1. Delegates and lambda functions Jim Warren, jim@cs.auckland.ac.nz COMPSCI 280 S2 2015 Enterprise Software Development

2. Today’s learning objectives  To understand and be able to apply C# language features in a variety of contexts for  Development of delegates  Use of lambda functions COMPSCI 2802

3. Delegates COMPSCI2803  Delegates are object-oriented, type-safe, and secure function pointers  A delegate is a reference type that defines a method signature  A delegate instance holds one or more methods  Methods can be static or non-static  Delegates allow methods to be passed as parameters.  Delegates can be used to define callback methods.  Delegates can be chained together; for example, multiple methods can be called on a single event.  It is a foundation of event handling. BOSS Job Finished??? Call back when Finished

4. Creating Delegates COMPSCI2804  The type of a delegate is defined by the name of the delegate  Each delegate is limited to referencing methods of a particular kind only. The type is indicated by the delegate declaration – the input parameters and return type  Example:  Two methods with the same signature as the delegate declared above  Creating a New Delegate Object  Use the new operator  The argument is the method call, but without the arguments to the method  Delegate objects are immutable. (can’t change)  A delegate can reference static or instance method public delegate void Print(string s); public static void printToLower (string s) { Console.WriteLine("static: " + s.ToLower()); } public void printToUpper (string s) { Console.WriteLine("instance: " + s.ToUpper()); } Print v1 = new Print(printToLower); Print v2 = new Print(new Program().printToUpper ); Static method

5. Using Delegates COMPSCI2805  Once a delegate is instantiated, a method call made to the delegate will be passed by the delegate to that method and will call the underlying method  Use the name of the delegate, followed by the parenthesized arguments to be passed to the delegate v1("This is a test"); v2("This is a test"); Static method output: static: this is a test output: instance: THIS IS A TEST

6. Using Arrays COMPSCI2806  Create an array of delegate objects  Instantiate each delegate object with various instance methods defined above  Note: In C#, if we reference a method on an object (omitting the parentheses), C# instead treats the method name like a field, returning the object representing that method.  Call each delegate object by using a loop -> invoke the underlying method Print[] arr = new Print[2]; arr[0] = new Print(printToLower); arr[1] = new Print(new Program().printToUpper); foreach (Print p in arr) p("In an array"); public delegate void Print(string s); static: in an array instance: IN AN ARRAY

7. Named & Anonymous methods COMPSCI2807  Named Method  A delegate can be associated with a named method.  When you instantiate a delegate using a named method, the method is passed as a parameter  The method that you pass as a delegate parameter must have the same signature as the delegate declaration.  A delegate instance may encapsulate either a static or an instance method.  Anonymous Method  In C# 2.0 (from 2005), you can declare a delegate using an anonymous method public delegate void Print(string s); Print v1 = printToLower; v1("This is another test"); Print v2 = new Program().printToUpper; v2("This is another test"); Named method Print a1 = delegate(string j){ Console.WriteLine("Anonymous:" + j); }; a1("Well done"); Anonymous method

8. Multicast delegates COMPSCI2808  Delegate objects can be assigned to one delegate instance to be multicast using the + operator.  A delegate can simultaneously reference multiple methods, and it will invoke all underlying methods  Methods are invoked sequentially, in the order added.  Conditions:  Only delegates of the same type can be composed.  The - operator can be used to remove a component delegate from a composed delegate. public delegate void Print(string s); Print mp2 = null; mp2 += printToLower; mp2 += new Program().printToUpper; mp2("Hello World"); Output: static: hello world Instance: HELLO WORLD A delegate object

9. Event COMPSCI2809  Event Basic:  An event is a message sent by an object to signal the occurrence of an action.  The action could be caused by user interaction, such as a mouse click, or it could be triggered by some other program logic.  The object that raises the event is called the event sender.  The object that captures the event and responds to it is called the event receiver.  The event sender class doesn’t know which object or method will receive (handle) the events it raises. What is needed is an intermediary (or pointer- like mechanism, delegate) between the source and the receiver.  An event is a member of a delegate type that enables an object or class to provide notifications. SenderReceiver Delegate

