delegate(代理，委托)

下面的例子描述了delegate的声明，初始化和调用：

public partial class Form1 : Form

{

// declares a delegate, it has one parameter.

delegate void help(string message);

public Form1()

{

InitializeComponent();

// anonymous method.

this.MouseClick += delegate(object sender, MouseEventArgs e)

{

MessageBox.Show("hello world");

};

}

private void Form1_Load(object sender, EventArgs e)

{

// 2 ways to initialize the delegate

// 1.decides what to do with this delegate, here using a named method "ShowMessage".

help h1 = new help(Form1.ShowMessage);   // also we can write it as "help h1 = Form1.ShowMessage", they are the same meaning.

// 2.decides what to do with this delegate, here using a anonymous method.

help h2 = delegate(string message) { MessageBox.Show(message); };

// 2 ways to call the delegate

// 1.call the delegate

h1.Invoke("message from delegate1");

// 2.call the delegate

h2("message from delegate2");

}

private static void ShowMessage(string message)

{

MessageBox.Show(message);

}

}
接下来总结一下常用的事件的注册方法，这里我列出了四种方式：

    public partial class Form1 : Form
    {
        // 1. delegate
        public delegate void TestEvent1Handler(object sender, EventArgs e);
        public event TestEvent1Handler TestEvent1;

        // 2. delegate with parameter
        public delegate void TestEvent2Handler(string para);
        public event TestEvent2Handler TestEvent2;

        // 3. EventHandler
        public event EventHandler TestEvent3;

        // 4. EventHandler with customized eventargs
        public event EventHandler<MyEventArgs> TestEvent4;


        public Form1()
        {
            InitializeComponent();
        }


        private void Form1_Load(object sender, EventArgs e)
        {
            // 1.
            if (TestEvent1 != null)
                TestEvent1(sender, e);
            // 2. 
            if (TestEvent2 != null)
                TestEvent2("new name from 2");
            // 3.
            if (TestEvent3 != null)
                TestEvent3(sender, e);
            // 4.
            if (TestEvent4 != null)
                TestEvent4(this, new MyEventArgs("new name from 4"));
        }
    }

    /// <summary>
    /// Customized EventArgs.
    /// </summary>
    public class MyEventArgs : EventArgs
    {
        string name;
        public MyEventArgs(string name)
        {
            this.name = name;
        }
        public string Name { get { return name; } }
    }

注册这些事件如下：

class Class1
    {
        public Class1()
        {
            Form1 form = new Form1();
            form.TestEvent1 += new Form1.TestEvent1Handler(form_TestEvent1);
            form.TestEvent2 += new Form1.TestEvent2Handler(form_TestEvent2);
            form.TestEvent3 += new EventHandler(form_TestEvent3);
            form.TestEvent4 += new EventHandler<MyEventArgs>(form_TestEvent4);
            form.Show();
        }

        void form_TestEvent4(object sender, MyEventArgs e)
        {
            MessageBox.Show("Test event 4: " + e.Name);
        }

        void form_TestEvent3(object sender, EventArgs e)
        {
            MessageBox.Show("Test event 3");
        }

        void form_TestEvent2(string para)
        {
            MessageBox.Show("Test event 2: " + para);
        }

        void form_TestEvent1(object sender, EventArgs e)
        {
            MessageBox.Show("Test event 1");
        }
    }

下面介绍三个常用的delegate：Func，Action和Predicate

1. Func<T,TResult> delegate:

语法：public delegate TResult Func<T, TResult>(T arg);

用它封装一个方法，该方法只有一个参数，T是参数的类型，TResult是返回值的类型。

例如：

       private void Test()
        {
            // 1. normal
            Func<string, string> d1 = ToUpper;
            // 2. anonymous method
            Func<string, string> d2 = delegate(string s) { return s.ToUpper(); };
            // 3. lambda expression
            Func<string, string> d3 = s => { return s.ToUpper(); };
            Func<string, string> d4 = s => s.ToUpper();

            Console.WriteLine(d1("china"));
            Console.WriteLine(d2("china"));
            Console.WriteLine(d3("china"));
            Console.WriteLine(d4("china"));
        }

        string ToUpper(string s)
        {
            return s.ToUpper();
        }

2. Action<T>, Action<T1, T2>，Action 等等
 它封装一个方法，带有一个或多个参数，但是没有返回值。

 它与Func的区别就是Func有返回值，Action无返回值。

3. Predicate<T>

predicate的意思是断定为，预言成..., 它封装了一个方法，带有一个参数，返回值是bool型。