Added the default operator snippets (#1132)

Author: pkulikov

csharp/language-reference/operators/DefaultOperator.cs

@@ -0,0 +1,63 @@
+using System;
+
+namespace operators
+{
+    public static class DefaultOperator
+    {
+        public static void Examples()
+        {
+            WithOperand();
+            DefaultLiteral();
+        }
+
+        private static void WithOperand()
+        {
+            // <SnippetWithOperand>
+            Console.WriteLine(default(int));  // output: 0
+            Console.WriteLine(default(object) is null);  // output: True
+
+            void DisplayDefaultOf<T>()
+            {
+                var val = default(T);
+                Console.WriteLine($"Default value of {typeof(T)} is {(val == null ? "null" : val.ToString())}.");
+            }
+
+            DisplayDefaultOf<int?>();
+            DisplayDefaultOf<System.Numerics.Complex>();
+            DisplayDefaultOf<System.Collections.Generic.List<int>>();
+            // Output:
+            // Default value of System.Nullable`1[System.Int32] is null.
+            // Default value of System.Numerics.Complex is (0, 0).
+            // Default value of System.Collections.Generic.List`1[System.Int32] is null.
+            // </SnippetWithOperand>
+        }
+
+        private static void DefaultLiteral()
+        {
+            // <SnippetDefaultLiteral>
+            T[] InitializeArray<T>(int length, T initialValue = default)
+            {
+                if (length < 0)
+                {
+                    return default;
+                }
+
+                var array = new T[length];
+                for (var i = 0; i < length; i++)
+                {
+                    array[i] = initialValue;
+                }
+                return array;
+            }
+
+            void Display<T>(T[] values) => Console.WriteLine($"[ {string.Join(", ", values)} ]");
+
+            Display(InitializeArray<int>(3));  // output: [ 0, 0, 0 ]
+            Display(InitializeArray<bool>(4, default));  // output: [ False, False, False, False ]
+
+            System.Numerics.Complex fillValue = default;
+            Display(InitializeArray(3, fillValue));  // output: [ (0, 0), (0, 0), (0, 0) ]
+            // </SnippetDefaultLiteral>
+        }
+    }
+}

csharp/language-reference/operators/Program.cs

@@ -62,6 +62,10 @@ static void Main(string[] args)
             LambdaOperator.Examples();
             Console.WriteLine();
 
+            Console.WriteLine("=========== default operator examples ==========");
+            DefaultOperator.Examples();
+            Console.WriteLine();
+
             Console.WriteLine("========== delegate operator examples ==========");
             DelegateOperator.Examples();
             Console.WriteLine();