[Unity]How to refer to a variable in other files

I show How to refer to a variable in other files.

(refer to a variable of class being componentization)

2 GameObjects, 2 files Each objects have.

How to refer to the value of speed variable in GameManager class.

1.  In Source code

public class GameManager : MonoBehaviour {

PlayerControl pc;

void Start () {

GameObject obj = GameObject.Find ("Player");

if (obj != null)

pc = obj.GetComponent<PlayerControl>();

Debug.Log ("speed : " + pc.speed);

}

void Update () {

}

}

2. in Unity Editor

1) Variable definition of PlayerControl class type

public class GameManager : MonoBehaviour {

public PlayerControl pc;

void Start () {

Debug.Log ("speed : " + pc.speed);

}

void Update () {

}

}

2)

you play the game, you cans see  an error like below

You have to connect reference variable of "pc".

NullReferenceException: Object reference not set to an instance of an object
GameManager.Start () (at Assets/Scripts/GameManager.cs:8)

Choose GUI Obejct, drag and drop over the variable "pc" of  GameManager component.

and then you use the variable of "speed" of PlayerControl class the same as number 1.