Merge pull request #11 from WeihanLi/main

enhancement on Caching singletons sample code

Author: davidfowl

1.md

@@ -198,7 +198,7 @@ public class Factory
 
     private static class Cache<T>
     {
-        public static T Instance = new T();
+        public static readonly T Instance = new T();
     }
 }
 ```
@@ -215,9 +215,9 @@ public class Factory
 
     private static class Cache<T>
     {
-        public static T Instance = CallExpensiveMethodToBuildANewInstance();
+        public static readonly T Instance = CallExpensiveMethodToBuildANewInstance();
 
-        public static T CallExpensiveMethodToBuildANewInstance()
+        private static T CallExpensiveMethodToBuildANewInstance()
         {
             // Imagine a really complex process to construct T
             return instance.