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This question was closed Aug 06, 2017 at 09:47 PM by Olmstead for the following reason:

The question is answered, right answer was accepted

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Question by Olmstead · Aug 06, 2017 at 09:17 PM · c#state machine

State Machine help, I am getting a NullReferanceException: Object reference not set to an instance of an object when trying to test the project.

For this project, I am building a traffic light that uses a data model and a state machine. The following code is in my data model:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class TrafficLight : MonoBehaviour
 {
     //Green light timer
     [SerializeField]
     private float greenLight = 5.0f;
 
     //Yellow light timer
     [SerializeField]
     private float yellowLight = 5.0f;
 
     //Red light timer
     [SerializeField]
     private float redLight = 5.0f;
 
     private float timePassed;
 
     void Start()
     {
         ResetTimeSinceLastTransition();
         GetComponent<Renderer>().material.color = Color.grey;
     }
 
     void Update()
     {
         timePassed += Time.deltaTime;
     }
 
     void ResetTimeSinceLastTransition()
     {
         timePassed = 0.0f;
     }
 
     public float GetTime()
     {
         return timePassed;
     }
 
     public float GetGreenTime()
     {
         return greenLight;
     }
 
     public float GetYellowTime()
     {
         return yellowLight;
     }
 
     public float GetRedTime()
     {
         return redLight;
     }
 
 }//Data Model

Here is my state machine script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System;
 
 public class TrafficLightStateMachine : MonoBehaviour
 {
 
     private TrafficLight target;
     private TrafficStates curState = TrafficStates.greenLight;
     private Dictionary<TrafficStates, Action> fsm = new Dictionary<TrafficStates, Action>();
 
     enum TrafficStates
     {
         greenLight,
         yellowLight,
         redLight
         
     }
 
     void Start()
     {
         //access to the Data Model
         GetComponent<TrafficLight>();
         //dictionary
         fsm.Add(TrafficStates.redLight, new Action(StateRed));
         fsm.Add(TrafficStates.yellowLight, new Action(StateYellow));
         fsm.Add(TrafficStates.greenLight, new Action(StateGreen));
         //default state
         SetState(TrafficStates.greenLight);
     }
     
     void Update()
     {
         fsm[curState].Invoke();
     }
 
     void StateGreen()
     {
         if (target.GetGreenTime() > target.GetTime())
         {
             GameObject yellowObj = GameObject.Find("YellowLight");
             yellowObj.GetComponent<Renderer>().material.color = Color.yellow;
             SetState(TrafficStates.yellowLight);
         }
     }
 
     void StateYellow()
     {
         if (target.GetYellowTime() > target.GetTime())
         {
             GameObject redObj = GameObject.Find("RedLight");
             redObj.GetComponent<Renderer>().material.color = Color.red;
             SetState(TrafficStates.redLight);
         }
     }
 
     void StateRed()
     {
         if (target.GetRedTime() > target.GetTime())
         {
             GameObject greenObj = GameObject.Find("GreenLight");
             greenObj.GetComponent<Renderer>().material.color = Color.green;
             SetState(TrafficStates.greenLight);
         }
     }
 
     void SetState(TrafficStates newState)
     {
         curState = newState;
     }
 
 }//State Machine

When I run the project in Unity, I am getting the following error: NullReferenceException: Object reference not set to an instance of an object TrafficLightStateMachine.StateGreen () (at Assets/Scripts/TrafficLightStateMachine.cs:41) TrafficLightStateMachine.Update () (at Assets/Scripts/TrafficLightStateMachine.cs:36)

When I follow the error in Unity, it takes me to this line for the problem:

         if (target.GetGreenTime() > target.GetTime())

Any help would be greatly appreciated.

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Best Answer

Answer by kalen_08 · Aug 06, 2017 at 09:34 PM

you never initialize "target" all you have is GetComponent(); what you need is target = gameObject.GetComponent (); @Olmstead

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avatar image kalen_08 · Aug 06, 2017 at 09:35 PM 0
Share

line 24 in Start ()

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Answer by Olmstead · Aug 06, 2017 at 09:46 PM

That did it, now it runs. Thank you! @kalen_08

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