C++/CLI passing additional arguments to event handling method - event-handling

Question

I'm trying to write a C++/CLI forms application that creates a lot of buttons at runtime: I have a vector of strings and for each string a button is being created:
std::vector<std::string> strings;
/*
string being initialized with values from file
*/
for ( std::vector<std::string>::iterator it = heroes.begin(); it != heroes.end(); ++it ) {
Button ^ button = gcnew Button;
/*
button being customized depending on the string
*/
buttonPannel->Controls->Add(button);
}
Now what I want to do is add an event handler for each button in a way that the string used to customize the button would be passed to handling method.
In c# I would have written something like
button->Click += new EventHandler((sender, args) => button_Click(s, e, *it));
How do I achieve this in C++/CLI?

Answer 1

You could do the exact equivalent of your C# code, but I'd rather make use of an existing property on the Button class to hold the extra data you need.
In this case, the Tag property seems appropriate: its purpose is to hold any extra data you need that is closely associated with the control, so this seems on-point for your string that drives the program logic. (You may need to make it a managed String^ object, rather than a std::string, but that's an easy conversion.)
void Form1::CreateButtons()
{
for (std::vector<std::string>::iterator it = heroes.begin(); it != heroes.end(); ++it)
{
Button ^ button = gcnew Button;
button->Tag = marshal_as<String^>(*it);
button->Click += gcnew EventHandler(this, &Form1::button_Click);
buttonPanel->Controls->Add(button);
}
}
void Form1::button_Click(Object^ sender, EventArgs^ e)
{
Control^ senderControl = dynamic_cast<Control^>(sender);
String^ heroName = nullptr;
if(senderControl != nullptr)
heroName = dynamic_cast<String^>(senderControl->Tag);
if(heroName == nullptr)
{
// Something went wrong. Bail out.
return;
}
// ...
}
If you really do want to do the equivalent of your C# code: Your C# lambda is doing variable capture on the it variable. We can do variable capture in C++/CLI, it's just a lot more manual.
(Note: Your C# example is capturing the iterator, not the string, not sure if that's what was intended. I wrote this to capture the string object instead.)
ref class EventHandlerStringCapture
{
public:
EventHandlerStringCapture(std::string str,
Action<Object^, EventArgs^, std::string>^ handler)
{
this->str = str;
this->handler = handler;
}
void eventHandler(Object^ sender, EventArgs^ e)
{
this->handler(sender, e, this->str);
}
private:
std::string str;
Func<Object^, EventArgs^, std::string>^ handler;
}
void Form1::CreateButtons()
{
for (std::vector<std::string>::iterator it = heroes.begin(); it != heroes.end(); ++it)
{
Button ^ button = gcnew Button;
// The variable to capture.
std::string str = *it;
// The actual event handler: a method in the current class.
Action<Object^, EventArgs^, std::string>^ actualHandler =
gcnew Action<Object^, EventArgs^, std::string>(this, &Form1::button_Click);
// Pass both the variable to capture and the
// actual event handler to a helper object.
EventHandlerStringCapture^ ehsc =
gcnew EventHandlerStringCapture(str, actualHandler);
// Grab the two-parameter event handler from the helper object,
// and make that the click handler.
button->Click +=
gcnew EventHandler(ehsc, &EventHandlerStringCapture::eventHandler);
buttonPanel->Controls->Add(button);
}
}
void Form1::button_Click(Object^ sender, EventArgs^ e, std::string heroName)
{
// ...
}
(Note: I'm not at a compiler, so there may be syntax errors.)
Obviously, using an existing property on the button object is simpler, but that's the C++/CLI equivalent to what the C# compiler does behind the scenes.