1. Abed El-Azeem Bukhari (MCPD,MCTS and MCP) el-bukhari.com

2. Delegates, Lambdas, and Events Prepared By : Abed ElAzeem Bukhari What ‘s in this chapter? ➤ Delegates ➤ Lambda expressions ➤ Events

3. Delegates private delegate string GetAString(); static void Main() { int x = 40; GetAString firstStringMethod = x.ToString; Console.WriteLine("String is {0}", firstStringMethod ()); firstStringMethod = new GetAString( x.ToString ); Console.WriteLine("String is {0}", firstStringMethod()); }

4. Delegates cont. SimpleDelegate/MathOperations.cs SimpleDelegate/Program.cs

5. Action<T> and Func<T> delegates delegate double DoubleOp(double x); Func<double, double> [] operations = { MathOperations.MultiplyByTwo, MathOperations.Square }; static void ProcessAndDisplayNumber( Func<double, double> action , double value) { double result = action (value); Console.WriteLine( &quot;Value is {0}, result of operation is {1}&quot;, value, result); }

6. BubbleSorter example BubbleSorter/BubbleSorter.cs BubbleSorter/Employee.cs

7. BubbleSorter example bool swapped = true; do { swapped = false; for (int i = 0; i < sortArray.Length — 1; i++) { if (sortArray[i] < sortArray[i+1])) // problem with this test { int temp = sortArray[i]; sortArray[i] = sortArray[i + 1]; sortArray[i + 1] = temp; swapped = true; } } } while (swapped); BubbleSorter/BubbleSorter.cs BubbleSorter/Employee.cs BubbleSorter/Program.cs

8. Multicast delegates Action <double> operations = MathOperations.MultiplyByTwo; operations += MathOperations.Square; MulticastDelegates/Program.cs MulticastDelegates/MathOperations.cs MulticastDelegateWithIteration/Program.cs Action < double > operation1 = MathOperations.MultiplyByTwo; Action < double > operation2 = MathOperations.Square; Action < double > operations = operation1 + operation2; //Multicast delegates also recognize the operators – and - = to remove method calls from the delegate.

9. Multicast delegates cont static void Main() { Action d1 = One; d1 += Two; Delegate[] delegates = d1. GetInvocationList (); foreach (Action d in delegates) { try { d(); } catch (Exception) { Console.WriteLine(&quot;Exception caught&quot;); } } }

10. Anonymous methods AnonymousMethods/Program.cs

11. Lambda Expressions Since C# 3.0, you can use a new syntax for assigning code implementation to delegates: Lambda expressions. Lambda expressions can be used whenever you have a delegate parameter type. The previous example using anonymous methods is changed here to use a Lambda expression. LambdaExpressions/Program.cs

12. Lambda Expressions Parameters Func<string, string> oneParam = s => String.Format( &quot;change uppercase {0}&quot;, s.ToUpper()); Console.WriteLine(oneParam(&quot;test&quot;)); Func<double, double, double> twoParams = (x, y) => x * y; Console.WriteLine(twoParams(3, 2)); Func<double, double, double> twoParamsWithTypes = (double x, double y) => x * y; Console.WriteLine(twoParamsWithTypes(4, 2));

13. Variables outside of the lambda expression int someVal = 5; Func < int, int > f = x => x + someVal; someVal = 7; Console.WriteLine(f(3)); // The result here invoking f(3) is 10

14. Variables outside of the lambda expression cont With the Lambda expression x = > x + someVal the compiler creates an anonymous class that has a constructor to pass the outer variable. The constructor depends on how many variables you access from the outside public class AnonymousClass { private int someVal; public AnonymousClass(int someVal) { this.someVal = someVal; } public int AnonymousMethod(int x) { return x + someVal; } }

15. Variables outside of the lambda expression cont Lambda expressions can be used any place where the type is a delegate. Another use of Lambda expressions is when the type is Expression or Expression <T> . Here the compiler creates an expression tree. This feature is discussed in “ Language Integrated Query. ( LINQ ) Lecture”

16. Events Events are based on delegates and offer a publish/subscribe mechanism to delegates. You can find events everywhere across the framework. In Windows applications, the Button class offers the Click event. This type of event is a delegate. A handler method that is invoked when the Click event is fired needs to be defined, with the parameters as defined by the delegate type.

17. Event Publisher EventsSample/CarDealer.cs The delegate EventHandler < TEventArgs > is defi ned as follows: public delegate void EventHandler <TEventArgs> (object sender, TEventArgs e) where TEventArgs: EventArgs

18. Event Publisher cont private delegate EventHandler <CarInfoEventArgs> newCarInfo; public event EventHandler <CarInfoEventArgs> NewCarInfo { add { newCarInfo += value; } remove { newCarInfo = value; } }

19. Event Publisher cont Before firing the event, it is necessary to check whether the delegate NewCarInfo is not null . If no one subscribed, the delegate is null : public void NewCar(string car) { Console.WriteLine(&quot;CarDealer, new car {0}&quot;, car); if (NewCarInfo != null ) { NewCarInfo(this, new CarInfoEventArgs(car)); } }

20. Event listener EventsSample/Consumer.cs EventsSample/Program.cs

21. Weak events WeakEventsSample/WeakCarInfoEventManager.cs Event Listner WeakEventsSample/Consumer.cs WeakEventsSample/Program.cs

22. Thanks For Attending Abed El-Azeem Bukhari (MCPD,MCTS and MCP) el-bukhari.com