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我有以下情况：一个通过串行端口与 I/O 设备“对话”的 C++ 库。C++ 库在设备上进行一些轮询，并在设置输入时调用回调函数：

#if defined( _MSC_VER ) || defined( __MINGW32__ ) || defined( __MINGW64__ )
#   define LIB_CALLBACK __stdcall
#   define LIB_CALL __cdecl
#   if defined( LIB_EXPORTS )
#       define LIB_API __declspec( dllexport )
#   else
#       define LIB_API __declspec( dllimport )
#   endif
#else
#   define LIB_API
#endif // WIN32

#if defined( __cplusplus )
extern "C" {
#endif

typedef int libInput;
typedef void (* LIB_CALLBACK libOnInput )  ( libInput buttons );

LIB_API void LIB_CALL libRegisterInput( libInput f );

#if defined( __cplusplus )
}
#endif

这是线程函数

libOnInput g_libOnInputFunc = nullptr;

LIB_API void LIB_CALL libRegisterInput( libInput f )
{
    libOnInputFunc = f;
}

// while true
if ( g_iocOnInputFunc )
{
    libInput = // ...
    // the event is called asyncronously. I don't want block the current thread
    std::thread t( g_libOnInputFunc, in );
    t.detach();
}

现在这个库必须在 VB.net 应用程序中使用。所以我用 C# 包装了我的库

public delegate void OnInputDownDelegate(int input);

[DllImport("mylib.dll", CallingConvention = CallingConvention.Cdecl)]
internal static extern void libRegisterInput( OnInputDownDelegate f );

public void RegisterOnInput(OnButtonDownDel f)
{
    libRegisterInput(f);
}

现在在 Visual Basic 上，我直接使用 C# 包装器库：

Imports CSharpLib

Public Class Form1


Private Shared myLib As CSharpLib.CSharpLib

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    myLib = New CSharpLib.CSharpLib()
    myLib.RegisterOnButtonDown(New CSharpLib.OnInputDownDelegate(AddressOf OnInput))
End Sub

Private Sub OnInput(ByVal input As Integer)
    MessageBox.Show(input.ToString())
End Sub

End Class

但是当我在设备上按下按钮运行应用程序时，mylib.dll 会捕获输入并想要调用回调函数但应用程序崩溃。似乎我正在调用空引用的委托。

这是错误代码：

Eccezione non gestita di tipo 'System.NullReferenceException' in Modulo sconosciuto.

Informazioni aggiuntive: Riferimento a un oggetto non impostato su un'istanza di oggetto.

翻译过来是：

Exception of type 'System.NullReferenceException' in Unknown Module

Extra information: Reference to an object not setted on an object instance
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1 回答 1

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我必须存储委托的引用以避免 GC 删除委托的引用：

public delegate void OnInputDownDelegate(int input);
private OnInputDownDelegate mDelegate;

[DllImport("mylib.dll", CallingConvention = CallingConvention.Cdecl)]
internal static extern void libRegisterInput( OnInputDownDelegate f );

public void RegisterOnInput(OnButtonDownDel f)
{
    mDelegate = f;
    libRegisterInput( mDelegate );
}
于 2013-09-04T11:10:18.893 回答