MSDN Magazine - January 2008 - (Page 24)

Guerrilla .NET February 4th - Los Angeles Guerrilla Connected Systems March 17th - Los Angeles Call 800 699 1932 to register today! or visit www.develop.com New - Hands-on .NET 3.0 Training What’s New in .NET 3.0 February 12th - Los Angeles However, if the native code keeps a copy of the pointer beyond the span of the call and intends to call back through that pointer later, you might need to use GCHandle to explicitly prevent the garbage collector from collecting the delegate. We must warn you that a pinned GCHandle could have a significantly negative impact on program performance. Fortunately, in this case, you don’t need to allocate a pinned GC handle, because the thunk is allocated in the unmanaged heap and is referencing the delegate indirectly through a reference known to the GC. Therefore, it is not possible for the thunk to move around, and native code should always be able to call the delegate through the unmanaged pointer if the delegate itself is alive. Marshal.GetFunctionPointerForDelegate can convert a delegate to a function pointer, but it doesn’t do anything to guarantee the lifetime of the delegate. Consider the following function declaration: public delegate void PrintInteger(int n); [DllImport(@”MarshalLib.dll”, EntryPoint=”CallDelegate”)] public static extern void CallDelegateDirectly( IntPtr printIntegerProc); Windows Communication Foundation January 8th - Los Angeles February 5th - New York February 26th - March 1st - Boston Windows Presentation Foundation January 14th - New York January 28th - Boston February 25th - Boston March 10th - Los Angeles If you call Marshal.GetFunctionPointerForDelegate for it and store the returned IntPtr, then pass the IntPtr to the function you are going to call, like so: IntPtr printIntegerCallback = Marshal.GetFunctionPointerForDelegate( new Lib.PrintInteger(MyPrintInteger)); GC.Collect(); GC.WaitForPendingFinalizers(); GC.Collect(); CallDelegateDirectly(printIntegerCallback); Windows WorkFlow Foundation January 28th - Los Angeles March 10th - Los Angeles March 10th - Boston NEW COURSE Code Smarter With Design Patterns in .NET On-Site Training For groups of students, DevelopMentor’s On-Site course delivery may offer the most cost-effective solution – at your location, at the time you need it. OnSites also offer the ability to customize a course to your group’s needs, including course content, labs that run on your programming platform and lab exercises tailored to your business or project. Call us today to discuss your training needs! US -800.699.1932 UK -(0) 8000.562.265 Europe-(0)01242.525.108 msdnmagazine CLR Inside Out It is possible that the delegate will be collected before you call CallDelegateDirectly, and you will get an MDA error that CallbackOnCollectedDelegate was detected. To fix that, you can either store a reference to the delegate in memory or allocate a GC handle. If native code returns an unmanaged function pointer to the CLR, it is the responsibility of native code to keep the actual function code around. Usually this isn’t a problem unless the code is in a dynamically loaded DLL or generated on the fly. P/Invoke Interop Assistant Understanding and remembering all the attributes and rules described so far may seem a bit daunting. After all, most developers of managed code just need to be able to quickly figure out the P/Invoke signature for a Win32® API function, paste it in their code, and be done with it. That’s where the P/Invoke Interop Assistant (available at the MSDN Magazine Web site) can help. This tool can efficiently assist with conversions from C++ to managed P/Invoke signatures as well as those in the opposite direction. It even comes with a database of Win32 functions, data types, and constants, so the common task of adding a Win32 P/Invoke to your C# or Visual Basic source file is made very easy. The tool package includes two command-line tools, SigImp and SigExp, which can be used for batch file processing. A GUI tool is also found in the package, which includes the functionalities of both tools. The GUI tool is handy for doing simple conversions. It contains http://www.develop.com http://www.develop.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2008

MSDN Magazine - January 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Cutting Edge IIS 7.0 - Enhance Your Apps with the Integrated ASP.NET Pipeline World Ready - Around the World with ASP.NET AJAX Applications WCF Syndication - HTTP Programming with WCF and the .NET Framework 3.5 Wicked Code Foundations Extreme ASP.NET {End Bracket}

MSDN Magazine - January 2008

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