common property and methods between classes

Hi Minhaj Hussain,
and thanks for response.
you said that "through Interface IB it is much better, now your code is loosely coupled....."
but if i not use Interface:

//------------------------------------------------------
// CLASS 'A'
//------------------------------------------------------
public class A
{

public A Instance{get{return this;}}
public B b;

public float FieldA;

public void DoSomethingA()
{
FieldA = b.DoSomethingB ();
}

}

//------------------------------------------------------
//  CLASS 'B'
//------------------------------------------------------

public class B
{

public B Instance{get{return this;}}
public A a {get;set;}

public virtual float DoSomethingB()
{
return a.FieldA * 0.5F;
}
}

//------------------------------------------------------
// CLASS B2
//------------------------------------------------------
public class B2:B
{

public override float DoSomethingB ()
{
return a.FieldA * 2.0F;
}

}


//------------------------------------------------------
// CLASS TEST
//------------------------------------------------------

public class test
{

public A a = new A();
public B b;

public test (B b)
{

this.b = b;

b.a = a.Instance;
a.b = b.Instance;
// other


}
}



public static void Main (string[] args)
{

test t = new test (new B());

t.a.FieldA = 5;
t.a.DoSomethingA();

Console.WriteLine (t.a.FieldA ); // return 2.5


test t1 = new test (new B2()); // return 10
t1.a.FieldA = 5;
t1.a.DoSomethingA();

Console.WriteLine (t1.a.FieldA );


}


the results is the same, but now code is highly coupled?
Can you explain more about this concept? (I am a beginner in c #)

About virtual method i need to specialize class B. there is another best way to do this?

thanks in advance