Add singleton example

Singleton/LazySingleton.cs

@@ -0,0 +1,15 @@
+public sealed class LazySingleton
+{
+    private LazySingleton() { }
+    private static readonly Lazy<LazySingleton> _lazy = new Lazy<LazySingleton>(() => new LazySingleton());
+
+    public static LazySingleton GetInstance(string value)
+    {
+        var instance = _lazy.Value;
+        instance.Value = value;
+
+        return instance;
+    }
+
+    public string Value { get; set; }
+}

Singleton/NonSingleton.cs

@@ -0,0 +1,9 @@
+public sealed class NonSingleton
+{
+    public NonSingleton(string value)
+    {
+        Value = value;
+    }
+
+    public string Value { get; set; }
+}

Singleton/Program.cs

@@ -0,0 +1,37 @@
+var nonSingleton1 = new NonSingleton("A");
+var nonSingleton2 = new NonSingleton("B");
+Console.WriteLine(nonSingleton1.Value);
+Console.WriteLine(nonSingleton2.Value);
+TestSingleton(nonSingleton1, nonSingleton2);
+
+var simpleSingleton1 = SimpleSingleton.GetInstance("A");
+var simpleSingleton2 = SimpleSingleton.GetInstance("B");
+Console.WriteLine(simpleSingleton1.Value);
+Console.WriteLine(simpleSingleton2.Value);
+TestSingleton(simpleSingleton1, simpleSingleton2);
+
+
+var threadSafeSingleton1 = ThreadSafeSingleton.GetInstance("A");
+var threadSafeSingleton2 = ThreadSafeSingleton.GetInstance("B");
+Console.WriteLine(threadSafeSingleton1.Value);
+Console.WriteLine(threadSafeSingleton2.Value);
+TestSingleton(threadSafeSingleton1, threadSafeSingleton2);
+
+
+var lazySingleton1 = LazySingleton.GetInstance("A");
+var lazySingleton2 = LazySingleton.GetInstance("B");
+Console.WriteLine(lazySingleton1.Value);
+Console.WriteLine(lazySingleton2.Value);
+TestSingleton(lazySingleton1, lazySingleton2);
+
+static void TestSingleton<T>(T object1, T object2) where T : class
+{
+    if (object1 == object2)
+    {
+        Console.WriteLine($"{typeof(T).FullName}: These are singletons");
+    }
+    else
+    {
+        Console.WriteLine($"{typeof(T).FullName}: These are NOT singletons");
+    }
+}

Singleton/SimpleSingleton.cs

@@ -0,0 +1,18 @@
+public sealed class SimpleSingleton
+{
+    private SimpleSingleton() { }
+
+    private static SimpleSingleton _instance;
+
+    public static SimpleSingleton GetInstance(string value)
+    {
+        if (_instance == null)
+        {
+            _instance = new SimpleSingleton();
+            _instance.Value = value;
+        }
+        return _instance;
+    }
+
+    public string Value { get; set; }
+}

Singleton/Singleton.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net7.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

Singleton/ThreadSafeSingleton.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading;
+
+public sealed class ThreadSafeSingleton
+{
+    private ThreadSafeSingleton() { }
+
+    private static readonly object _lock = new object();
+
+    private static ThreadSafeSingleton _instance;
+
+    public static ThreadSafeSingleton GetInstance(string value)
+    {
+        if (_instance == null)
+        {
+            lock (_lock)
+            {
+                if (_instance == null)
+                {
+                    _instance = new ThreadSafeSingleton();
+                    _instance.Value = value;
+                }
+            }
+        }
+        return _instance;
+    }
+
+    public string Value { get; set; }
+}