C# for Beginner Part 98 to 100

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Part 98   Anonymous methods in c#

What is an anonymous method?

Anonymous method is a method without a name. Introduced in C# 2.0,they provide us a way of creating delegate instances without having to write a separate method.

class Program    {        static void Main(string[] args)        {            List<Person> persons = new List<Person>() {                 new Person{ID=101,Name="lin1"},                new Person{ID=102,Name="lin2"},                new Person{ID=103,Name="lin3"}            };            Person person = persons.Find(                delegate(Person p)          //this is an anonymous method.                {                    return p.ID == 101;                }                );            Console.WriteLine("person id={0},name={1}", person.ID, person.Name);        }    }    class Person    {        public int ID { get; set; }        public string Name { get; set; }    }

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Another example:Subscribing for an event handler.

public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        private void Form1_Load(object sender, EventArgs e)        {            Button button = new Button();            button.Text = "click me";            this.Controls.Add(button);            //button.Click += button_Click;            button.Click += delegate(object send, EventArgs ea) {                MessageBox.Show("you click me by anonymous method");            };        }        void button_Click(object sender, EventArgs e)        {            MessageBox.Show("you just click me");            throw new NotImplementedException();        }    }

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Part 99   Lambda expression in c#

 class Program    {        static void Main(string[] args)        {            List<Person> persons = new List<Person>() {                 new Person{ID=101,Name="lin1"},                new Person{ID=102,Name="lin2"},                new Person{ID=103,Name="lin3"}            };            Person person = persons.Find(                delegate(Person p)          //this is an anonymous method.                {                    return p.ID == 101;                }                );            Person p1 = persons.Find(p=>p.ID==101);//using lambda expression            Person p2 = persons.Find((Person p)=>p.ID==101);//you can also explicitly the input type but no required            Console.WriteLine("person id={0},name={1}", person.ID, person.Name);        }    }    class Person    {        public int ID { get; set; }        public string Name { get; set; }    }

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=> is called lambda operator and read as Goes To. Notice that with a lambda expression you don‘t have to use the delegate keyword explicitly and don‘t have to specify the input parameter type explicity. The parameter type is inferred(推倒出來). lambda expressions are more convenient to use than anonymous methods. lambda expressions are particularly helpful for writing LINQ query expressions.

In most of the cases lambda expressions supersedes(替代) anonymous methods. To my knowlege, the only time I prefer to use anonymous methods over lambdas is, when we have to omit(省略) the parameter list when it‘s not used within the body.

Anonymous methods allow the parameter list to be omitted entirely when it‘s not used within the body,where as with lambda expressions this is not the case.

For example, with anonymous method notice that we have omitted the parameter list as we are not using them within the body

Button.Click += delegate{MessageBox.Show("hello world.");};

The above code can be rewritten using lambda expression as shown below.Notice that with lambda we cannot omit the parameter list.

Button.Click+=(sender,e)=>{MessegeBox.Show("hello world.");};Button.Click+=()=>{MessegeBox.Show("hello world.");};//if omit parameter list it will get a compilar error.

Part 100   Func delegate in c#

What is Func<T,TResult> in C#?

In simple terms,Func<T,TResult> is just generic delegate. Depending on the requirement,the type parameters(T and TResult) can be replaced with the corresponding(對應的) type arguments.

For example,Func<Employee,string> is a delegate that represents(代表) a function expecting(期待) Employee object as an input parameter and returns a string.

class Program    {        static void Main(string[] args)        {            List<Employee> employees = new List<Employee>() {                 new Employee{ID=101,Name="lin1"},                new Employee{ID=102,Name="lin2"},                new Employee{ID=103,Name="lin3"}            };            //Func<Employee, string> selector = e => "name=" + e.Name;            IEnumerable<string> employeeNames = employees.Select(e => "name=" + e.Name);            foreach (string name in employeeNames)            {                Console.WriteLine(name);            }        }    }    class Employee    {        public int ID { get; set; }        public string Name { get; set; }    }

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What is the difference between Func delegate and lambda expression?

They‘re the same, just two different ways to write the same thing. The lambda syntax is newer, more concise(簡潔) and easy to write.

What if I have to pass two or more input parameters?

As of this recording there are 17 overloaded versions of Func, which enables us to pass variable number and type of input parameters. In the example below, Func<int,int,string> represents a function that expects 2 int input parameters and returns a string.

class Program    {        static void Main(string[] args)        {                       Func<int, int, string> funcDelegate = (num1, num2) => (num1 + num2).ToString();            string result = funcDelegate(10,20);            Console.WriteLine("sum="+result);        }    }

Finally, I completely learning the  C# language, go to bed, good night guy.

C# for Beginner Part 98 to 100