10. Example: COMPSCI28010  Declare a delegate that takes two parameters:  the source that raised the event, and  the data for the event  In the Button class (e.g. in a Windows Forms Application)  Defines a click event of type EventHandler  Inside the Button class, the click member is exactly like a field of type EventHandler  Outside the Button class, the click member can only be used on the left- hand side of the += and -= operators.  The += operator adds a handler for the event and -= removes a handler for the event public delegate void EventHandler(object sender, System.EventArgs e); public class Button { public event EventHandler Click; public void Reset() { Click = null; } …

11. Example: COMPSCI28011  In the Form1 class (Windows application)  Create a button object  Create a event handler method  Connect the event handler method to the click event of the button object  You can also reuse the same method for multiple events public Form1() {... button1.Click += new EventHandler(button1_Click); } public void button1_Click(object sender, EventArgs e){ Console.WriteLine("Clicked"); } Event handler method button2.Click += new EventHandler(button1_Click); button3.Click += new EventHandler(button1_Click); A lot of this is set up for you when your create a new Windows Forms Application in VS and drag a button onto it.

12. Lambda expressions  A lambda expression is  An anonymous function used to create delegates or expression tree* types  Denoted with =>  Can pronounce as ‘maps to’, ‘such that’ or ‘for which’  Same order of operations precedence as assignment with =  Lambda expressions let you write local functions that can be passed as arguments or returned as the value of function calls  Particularly helpful for writing LINQ query expressions  You can assign a lambda expression to a delegate type * See next slide COMPSCI 28012 Borrowing liberally from http://msdn.microsoft.com/en- us/library/bb397687.aspx

13. Expression trees COMPSCI 28013 (a+b)*c+7 http://commons.wikimedia.org/wiki/File:Exp- tree-ex-11.svg  Holding an expression in a dynamic structure  For C# this is as a nested series of objects  E.g. a binary operator like + can have Left and Right terms, each of which themselves might be variables, constants or other operators  A lambda expression is just ‘syntactic sugar’  It’s an easier way to write stuff that the compiler turns into delegate definitions or expression trees  Great further explanation at https://www.youtube.com/watch?v=P60pt5xlms0 and https://www.youtube.com/watch?v=0YJECE45jhk  Author Jamie King almost sounds drunk, but I think that’s just his natural speaking style

14. Syntactic sugar  So as Jamie King illustrates on YouTube Func test= i => i > 5;  Is equivalent to static bool qwerty(int i) { return i > 5; } and then Func test=qwerty;  Either way, you can do  Console.Writeline(test(3)); Console.Writeline(test(8)); Handout 07COMPSCI 28014 False True

15. Definition  Expression lambda  In general looks like (x, y) => x==y  When there’s only one parameter you can skip the parentheses  Statement lambda  Statements in curly braces for the right-hand side of the lambda  The above example is a function of type void, but you could include one or more return statements in the code, too  E.g. to perform a logical test for a bool delegate or compute a number for an int delegate COMPSCI 28015

16. Use in queries  Very convenient in interaction with LINQ  The type of the first input (e.g. the ‘n’ above) is inferred to be the type of the elements in the source sequence  So if we are using a database connection we may have the result of referencing a table or from a query, such as IEnumerable (iterator on a set of Customer instances)  And then we can do a query like: customers.Where(c => c.City == "London");  In this case the ‘c’ has access to the Customer object’s properties (e.g. the City property, which might’ve been sourced from a field of that name in a customer table under MySQL) COMPSCI 28016 Using an array as the data source for the query in this case Read “Customers, c, such that c’s city is London”

17. Conclusion  The ability to have variables reference functions is powerful  Allows us to achieve a high degree of modularity between a ‘supervisor’ procedure and the procedures delegated to do subtasks  Provides a powerful framework for event handling  Useful for many things, including responding to user actions in a Graphical User Interface (GUI)  Lambda expressions are a convenient syntax for plugging anonymous functions into your code  Useful for many things including implementing query criteria  Keep (or get!) going with assignment 2  Next – we’ll look at further useful VS and C# features COMPSCI 28017