public class A
{
    public int AA;
    public int AB;

    public static implicit operator A(B v)
    {
        A obj = new A
        {
            AA = (A)v.BA,
            AB = 0
        };

        return obj;
    }

    public static explicit operator B(A v)
    {
        B obj = new B
        {
            BA = (B)v.AA
        };

        return obj;
    }
}

public class B
{
    public int BA;
}