optimise files

CS/15_5/Program.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Text;
+
+namespace _15_5
+{
+    internal class Program
+    {
+        static void Main(string[] args)
+        {
+            StringBuilder sb = new StringBuilder("ABC", 50);
+            sb.Append(new char[] { 'D', 'F', 'E' });
+            sb.AppendFormat("GHI{0}{1}", 'J', 'k');
+            Console.WriteLine("{0} chars,内容为{1}", sb.Length, sb.ToString());
+            sb.Insert(0, "Alphabet---");
+            sb.Replace('k', 'K');
+            Console.WriteLine("{0} chars,内容为{1}", sb.Length, sb.ToString());
+        }
+    }
+}