add generic pattern match sample (#803)

* add generic pattern match sample

Supports dotnet/docs#10334

* add last output line.

Author: BillWagner

snippets/csharp/language-reference/keywords/switch/type-pattern3.cs

@@ -0,0 +1,56 @@
+// <Snippet1>
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+
+class Example
+{
+    static void Main(string[] args)
+    {
+        int[] values = { 2, 4, 6, 8, 10 };
+        ShowCollectionInformation(values);
+        
+        var names = new List<string>();
+        names.AddRange( new string[] { "Adam", "Abigail", "Bertrand", "Bridgette" } );
+        ShowCollectionInformation(names);
+
+        List<int> numbers = null;
+        ShowCollectionInformation(numbers);
+    }
+   
+    private static void ShowCollectionInformation<T>(T coll)
+    {
+        switch (coll)
+        {
+            case Array arr:
+               Console.WriteLine($"An array with {arr.Length} elements.");
+               break;
+            case IEnumerable<int> ieInt:
+               Console.WriteLine($"Average: {ieInt.Average(s => s)}");
+               break;   
+            case IList list:
+               Console.WriteLine($"{list.Count} items");
+               break;
+            case IEnumerable ie:
+               string result = "";
+               foreach (var e in ie) 
+                  result += "${e} ";
+               Console.WriteLine(result);
+               break;   
+            case object o:
+               Console.WriteLine($"A instance of type {o.GetType().Name}");
+               break;   
+            default:
+                Console.WriteLine("Null passed to this method.");
+                break;
+        }
+    }
+}
+// The example displays the following output:
+//     An array with 5 elements.
+//     4 items
+//     Null passed to this method.
+// </Snippet1>
+
+