MonoBehaviour.Reset()

Description

Reset to default values.

Reset is called when the user hits the Reset button in the Inspector's context menu or when adding the component the first time. This function is only called in editor mode. Reset is most commonly used to give good default values in the inspector.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public GameObject target;
    void Reset() {
        if (!target)
            target = GameObject.FindWithTag("Player");
        
    }
}