Delegate Variance in c++/cli

I am in the process of converting working C# code into C++/CLI, and I'm having trouble u开发者_运维技巧nderstanding why it does not compile.

The error I received:

void MyNamespace::Handler::DataChanged(System::Object ^,System::EventArgs ^)' : the specified function does not match the delegate type 'void (System::Object ^,System::Data::DataRowChangeEventArgs ^)'

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);
     }
 }

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);
}