c++/cli 中的委托方差

发布于 2024-11-25 17:29:24 字数 237 浏览 1 评论 0原文

我正在将工作 C# 代码转换为 C++/CLI，但我无法理解它为何无法编译。

我收到的错误：

void MyNamespace::Handler::DataChanged(System::Object ^,System::EventArgs ^)'：指定的函数与委托类型'void (System::Object ^,System ::Data::DataRowChangeEventArgs ^)'

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后eg是否自 2024-12-02 17:29:24

看起来 C++/CLI 不像 C# 和 VB 那样支持委托的参数变化，请参阅此 Microsoft connect 错误报告。

作为解决方法，您可以将对处理程序的调用包装在一个包装器中，该包装器采用 DataRowChangeEventArgs 并调用您的处理程序：

public ref class MyClass
{
....
public:
    void MyClass::Delegates(DataTable ^table)
    {
        Handler ^handler = gcnew Handler();
        DataRowChangeEventForwarder& forwarder = 
             gcnew DataRowChangeEventForwarder(
             new EventHandler(handler, &MyNamespace::Handler::DataChanged)));
        table->RowChanged += gcnew DataRowChangeEventHandler (forwarder, &MyNamespace::MyClass::RowChangedDelegate);
    }
 }

 public ref class DataRowChangeEventForwarder 
 {
 private:
     EventHandler^ eventHandler;
 public:
     EventForwarder(EventHandler^ eventHandler) 
     {
          this->eventHandler = eventHander;
     }

     void MyClass::RowChangedDelegate(Object ^sender, DataRowEventArgs ^arg)
     {
          handler->DataChanged(sender, arg);
     }
 }

Looks like C++/CLI doesn't support parameter variance for delegates as C# and VB do, see this Microsoft connect bug report.

As a work around your can wrap your call to handler in a wrapper that takes a DataRowChangeEventArgs and calls your handler:

public ref class MyClass
{
....
public:
    void MyClass::Delegates(DataTable ^table)
    {
        Handler ^handler = gcnew Handler();
        DataRowChangeEventForwarder& forwarder = 
             gcnew DataRowChangeEventForwarder(
             new EventHandler(handler, &MyNamespace::Handler::DataChanged)));
        table->RowChanged += gcnew DataRowChangeEventHandler (forwarder, &MyNamespace::MyClass::RowChangedDelegate);
    }
 }

 public ref class DataRowChangeEventForwarder 
 {
 private:
     EventHandler^ eventHandler;
 public:
     EventForwarder(EventHandler^ eventHandler) 
     {
          this->eventHandler = eventHander;
     }

     void MyClass::RowChangedDelegate(Object ^sender, DataRowEventArgs ^arg)
     {
          handler->DataChanged(sender, arg);
     }
 }

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南风几经秋 2024-12-02 17:29:24

这是该错误的完整可编译再现（可能是注释，但格式会很糟糕）：

public ref struct A
{
};

public ref struct B : A
{
};

public ref struct C
{
    delegate void DType(B^);
    void F(A^ a) { };
};

int main(void)
{
    C^ c = gcnew C;
    C::DType^ del = gcnew C::DType(c, &C::F);
}

Here is a complete compile-ready reproduction of the bug (would be a comment, but the formatting would be horrid):

public ref struct A
{
};

public ref struct B : A
{
};

public ref struct C
{
    delegate void DType(B^);
    void F(A^ a) { };
};

int main(void)
{
    C^ c = gcnew C;
    C::DType^ del = gcnew C::DType(c, &C::F);
}

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