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Hooking Up to a Windows Callback
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The Win32 API sometimes uses callback functions to pass information to the caller asynchronously. The closest analogy to a callback function in C# (and in the CLR) is a delegate, so the runtime interop layer can map a delegate to a callback. Here s an example that does this for the SetConsoleHandler() API (the one used to catch Ctrl+C): using System; using System.Threading; using System.Runtime.InteropServices; class ConsoleCtrl { public enum ConsoleEvent { CTRL_C = 0, // From wincom.h CTRL_BREAK = 1, CTRL_CLOSE = 2, CTRL_LOGOFF = 5, CTRL_SHUTDOWN = 6 }
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CHAPTER 33 INTEROP
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public delegate void ControlEventHandler(ConsoleEvent consoleEvent); public event ControlEventHandler ControlEvent; // save delegate so the GC doesn't collect it. ControlEventHandler eventHandler; public ConsoleCtrl() { // save this to a private var so the GC doesn't collect it... eventHandler = new ControlEventHandler(Handler); SetConsoleCtrlHandler(eventHandler, true); } private void Handler(ConsoleEvent consoleEvent) { if (ControlEvent != null) ControlEvent(consoleEvent); } [DllImport("kernel32.dll")] static extern bool SetConsoleCtrlHandler(ControlEventHandler e, bool add); } class Test { public static void MyHandler(ConsoleCtrl.ConsoleEvent consoleEvent) { Console.WriteLine("Event: {0}", consoleEvent); } public static void Main() { ConsoleCtrl cc = new ConsoleCtrl(); cc.ControlEvent += new ConsoleCtrl.ControlEventHandler(MyHandler); Console.WriteLine("Enter 'E' to exit"); Thread.Sleep(15000); } } The ConsoleCtrl class encapsulates the API function. It defines a delegate that matches the signature of the Win32 callback function and then uses that as the type passed to the Win32 function. It exposes an event that other classes can hook up to. The one subtlety of this example has to do with the following line: ControlEventHandler eventHandler; // sleep 15 seconds
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CHAPTER 33 INTEROP
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This line is required because the interop layer will pass a pointer to the delegate to the Win32 function, but the garbage collector has no way to know that pointer exists. If the delegate isn t stored in a place the garbage collector can find, it s collected the next time the garbage collector runs, which is bad.
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Design Guidelines
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The following sections highlight a few guidelines to help you decide what method of interop to use and how to use it.
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The two options for doing interop with existing C libraries are calling functions directly from C# using platform invoke and using C++/CLI to encapsulate the C functions in a nice managed class written in C++. Which is the better choice depends upon the interface being called. It s easy to tell by how much effort it takes to get it working. If it s straightforward, then doing it in C# is easy. If you find yourself wondering how to do that in C#, or you start using a lot of unsafe code, then it s likely that doing it using the Managed Extensions is a better choice. This is especially true for complex interfaces, where a structure contains pointers to other structures or the sizes of a structure aren t fixed. In such cases, you ll need to do a translation from the C-style way of doing things to the .NET-managed way of doing things. This might involve grabbing a value out of a union or walking through a structure of variable size and then putting the values into the appropriate variable on a collection class. Doing such an operation is a lot easier in C++ than it is in C#, and you likely already have working C++ code on which you can base your C# code.
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Marshalling in C#
When defining functions in C#, consider the following guidelines: In general, the data marshalling layer does the right thing. Choose the type that s closest to the type you want. For opaque types (such as pointers) where all you really care about is the size of the variable, just use an IntPtr. To control data marshalling, use the MarshalAs attribute. This is most often used to control string marshalling. Rather than using a pointer type for a parameter, define it using ref or out. Read about data marshalling in the .NET Framework s developer specifications. If things get ugly, switch to using C++/CLI. Switching to C++/CLI can range from writing a small wrapper that s then called from C# to using C++/CLI for large portions of an application the optimum technique depends on the skills of the developers available.
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