Update

Author: milanm

src/DesignPatterns.sln

@@ -39,7 +39,7 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Prototype", "Prototype\Prot
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Bridge", "Bridge\Bridge.csproj", "{96EDCEDC-8C6D-438E-8E69-7DD8C03D3D5E}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Facade", "Facade\Facade.csproj", "{02577667-C912-453E-B3D0-F20D5ADF9250}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Facade", "Facade\Facade.csproj", "{02577667-C912-453E-B3D0-F20D5ADF9250}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution

src/Observer/IObserver.cs

@@ -1,4 +1,4 @@
 public interface IObserver
 {
-    void Update(string message);
+    void Update(string? message);
 }

src/Observer/MessagePublisher.cs

@@ -1,7 +1,7 @@
 public class MessagePublisher : IMessagePublisher
 {
     private List<IObserver> _observers = new List<IObserver>();
-    private string _latestMessage;
+    private string? _latestMessage;
 
     public void Subscribe(IObserver observer)
     {

src/Observer/UserSubscriber.cs

@@ -7,7 +7,7 @@ public UserSubscriber(string name)
         _name = name;
     }
 
-    public void Update(string message)
+    public void Update(string? message)
     {
         Console.WriteLine($"{_name} received message: {message}");
     }