Callback Sample

This sample demonstrates how to pass delegates to an unmanaged function expecting function pointers. A delegate is a class that can hold a reference to a method and is equivalent to a type-safe function pointer or a callback function.

Note Note

When you use a delegate inside a call, the common language runtime protects the delegate from being garbage collected for the duration of that call. However, if the unmanaged function stores the delegate to use after the call completes, you must manually prevent garbage collection until the unmanaged function finishes with the delegate. For more information, see the HandleRef Sample and GCHandle Sample.

The Callback sample uses the following unmanaged functions, shown with their original function declaration:

  • TestCallBack exported from PinvokeLib.dll.

    void TestCallBack(FPTR pf, int value);
    
  • TestCallBack2 exported from PinvokeLib.dll.

    void TestCallBack2(FPTR2 pf2, char* value);
    

PinvokeLib.dll is a custom unmanaged library that contains an implementation for the previously listed functions.

In this sample, the LibWrap class contains managed prototypes for the TestCallBack and TestCallBack2 methods. Both methods pass a delegate to a callback function as a parameter. The signature of the delegate must match the signature of the method it references. For example, the FPtr and FPtr2 delegates have signatures that are identical to the DoSomething and DoSomething2 methods.

public delegate bool FPtr(int value);
public delegate bool FPtr2(String^ value);

public ref class LibWrap
{
public:
    // Declares managed prototypes for unmanaged functions.
    [DllImport("..\\LIB\\PinvokeLib.dll")]
    static void TestCallBack(FPtr^ cb, int value);

    [DllImport("..\\LIB\\PinvokeLib.dll")]
    static void TestCallBack2(FPtr2^ cb2, String^ value);
};

public ref class App
{
public:
    static void Main()
    {
        FPtr^ cb = gcnew FPtr(&App::DoSomething);
        LibWrap::TestCallBack(cb, 99);
        FPtr2^ cb2 = gcnew FPtr2(&App::DoSomething2);
        LibWrap::TestCallBack2(cb2, "abc");
    }

    static bool DoSomething(int value)
    {
        Console::WriteLine("\nCallback called with param: {0}", value);
        // ... 
        return true;
    }

    static bool DoSomething2(String^ value)
    {
        Console::WriteLine("\nCallback called with param: {0}", value);
        // ... 
        return true;
    }
};
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