1

There are a variable in class A.

In the class B creates an public instance of class A.

In the class C I can’t access public instance from class B.

Why so?

ClassA:

int x;

public int X {
	get { return this.x; }
}

// Use this for initialization
void Start () {
	x = 5;
}

ClassB:

public GameObject abc;

public ClassA classA; // it is public

void Start () {
	classA = abc.GetComponent<ClassA> ();
}

void Update () {
	Debug.Log ("Class B:" + classA.X);
}

ClassC:

void Update () {
            // I have error here
            // The name `classA' does not exist in the current context
	Debug.Log ("Class B:" + classA.X);
}
2

ClassC doesn’t know what classA is. Only ClassB has the reference.

ClassC would need a public reference just like ClassB

ClassC:

public ClassA classA; // it is public
void Update () {
Debug.Log ("classA.X: " + classA.X);

If you’re trying to access ClassB’s reference of ClassA, then ClassC will need a reference of ClassB:

ClassC:

public ClassB classB; // it is public
void Update () {
Debug.Log ("classB.classA.X: " + classB.classA.X);

This is a general programming question, not really anything to do with Unity.