public interface IClassA : IClass {
     int A { get; set; }
}

public interface IClassB : IClass {}

public class ClassA : IClassA {
    public int A { get; set; }
    public int B { get; set; }
    public Class1 Class1 { get; set; }

    [DefaultConstructor]
    public ClassA() {}

    public ClassA(int a) {
         A = a;
    }
}

public class ClassB : IClassB {
     public IClassA ClassA;

     public ClassB(IClassA classA) {
         ClassA = classA;
     }
}

public class ClassM : IClassA {
    public int A { get; set; }

    public ClassM(int a) {
        A = a;
    }

    public ClassM(int a, int b) {
        A = a + b;
    }
}