error CS0051: Inconsistent accessibility

Hi All,
I am trying out a sample for observer pattern in C#. I have added the snippet for your reference.
I am getting errors
error CS0051: Inconsistent accessibility: parameter type 'ObserverPattern01.BaseObserver' is less accessible than method 'ObserverPattern01.ConcreteSubject1.Register(ObserverPattern01.BaseObserver)'
\ObserverPattern01\Program.cs(53,21): error CS0051: Inconsistent accessibility: parameter type 'ObserverPattern01.BaseObserver' is less accessible than method 'ObserverPattern01.ConcreteSubject1.UnRegister(ObserverPattern01.BaseObserver)'

Can  you please help me resolve this issue?

Thanks in Advance

interface BaseObserver
    {
        void update();
        char[] Name();
    }
    interface BaseSubject
    {
        void Register(BaseObserver obs);
        void UnRegister(BaseObserver obs);
        void Notify();        
    }
    public class ConcreteObserver1 : BaseObserver  
    {
        public void update()
        {
            System.Console.WriteLine("ConcreteObserver1: I know Subject changed");
        }
        public char[] Name()
        {
            return "ConcreteObserver1";
        }
    }
public class ConcreteSubject1 : BaseSubject
    {
        public void Register(BaseObserver obs)
        {
            Console.WriteLine("Registered: " + obs.Name());            
        }
        public void UnRegister(BaseObserver obs)
        {
            Console.WriteLine("UnRegistered: " + obs.Name());
        }
        public void Notify()
        {
        }
        private ArrayList observers;
    }

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