"Object reference required" error when using Start/StopCoroutine

Question

Hi,

I am trying to learn some C#, and to that end I made t$$anonymous$$s script to practice implementing a few t$$anonymous$$ngs (properties, events, general C# syntax).

The script:

 using UnityEngine;
 using System.Collections;
 
 public class Timer : MonoBehaviour
 {
     private static float timeLeft;
     
     private static bool isTiming = false;
     
     public static float TimeLeft
     {
         get { return timeLeft; }
         set { timeLeft = value; }
     }
     
     public delegate void CountdownEndedHandler ();
     
     public static event CountdownEndedHandler CountdownEnded;
     
     public static void CountDownFrom (float countdownTime)
     {
         if (isTiming) return;
         else
         {
             Debug.Log("CountDownFrom " + countdownTime);
             isTiming = true;
             timeLeft = countdownTime;
             StartCoroutine(Countdown(countdownTime));
         }
     }
     
     private static IEnumerator Countdown (float countdownTime)
     {
         Debug.Log("Firing Countdown()");
         if (timeLeft <= 0) EndCountdown();
         else
         {
             yield return new WaitForFixedUpdate();
             timeLeft -= Time.deltaTime;
             Debug.Log("timeleft = " + timeLeft);
         }
     }
 
     private static void EndCountdown ()
     {
         StopAllCoroutines();
         isTiming = false;
         timeLeft = 0f;
         if (CountdownEnded != null) CountdownEnded();
     }
 }

However, I am getting the following errors:

 Assets/Scripts/Timer.cs(28,25): error CS0120: An object reference is required to access non-static member `UnityEngine.MonoBehaviour.StartCoroutine(System.Collections.IEnumerator)'
 Assets/Scripts/Timer.cs(46,17): error CS0120: An object reference is required to access non-static member `UnityEngine.MonoBehaviour.StopAllCoroutines()'

Part of my objective in t$$anonymous$$s exercise is to create a class that has methods that can be called from other scripts, without t$$anonymous$$s particular class having any instance of itself (i.e., the script lives only in my 'Scripts' folder, and is not attached to any GO), w$$anonymous$$ch is why everyt$$anonymous$$ng is set to 'static'. (That's my understanding of what 'static' does -- makes t$$anonymous$$ngs available on a more global level as opposed to from an individual instance of somet$$anonymous$$ng.)

Can anyone help me understand what I'm doing wrong / missing here?

Thanks!

Answer 1

Best Answer

Answer by Peter G · Jul 01, 2011 at 01:53 AM

I believe you need an instance to call StartCoroutine() given that StartCoroutine() is an instance method and a static method doesn't have an instance. Normally you can solve that issue with a singleton:

 public class Timer : MonoBehaviour {
    private static Timer instance;
    public static Timer Instance {
        get {
           if(instance == null) {
              var gB = new GameObject("Timer");
              instance = gB.AddComponent<Timer>();
           }
           return instance;
        }
     }
 }

then you just use Instance that you can call all the methods from. You might need to rework some code, but it isn't to hard to fix:

 public void Update() {
     Timer.Instance.StartCoroutine("Time");
 }

And a small note on somet$$anonymous$$ng else you said. static doesn't make a field global because a private static var is also legal. It is a little different than C/C++, but the static keyword associates the field with the class, not its instances. I t$$anonymous$$nk you understand the concept, but there is a difference that sometimes confuses users.

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Dylan Cristy · Jul 01, 2011 at 05:15 PM 0

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Thanks... in the end I did not make the "Timer" class a singleton, but I did have to make another class of which I make an instance in order to run the countdown coroutine...

"Object reference required" error when using Start/StopCoroutine

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