Getting the Most from Lambda Expressions

2. Getting the Most from Lambda Expressions Deborah Kurata President InStep Technologies, Inc Session Code: DTL315

3. Deborah Kurata is... • Consultant and President of:InStep Technologies, Inc. • Author of: • Doing Objects in VB 2005 • Best Kept Secrets in .NET • Software designer/developer • INETA Speaker • Microsoft MVP • DeborahK@insteptech.com

4. InStep Technologies is... • Consulting • Mentoring services to get you started and keep you going with .NET • Strategic software consulting and training services • Application architecture and design • Custom software development of Windows and Web-based applications • Web site: www.insteptech.com

5. You Are... • Using Generic Lists • List(Of Customer) • List<Customer> • Using Lambda Expressions • Just getting started/new to them • Use them once in a while • Use them everyday

6. This Presentation is … • Fun with Lambda Expressions • What are Lambda Expressions? • Lambdas as Generic Delegates • Lambdas as Callable Entities • Lambdas as Callbacks

7. Generic Lists • Class Customer • Manages a single customer • Customer properties • Customer methods • Class Customers • Manages the list of customers • List(Of Customer) • List<Customer>

8. demo Fun With Lambda Expressions

9. Lambda Calculus • “Formal system designed to investigate function definition, function application, and recursion.” • Wikipedia

10. What are Lambda Expressions? • New in Visual Basic 9 and C# 3.0 (VS 2008) • Unnamed, inline functions • Single statement • Multiple statement (C# only; coming in VB 10) • Used wherever delegates are required

11. What is a Delegate? • An object that holds a reference to a method with a particular parameter list and return type • Like object-oriented, type-safe function pointer • Make it possible to treat methods as entities • Assign to variables • Pass as parameters

12. Classic Delegate Example: Events // Passes the method to the new delegate HelloButton.Click += new EventHandler(HelloButton_Click); ' AddressOf assigns the address of the ' method to the delegate AddHandler HelloButton.Click, _ AddressOf HelloButton_Click

13. Using Lambda Expressions // C# HelloButton.Click += (s, ev) => MessageBox.Show("Hello World"); ' VB: AddHandler HelloButton.Click, _ Function(s, ev) _ MessageBox.Show("Hello World")

14. Defining Delegates • Named method • Anonymous method (C# 2.0) • Lambda expression (C# 3.0 and VB 9)

15. Lambda Expression Syntax - VB ' General syntax: Dim foundCustomer as Customer = _ allCust.First(Function(c as Customer) _ c.CustomerId = 4) ' With inferred typing: Dim foundCustomer = _ allCust.First(Function(c) _ c.CustomerId = 4)

16. Lambda Expression Syntax – C# ' General syntax: Customer foundCustomer = allCust.First((Customer c) => c.CustomerId == 4); ' With inferred typing: varfoundCustomer = allCust.First(c => c.CustomerId == 4);

17. Lambda Multiline Syntax – C# var foundCustomer = allCust.First(c => { Debug.WriteLine(c.FullName); if (c.CustomerId == 4) return true; else return false; });

18. Features that Use Delegates • Event handlers • Enumerable class methods • Extension methods of IEnumerable(T) • Examples: Aggregate, All, Any, FirstOrDefault • LINQ methods • Used by LINQ • Examples: OrderBy, Where, GroupBy

19. Lambdas as Generic Delegates • Generic delegates were new in .NET 2.0 • Predicate Delegates • Action Delegates • Func Delegates

20. Predicate Delegate • Predicate: “An operator or function which returns a Boolean value” - Wikipedia • Encapsulates a method that evaluates to True or False • Takes one parameter

21. Example: Predicate Delegate ' VB: Dim foundCustomer = _ Array.Find(custArray, Function(c) _ c.LastName.StartsWith("K")) // C#: varfoundCustomer = Array.Find(custArray, c => c.LastName.StartsWith("K"));

22. demo Predicate Delegates

23. Action Delegate • Encapsulates a method that does not return a value • Sub in VB • void in C# • Takes up to four parameters

24. Example: Action Delegate ' VB: Can only use named methods allCust.ForEach(AddressOfWriteToDebug) // C#: allCust.ForEach(c => Debug.WriteLine(c.FullName));

25. demo Action Delegates

26. Func Delegate • Encapsulates a method that returns a value • Takes up to 4 parameters and a return value • Last parameter is the return value • Func<int, int, string> • Func(Of Integer, Integer, String)

27. Example: Func Delegate ' VB: Dim total = allCust.Sum(Function(c) _ c.SalesTotal) // C#: var total = allCust.Sum(c => c.SalesTotal);

28. demo Func Delegates

29. Lambdas as Callable Entities • Lambda expressions can be assigned to a delegate variable • Lambda expression is executed when the delegate is executed

30. Example: Assignment ' VB: Infers the type Dim f = _ Function(x as Integer) (x + x).ToString() Debug.WriteLine(f(5)) Debug.WriteLine(f(10)) // C#: Func<int,string> f = x => (x + x).ToString(); Debug.WriteLine(f(5)); Debug.WriteLine(f(10));

31. Lambda Expression Execution • Lambda expressions are executed when they are called, not when they are constructed • Local variables used in a lambda expression are “captured” or “lifted” • Variable value used is the value at execution time • Variable lifetime extends to the lifetime of the delegate

32. Example: Local Variables // C#: int y = 0; Func<int,string> f = x => (x + y).ToString(); y = 10; Debug.WriteLine(f(5)); ' VB: Dim y As Integer = 0 Dim f As Func(Of Integer, String) = _ Function(x) (x + y).ToString() y = 10 Debug.WriteLine(f(5))

33. demo Callable Entities

34. Lambdas as Callbacks • Callback • “Executable code that is passed as an argument to other code” • Passes a function to a function • A delegate does not know or care about the class of the method it is referencing • No need to reference the class that defined the method

35. demo Callbacks

36. Getting the Most from Lambdas • Use Lambda expressions for • Writing inline functions • Calling LINQ methods • Calling extension methods that take a delegate • Creating callable functions • Performing callbacks

37. Resources • www.microsoft.com/teched Sessions On-Demand & Community • www.microsoft.com/learning • Microsoft Certification & Training Resources • http://microsoft.com/technet • Resources for IT Professionals • http://microsoft.com/msdn Resources for Developers www.microsoft.com/learning Microsoft Certification and Training Resources

38. Related Content DTL336 Future Directions for Visual Basic Presenter(s): Jonathan Aneja, Anders Hejlsberg DTL402 How LINQ Works: A Deep Dive into the Microsoft Visual Basic and C# Implementations Presenter: Jonathan Aneja

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