C#入门(3)

Delegates, Events, Lambda Expressions

最早的windows是使用c风格的函数指针来进行callback的,但是这样仅仅传递了一个内存中的地址,无法包含更多的信息,如类型安全等,而且容易引发crash.

所以.Net平台下的delegate可以指向静态方法或者是对象方法,一般包含三种重要信息:

需要调用的方法的地址
方法的参数
方法的返回值

代理可以支持同步方法和异步方法:
对于一个delegate,首先编译器会生成一个seal的类.具体如下:

public delegate string MyDelegate(bool a, bool b, bool c);

sealed class MyDelegate : System.MulticastDelegate
{
	public string Invoke(bool a, bool b, bool c);
	public IAsyncResult BeginInvoke(bool a, bool b, bool c,AsyncCallback cb, object state);
	public string EndInvoke(IAsyncResult result);
}

// This is only pseudo-code!
public sealed class DelegateName : System.MulticastDelegate
{
	public delegateReturnValue Invoke(allDelegateInputRefAndOutParams);
	public IAsyncResult BeginInvoke(allDelegateInputRefAndOutParams,AsyncCallback cb, object state);
	public delegateReturnValue EndInvoke(allDelegateRefAndOutParams,IAsyncResult result);
}

在生成的类中的三个方法中,Invoke()是负责同步(synchronized)的,该方法在使用正确的语法时,不需要显式调用.而剩下的两个BeginInvoke()和EndInvoke()是负责异步的(asynchronized),注意这里的BeginInvoke()后面会自动增加两个参数,一个是AsyncCallback,一个是object;而EndInvoke()的返回值则是delegate的返回值,参数则是BeginInvoke()的返回值.但是当使用out或者ref修饰的参数时,这样的参数也会传递给EndInvoke().

所以使用delegate声明,其实是继承了System.MulticastDelegate类和System.Delegate类.但是在程序中不能够显式继承自这两个类.下面给出了这两个类的部分:

public abstract class MulticastDelegate : Delegate
{
	// Returns the list of methods "pointed to."

	public sealed override Delegate[] GetInvocationList();
	// Overloaded operators.
	public static bool operator ==(MulticastDelegate d1, MulticastDelegate d2);
	public static bool operator !=(MulticastDelegate d1, MulticastDelegate d2);

    // Used internally to manage the list of methods maintained by the delegate.
	private IntPtr _invocationCount;
	private object _invocationList;
}

public abstract class Delegate : ICloneable, ISerializable
{
	// Methods to interact with the list of functions.
	public static Delegate Combine(params Delegate[] delegates);
	public static Delegate Combine(Delegate a, Delegate b);
	public static Delegate Remove(Delegate source, Delegate value);
	public static Delegate RemoveAll(Delegate source, Delegate value);
	// Overloaded operators.
	public static bool operator ==(Delegate d1, Delegate d2);
	public static bool operator !=(Delegate d1, Delegate d2);

	// Properties that expose the delegate target.
	public MethodInfo Method { get; }
	public object Target { get; }
}

下表给出具体属性的含义:

Member	Meaning
Method	This property returns a System.Reflection.MethodInfo object that represents details of a static method maintained by the delegate.
Target	If the method to be called is defined at the object level (rather than a static method), Target returns an object that represents the method maintained by the delegate. If the value returned from Target equals null, the method to be called is a static member.
Combine()	This static method adds a method to the list maintained by the delegate. In C#, you trigger this method using the overloaded += operator as a shorthand notation.
GetInvocationList()	This method returns an array of System.Delegate objects, each representing a particular method that may be invoked.
Remove() RemoveAll()	These static methods remove a method (or all methods) from the delegate’s invocation list. In C#, the Remove() method can be called indirectly using the overloaded -= operator.

Combine()是当给delegate增加回调函数时用的,调用+=会隐式调用Combine(),

使用delegates:
一般使用delegates的步骤为:

定义一种新的delegates类型,来支持回调
在使用delegates的类中定义一个该delegates的成员变量,可以是private也可以是public
定义一个辅助函数来使得调用者可以方便的调用回调
实现函数,在其中正确地调用delegates变量

Tips:

在c#中的界面开发中，由于父级控件的属性设置对于子级控件是有影响的，因此对于多级嵌套的层级控件，必须定义好add的顺序，先将后继控件加载然后加入到前级控件和先加载自身，然后加载后继控件是不同的。A-1-B-2-C和A-2-B-1-C，如这个表达式，ABC依次为嵌套关系。数字代表加载顺序。

这里的关键因素是一个属性,一般是tabControl或者是tabPage有的,叫AutoScaleMode,默认的是根据Font来进行改变,因此当父级元素和子级元素中的Font设置不一样,甚至是设置的一样,但是加入时仍然会按照字体的大小进行缩放处理.所以将该属性设为none可以避免不必要的麻烦.
这个Tips如果算是Bug,应该解了.