Projektdateien hinzufügen.

WindowsFormsApp1/OP/ClassObserver.cs

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+using System.Windows.Forms;
+
+namespace WindowsFormsApp1.OP
+{
+    public class ClassObserver: IObserver<int>
+    {
+        public void Update(ISubject<int> subject)
+        {
+            MessageBox.Show(subject.GetState().ToString());
+        }
+    }
+}

WindowsFormsApp1/OP/IObserver.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WindowsFormsApp1.OP
+{
+    public interface IObserver<T>
+    {
+        /// <summary>
+        /// Receive update from subject
+        /// </summary>
+        void Update(ISubject<T> subject);
+    }
+}

WindowsFormsApp1/OP/ISubject.cs

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+namespace WindowsFormsApp1.OP
+{
+    public interface ISubject<T>
+    {
+        /// <summary>
+        /// Attach an observer to the object.
+        /// </summary>
+        /// <param name="observer">Your Observer.</param>
+        void Subscribe(IObserver<T> observer);
+        /// <summary>
+        /// Detach an observer from an object.
+        /// </summary>
+        /// <param name="observer">Your observer.</param>
+        void Unsubscribe(IObserver<T> observer);
+        /// <summary>
+        /// Notify all observers about an event.
+        /// </summary>
+        void Notify();
+
+        void SetState(T state);
+        T GetState();
+    }
+}

WindowsFormsApp1/OP/PropertyObserver.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Forms;
+
+namespace WindowsFormsApp1.OP
+{
+    public class PropertyObserver : IObserver<string>
+    {
+        public PropertyObserver()
+        { }
+
+        public void Update(ISubject<string> subject)
+        {
+            MessageBox.Show(subject.GetState());
+        }
+    }
+}

WindowsFormsApp1/OP/Subject.cs

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+using System.Collections.Generic;
+
+namespace WindowsFormsApp1.OP
+{
+    /// <summary>
+    /// Owns some important state and notifies observers when the state changes.
+    /// </summary>
+    public class Subject<T> : ISubject<T>
+    { 
+        private protected List<IObserver<T>> _observers = null;
+        private object state { get;set; }
+
+        public Subject()
+        {
+            _observers = new List<IObserver<T>>();
+        }
+
+        public virtual void Subscribe(IObserver<T> observer)
+        {
+            _observers.Add(observer);
+        }
+        public virtual void Unsubscribe(IObserver<T> observer)
+        {
+            _observers.Remove(observer);
+        }
+        public void Notify()
+        {
+            foreach (IObserver<T> observer in _observers)
+            {
+                observer.Update(this);
+            }
+        }
+        public virtual void SetState(T state)
+        {
+            this.state = state;
+            Notify();
+        }
+        public virtual T GetState()
+        {
+            return (T)state;
+        }
+    }
+}