Modernise and fix warnings

Author: deanagan

Patterns/Observer/SpecialsSubject.cs

@@ -4,10 +4,10 @@ namespace Observer
 {
     public class SpecialsSubject : ISubject
     {
-        public delegate void Callback (string s);
-        public string SubjectState {get; set;}
+        public delegate void Callback(string s);
+        public required string SubjectState { get; set; }
 
-        private List<IObserver> _observers = new List<IObserver>();
+        private readonly List<IObserver> _observers = [];
 
         public void Attach(IObserver observer)
         {

Patterns/Proxy/Entries.cs

@@ -14,7 +14,7 @@ public bool Delete(int id)
         {
             return _products.Remove(id);
         }
-        public IProductInfo Get(int id)
+        public IProductInfo? Get(int id)
         {
             if (!_products.ContainsKey(id))
             {

Patterns/Proxy/IEntries.cs

@@ -4,6 +4,6 @@ namespace Proxy
     public interface IEntries
     {
         bool Delete(int id);
-        IProductInfo Get(int id);
+        IProductInfo? Get(int id);
     }
 }

Tests/ObserverTest.cs

@@ -11,11 +11,13 @@ public class ObserverShould
         public void UpdateObserverOnce_WhenSubjectHasSpecials()
         {
             // Arrange
-            var subject = new SpecialsSubject();
+            var subject = new SpecialsSubject
+            {
+                SubjectState = "Footwear Sale"
+            };
             var mockObserver = new Mock<IObserver>();
             // Act
             subject.Attach(mockObserver.Object);
-            subject.SubjectState = "Footwear Sale";
             subject.Notify();
             // Assert
             mockObserver.Verify(observer => observer.Update(subject), Times.Once());
@@ -25,7 +27,10 @@ public void UpdateObserverOnce_WhenSubjectHasSpecials()
         public void NotCallUpdate_WhenObserverNotAttachedToSubject()
         {
             // Arrange
-            var subject = new SpecialsSubject();
+            var subject = new SpecialsSubject
+            {
+                SubjectState = "Footwear Sale"
+            };
             var mockObserver = new Mock<IObserver>();
             // Act
             subject.SubjectState = "Footwear Sale";