Why can't I add a handler to PropertyChanged with EventHandler attribute type (by reference to method)

Question

I'm doing something like this:

void komar_EventHandler(object sender, EventArgs e)
{
    // Do somethig
}

...
{
    Komar k = new Komar();

    k.PropertyChanged += komar_EventHandler; // OK (why?)


    EventHandler methodRef = komar_EventHandler;

    k.PropertyChanged += methodRef;                              // Error !
    k.PropertyChanged += (PropertyChangedEventHandler)methodRef; // Error !


    // I know solution, but I don't like it :)
    k.PropertyChanged += (s, e) => { methodRef(s, e); };

}

Why can I add a handler with different type immediately, but I can't use reference to a method?

Read [this](https://msdn.microsoft.com/en-us/library/mt654057.aspx). — Maarten, Sep 21 '16 at 10:01
What is the error and what did your research show? http://stackoverflow.com/questions/28522523/cannot-implicitly-convert-type-system-eventhandler-to-system-eventhandlerobject — CodeCaster, Sep 21 '16 at 10:12

Thomas Levesque · Accepted Answer · 2016-09-21T13:23:19.927

k.PropertyChanged += komar_EventHandler; // OK (why?)

Because the compiler automatically converts the method group komar_EventHandler to a PropertyChangedEventHandler delegate. It's the same as if your code was:

k.PropertyChanged += new PropertyChangedEventHandler(komar_EventHandler);

k.PropertyChanged += methodRef;                              // Error !

The PropertyChanged event expects a PropertyChangedEventHandler, not an EventHandler. There's no implicit conversion between different delegate types.

k.PropertyChanged += (PropertyChangedEventHandler)methodRef; // Error !

An EventHandler can't be cast to a PropertyChangedEventHandler; it's just not a valid conversion. You can, however, create a PropertyChangedEventHandler from a EventHandler:

k.PropertyChanged += new PropertyChangedEventHandler(methodRef);

Nice answer however i'd comment the solution that OP doesn't like as well (with lambda) — Fabjan, Sep 21 '16 at 10:37

score 1 · Answer 2 · edited May 23 '17 at 12:19

1

Thomas Levesque you've got a point. I forgot about this:

k.PropertyChanged += komar_EventHandler; // OK (why?)
// because:
k.PropertyChanged += new PropertyChangedEventHandler(komar_EventHandler);

But the solution my question is, however, other.

Compiler need a method, so the solution is:

k.PropertyChanged += methodRef.Invoke;                               // OK !
k.PropertyChanged += (PropertyChangedEventHandler)methodRef.Invoke;  // OK !

Edit:
We can do it also this way:

k.PropertyChanged += new PropertyChangedEventHandler(methodRef);
// Debugger showed me that k.PropertyChanged has reference
// to the method methodRef.Invoke same as k.PropertyChanged += methodRef.Invoke;

edited May 23 '17 at 12:19

Community

1
1

answered Sep 21 '16 at 11:26

marbel82

925
1
18
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Yes, `methodRef.Invoke` works too, since it's a method with a compatible signature – Thomas Levesque Sep 21 '16 at 11:49
@ThomasLevesque Oh, I just saw that you have completed your answer before I wrote mine. So... you should win :) – marbel82 Sep 21 '16 at 12:38

Why can't I add a handler to PropertyChanged with EventHandler attribute type (by reference to method)

2 Answers2