CODEWITHSUNDEEP
c#delegatesaction
Gerard
Gerard

Reputation: 13407

Why does argument 'Delegate' accept only RoutedEventHandler delegate?

This works fine:

EventManager.RegisterClassHandler(typeof(DataGrid),    
      DataGrid.SelectionChangedEvent,    
      new RoutedEventHandler(ScrollToSelectedItem));

void ScrollToSelectedItem(object sender, RoutedEventArgs e)

This compiles but gives at runtime the exception: Handler type is mismatched:

EventManager.RegisterClassHandler(typeof(DataGrid),    
         DataGrid.SelectionChangedEvent,       
         new Action<object, RoutedEventArgs>(    
         (object sender, RoutedEventArgs e1) =>    
         ScrollToSelectedItem(sender, e1, false)));

  void ScrollToSelectedItem(object sender, RoutedEventArgs e, bool jump)

The third parameter of RegisterClassHandler is declared as Delegate handler.

Is it implicitly assumed that the handler is not only a delegate but a RoutedEventHandler delegate?
Then why has RegisterClassHandler not a more accurate signature?

EDIT just discovered that I can solve this via:

RoutedEventHandler htrue = (o, ea) => ScrollToSelectedItem(o, ea, true);
RoutedEventHandler hfalse = (o, ea) => ScrollToSelectedItem(o, ea, false);
void ScrollToSelectedItem(object sender, RoutedEventArgs e, bool jump)
EventManager.RegisterClassHandler(typeof(DataGrid),     
        DataGrid.SelectionChangedEvent, hfalse);

Upvotes: 1

Views: 424

Answers (2)

Servy
Servy

Reputation: 203828

Just because two different delegates have the same signature doesn't make them the same, for the same reason that the following two types aren't the same:

public class Foo { public int n; }
public class Bar { public int n; }

You can't assign a Foo to a Bar just because they have the same instance fields.

Action is a type, just like RoutedEventHandler, and there is no implicit conversion between them.

Then why has RegisterClassHandler not a more accurate signature?

As to why it doesn't have a more defined signature, I couldn't say, I didn't design the class. It does sound like it's not very well designed though, given your confusion. It probably should have had a specific delegate as the required parameter.

Upvotes: 2

RomSteady
RomSteady

Reputation: 398

Delegate (uppercase) is a class. delegate (lowercase) is a type.

Action<> returns a delegate (lowercase) which does not cast to Delegate (uppercase) as far as I can tell.

Upvotes: 0

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