C# In Depth: Chapter 6

diff --git a/CSharpInDepth/Chapter6/Chapter6/Main.cs b/CSharpInDepth/Chapter6/Chapter6/Main.cs
index 7212898..b156a6c 100644 (file)
--- a/CSharpInDepth/Chapter6/Chapter6/Main.cs
+++ b/CSharpInDepth/Chapter6/Chapter6/Main.cs
@@ -94,15 +94,6 @@ namespace Chapter6
 
                        /**
                         * 
-                        * Listings 6.8 Looping over the lines in a file using an iterator block.
-                        */
-                       foreach (string line in ReadLines ("TheLayofLeithian.txt")) {
-                               Console.WriteLine(line);
-                       }
-
-
-                       /**
-                        * 
                         * Listings 6.7 Demonstration of yield break working with try/finally.
                         */
                        DateTime stopFinally = DateTime.Now.AddSeconds (2);
@@ -110,10 +101,39 @@ namespace Chapter6
                                Console.WriteLine ("Received {0}", i);
                                if (i > 3) {
                                        Console.WriteLine ("Returning");
-                                       return;
+                                       // foreach must have a finally block, which calls Dispose in the iterator.
+                                       // When you call Dispose on an iterator created with an iterator block
+                                       // before it's finished iterating the state machine executes any finally
+                                       // blocks that are in the scope where the code is currently "paused".
+                                       // So this will call iterator's Dispose->will run finally (try{ yield return }finally{})
+                                       // If we have more than one block try{ yield return }finally{} in our iterator, Dispose
+                                       // will run the finally in the scope of the latest executed yield return.
+                                       //return;
+                                       break;
                                }
                                Thread.Sleep (300);
                        }
+
+
+                       /**
+                        * 
+                        * Listings 6.8 Looping over the lines in a file using an iterator block.
+                        */
+                       foreach (string line in ReadLines ("TheLayofLeithian.txt")) {
+                               Console.WriteLine (line);
+                       }
+
+                       /**
+                        * 
+                        * Listings 6.9 Implementing LINQ's Where method using iterator blocks.
+                        */
+                       IEnumerable<string> lines = ReadLines ("TheLayofLeithian.txt");
+                       Predicate<string> predicate = delegate(string line) {
+                               return line.StartsWith ("before");
+                       };
+                       foreach (string line in Where (lines, predicate)) {
+                               Console.WriteLine(line);
+                       }
                }
 
                static IEnumerable<int> CreateEnumerable ()
@@ -185,6 +205,25 @@ namespace Chapter6
                                }
                        }
                }
+
+               public static IEnumerable<T> Where<T> (IEnumerable<T> source, Predicate<T> predicate)
+               {
+                       if (source == null || predicate == null) {
+                               throw new ArgumentNullException();
+                       }
+                       return WhereImpl(source, predicate);
+               }
+
+               private static IEnumerable<T> WhereImpl<T> (IEnumerable<T> source, Predicate<T> predicate)
+               {
+                       foreach (T item in source)
+                       {
+                               if (predicate(item))
+                               {
+                                       yield return item;
+                               }
+                       }
+               }
        }