Using Invoke method from another class return NullReferenceException

Question

Hi,

I'm trying to use method in other class that uses Invoke method but I always get a NullReferenceException. If I use methodBOne directly, Invoke's method works well.

Can anyone explain me why this happens and any solution?

Example code:

 public class A{
   public void methodA(){
      B classBInstance = new B();
      classBInstance.methodBOne();
   }
 }

 public class B
 {
     public void methodBOne()
     {
         Invoke("methodBTw0", time);
     }
     public void methodBTw0()
     {
         //Do something
     }
 }

Thanks!

Answer 1

Answer by PatriceVignola · Aug 03, 2015 at 06:38 PM

What you didn't show in your code snippet is that class B is a MonoBehaviour (since it's able to call the Invoke method), and you should never call the constructor of a MonoBehaviour in Unity. The either have to drag the script to a prefab in a scene, or you instantiate it at runtime with the Instatiate() method.

For your code to work, you should have a reference to your class B in your class A, and then call classBInstance.methodBOne(). Remove the B classBInstance = new B(); line (Unity will warn you at runtime otherwise) since it can lead to unexpected behavior.

Add comment · Show 2 · Share

Skied · Aug 03, 2015 at 06:52 PM 0

Share

I don't know that of $$anonymous$$onoBehaviiour constructors. Thank you very much.

PatriceVignola · Aug 03, 2015 at 07:01 PM 0

Share

@Skied - There is not really a "$$anonymous$$onoBehaviour" constructor. Unlike other classes, Unity uses their own instantiating techniques under the hood to create an instance of the $$anonymous$$onoBehaviour objects, and how you tell Unity to do that is by dragging it on a prefab in your scene. Then, you add a public variable of the same type at the top of your class A and simply drag the prefab on the slot in the inspector:

 public class A : $$anonymous$$onoBehaviour
 {
      public B classBInstance;
 
      public void methodA(){
           classBInstance.methodBOne();
      }
 }
  
 public class B : $$anonymous$$onoBehaviour
 {
      public void methodBOne()
      {
          Invoke("methodBTw0", time);
      }
      public void methodBTw0()
      {
          //Do something
      }
 }

Answer 2

Answer by Positive7 · Aug 03, 2015 at 09:35 PM

 using UnityEngine;
 using System.Collections;
 
 public class A : MonoBehaviour {
     public B classBInstance;
 
     void Start(){
         classBInstance = GetComponent<B>();
         methodA();
     }
 
         public void methodA(){
             classBInstance.methodBOne();
         }
     }
 
 using UnityEngine;
 using System.Collections;
 
 public class B : MonoBehaviour
 {
     public void methodBOne()
     {
         Invoke("methodBTw0", 1f);
     }
     public void methodBTw0()
     {
         Debug.Log("Executed");
     }
 }

Add comment · Share

Using Invoke method from another class return NullReferenceException

2 Replies

Your answer

Welcome to Unity Answers

Follow this Question

Related Questions