }
    void Die()
    {
        Instantiate(deathParticals, transform.position, Quaternion.identity);
        transform.position = spawn;
    }
}

GameManager Class:

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

public class GameManager : MonoBehaviour {
    public static int currentScore;
    public static int highscore;

    public static int currentLevel = 0;
    public static int unlockedLevel; 

    public void completeLevel()
    {
        currentLevel += 1;
        Application.LoadLevel(currentLevel);
    }
}

Answer 1

As your GameManager class inherits from MonoBehaviour, this script needs to be attached to a game object. Then you need to get a reference to the the GameManager component, which can be done in several ways.

GameManager gameMananger = GameObject.Find("GameManager").GetComponent<GameManager>();

The above will work if your game object is called "GameManager", swap this for the name of your object if it is called something else.

Then gameMananger.completeLevel(); will now work.

You could shorten this to GameObject.Find("GameManager").GetComponent<GameManager>().completeLevel();

If the GameManager script is attached to the same gameObject as PlayerMovement, this is all you need GetComponent<GameManager>().completeLevel();

Answer 2

You need to instantiate the GameManager object.

public GameObject gameManager;
Instantiate(gameManager)

Or as Jim mentioned:

you need to mark completeLevel as static:

static public void completeLevel()
{
    ....
}

More found here: Unity Tutorial: gameManager