Added 01_filesystem_dfs for 07_trees in C# (#299)

07_trees/csharp/01_filesystem_dfs/Program.cs

@@ -0,0 +1,37 @@
+using System;
+using System.IO;
+using System.Linq;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            string dir = Path.Combine(Directory.GetCurrentDirectory(), "pics");
+            
+            PrintNames(dir);
+        }
+
+        private static void PrintNames(string dir)
+        {
+            var filesAndDirs = Directory.GetFileSystemEntries(dir).OrderBy(f => f, StringComparer.OrdinalIgnoreCase);
+
+            // loop through every file and folder in the current folder
+            foreach (var path in filesAndDirs)
+            {
+                if (File.Exists(path))
+                { 
+                    // if it is a file, print out the name
+                    Console.WriteLine(Path.GetFileName(path));
+                }
+                else
+                { 
+                    // if it is a folder, call this function recursively on it
+                    // to look for files and folders
+                    PrintNames(path);
+                }
+            }
+        }
+    }
+}