Initial data commit

Task/Singleton/C-sharp/singleton-4.cs

@@ -0,0 +1,11 @@
+public sealed class Singleton4 //Lazy: Yes ||| Thread-safe: Yes ||| Uses locking: No
+{
+    public static Singleton4 Instance => SingletonHolder.instance;
+
+    private class SingletonHolder
+    {
+        static SingletonHolder() { }
+
+        internal static readonly Singleton4 instance = new Singleton4();
+    }
+}