1. Chengyu Sun California State University, Los Angeles
CS4540 Special Topics in Web Development C# for Java Programmers: Advanced Language Features
Chengyu Sun
California State University, Los Angeles

2. Overview Indexers Extension methods Delegates Anonymous methods
Lambda expressions
event

3. Access List Elements By Index
List<string> translations = ….
for( int i=0 ; i < translations.Count ; ++i )
Console.WriteLine("{0}. {1}", i+1, transactions[i]);
List elements can be accessed like array elements
Where does it say this in the List API documentation??

4. Indexer Syntax public T this[int index] { get; set; }
A type parameter for a generic collection,
or a regular type for a non-generic one

5. About Indexer It's a property backed by a getter and/or a setter
The type of index doesn't have to be int
Internally the index property has a name which can be used by other languages
Default name Item
Can be customized with an IndexerName attribute
Some classes that have indexers: List, Dictionary, String

6. Add Methods To A Class
Example: add a method Substring(string a, string b) to String which returns the substring that starts with a and ends with b, e.g. "Amazon".Substring("a","z")  "Amaz"
How can we do this when String is a sealed class (like a final class in Java) which can not be in inherited

7. An Extension Method public static class MyExtensions {
public static string Substring(
this string s,
string begin,
string end)
{ … } }
var word = "Amazon";
Console.WriteLine( word.Substring("a", "z") );

8. About Extension Methods
Static methods invoked using instance method syntax
Must be defined in a non-generic, static class
Use with caution – static, utility methods are not replacements for proper OO design & implementation

9. The Need for Function (or Method) References
GUI programming
E.g. button click
Asynchronous programming
E.g. Ajax request
Some object has a field/property that is of function type
The field/property is assigned a function (a.k.a. event handler or callback function)
The function will be called when something happens

10. Function References in Some Language
C and C++ have function pointers
In JavaScript, functions are objects (i.e. no special treatment needed)
What's about Java?
No function references
Use objects that implement certain interfaces (e.g. ActionListener)  event handler/callback function must have a certain name (in addition to parameters and return types)

11. A C# Delegate // Define a delegate type that represents references to
// methods with a particular parameter list and return type
public delegate int BinaryOp(int op1, int op2);
// A method
static int Add(int a, int b) => a + b;
// Create a delegate object that references the method
BinaryOp bop = new BinaryOp(Program.Add);
// Call the method
int sum = bop(10, 20);

12. Multicast
C# delegates support multicast, i.e. references multiple methods and invoke them together
int Add2(int x, int y) => x + y;
bop += (new Program()).Add2;
Overloaded += operator that adds a method to
the list of methods referenced by the delegate

13. Simplify Use of Delegates
Generic delegates for methods take up to 16 arguments
Action<> for methods that return void
Func<> for methods that return a value
Method group conversion allows providing a method in place of a delegate

14. Anonymous Method Can access variables outside the method int c = 100;
BinaryOp bop = delegate(int a, int b) { return a+b+c; };
Must have a ; after the method body

15. Lambda Expression
(parameter list) => { statements; }
The simpler and preferred way to write anonymous methods, e.g.
int c = 100;
BinaryOp bop = (int a, int b) => { return a+b+c; };

16. Syntactical Variations of Lambda Expression
Action<> line = () => { Console.WriteLine(); };
() => Console.WriteLine();
Func<int,int> square = (int x) => { return x*x; };
(int x) => x*x;
(x) => x*x;
x => x*x;

17. Closure Example in JavaScript
function foo(){
let value=10;
return () => {console.log(value++);} }
var bar1 = foo();
var bar2 = foo();
bar1(); // ??
bar2(); // ??

18. Closure in C#
Like functions in JavaScript, anonymous methods and lambda expression in C# form closures, i.e. the combination of a function and the lexical environment within which that function was declared
Example??

19. Delegate Example: List.FindAll()
Use List.FindAll() to find all the people whose last name is Doe

20. Event Handling Example
Register two event handlers for the text entered event of MockTextField
1. Using regular delegate
The event handlers object must be private to prevent client code messing it up
2. Using event
The compiler automatically provides registration and unregistration methods and guard against undesirable actions

21. Readings
Pro C# 7: Chapter 10, 11