1. call, callback, delegate, event handler是什么关系？

  1. FuncA call FuncB ,顺序执行
  2. FuncA call FuncB with callback 参数Func，执行完后，返回FuncA
  3. delegate是用于封装 命名或匿名方法的引用类型，类似于C++中的函数指针，但却是类型安全的。
  4. event handler 是callback的一种：主框架 =》事件 =》事件处理函数 =》返回主框架

回调通常与原始调用者处于相同的抽象层

回调通常与原始调用者处于相同的抽象层

2. callback到底是什么？

Callbacks are extensibility points that allow a framework to call back into user code through a delegate. These delegates are usually passed to the framework through a parameter of a method.

callback 允许框架通过一个delegate 返回到用户代码中。

Events are a special case of callbacks that supports convenient and consistent syntax for supplying the delegate (an event handler). In addition, Visual Studio’s statement completion and designers provide help in using event-based APIs.

✓ CONSIDER using callbacks to allow users to provide custom code to be executed by the framework.

✓ CONSIDER using events to allow users to customize the behavior of a framework without the need for understanding object-oriented design.

✓ DO prefer events over plain callbacks, because they are more familiar to a broader range of developers and are integrated with Visual Studio statement completion.

X AVOID using callbacks in performance-sensitive APIs.

✓ DO use the new Func<...>, Action<...>, or Expression<...> types instead of custom delegates, when defining APIs with callbacks.

Func<...> and Action<...> represent generic delegates.

Expression<...> represents function definitions that can be compiled and subsequently invoked at runtime but can also be serialized and passed to remote processes.

✓ DO measure and understand performance implications of using Expression<...>, instead of using Func<...> and Action<...> delegates.

Expression<...> types are in most cases logically equivalent to Func<...> and Action<...> delegates.

The main difference between them is that the delegates are intended to be used in local process scenarios; expressions are intended for cases where it’s beneficial and possible to evaluate the expression in a remote process or machine.

✓ DO understand that by calling a delegate, you are executing arbitrary code and that could have security, correctness, and compatibility repercussions.

3. P/Invoke是什么，怎么用？

Platform Invocation Services (PInvoke) allows managed code to call unmanaged functions that are implemented in a DLL.

平台调用服务允许托管代码去调用dll中实现的非托管函数。

  1. How to: Implement Callback Functions
  2. EnumWindows function
  3. EnumWindowsProc callback function
public delegate bool CallBack(int hwnd, int lParam);

public class EnumReportApp
{
    [DllImport("user32")]
    public static extern int EnumWindows(CallBack x, int y);

    public static void Main()
    {
        CallBack myCallBack = new CallBack(EnumReportApp.Report);
        EnumWindows(myCallBack, 1024);
        // 1024这个参数经EnumWindows这个函数输入，
        // 再经EnumWindows传送到callback中，
        // callBack就可以处理
        // callBack返回值，传送到EnumWindows中，false结束枚举窗口dll函数
        // 代码执行桟==>重新回到Main函数中
        Console.Read();

        //执行流程：
        // Main函数 --> Dll函数 --> CallBack函数 -->Main函数
    }

    public static bool Report(int hwnd, int lParam)
    {
        Console.Write("Window handle is ");
        Console.WriteLine(hwnd+ "\t\t\t\t\t\t" + lParam);
        if (hwnd > 100000)
            return false;
        return true;
    }
}