Inheritance from object to object

Questions & Answers
1

I have been working with inheritance in classes for the first time now and some things confuse me.
Say I define some variables at the top of the class, give them some value in the Start() function and use them in a sub-class. Sometimes my variables cannot be found in the subclass.

For example, in the base class I type this:

using UnityEngine;
using System.Collections;

public class GeneralItemScript : MonoBehaviour {
	
	public InteractionScript iaScript;
	public GameObject player;
	
	// Use this for initialization
	void Start () {
		player = GameObject.Find ("Player");
		
		iaScript = player.GetComponent<InteractionScript>();
		
	}
	
	// Update is called once per frame
	void Update () {
	
	}
	
	
	virtual public void PerformInteraction() {
		
	}
}

And in the subclass I attempt to use this iaScript in a custom function ( PerformInteraction() ):

using UnityEngine;
using System.Collections;

public class PickupItemScript : GeneralItemScript {
	
	public bool b_isCarried;
	
	// Use this for initialization
	void Start () {
	
		b_isCarried = false;
		
	}
	
	// Update is called once per frame
	void Update () {
		
	}
	
	override public void PerformInteraction() {
		if (iaScript.SetCarriedItem(this.gameObject)) {
			b_isCarried = true;
		}
	}
	
	
	public void SetCarryLocation() {
		transform.parent = player.transform.Find("Arm");
	}
	
	public void StopCarry() {
		Vector3 tempVec = transform.position;
		transform.parent = null;
		transform.position = tempVec;
		b_isCarried = false;
	}
}

This earns me the error that I cannot find the object referenced.
What are the general rules/ do’s and don’ts of inheritance? And how does overriding functions work exactly?

EDIT: added entire scripts

2

Your Start() method isn’t properly overridden. Therefore the base implementation is never called. That is why your iascript remains null.

// in GeneralItemScript.cs
virtual protected void Start()
{
    player = GameObject.Find ("Player");
    iaScript = player.GetComponent<InteractionScript>();
}

// in PickupItemScript.cs
override protected void Start()
{
    base.Start();
    b_isCarried = false;
}

More about it (MSDN):

override (C# Reference)

base (C# Reference)