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c#events
hultqvist
hultqvist

Reputation: 18461

What the purpose/difference in using an event-type constructor

In all examples I can find as well as the automatically generated code i Visual Studio, events are set using the following code:

button1.Click += new System.EventHandler(this.button1_Click);

But I can also write it visually cleaner by omitting the constructor wrapper:

button1.Click += this.button1_Click;

Which also compile fine.

What is the difference between these two? And why is the first one mostly used/preferred?

Upvotes: 5

Views: 118

Answers (3)

Hans Passant
Hans Passant

Reputation: 942000

It is removing event handlers that inspired the sugar:

 button1.Click -= new System.EventHandler(this.button1_Click);

Using new to remove an event handler? Yes.

Even the full delegate constructor syntax is sugar. A delegate needs a Method and a Target. The target is automatically assigned without you specifying it. It will be "this". A bit unfortunate, it hides the fact that an event subscription adds a reference to your class object that can keep it from being garbage collected. It is explicit in the C++/CLI syntax:

 button1->Click += gcnew System::EventHandler(this, &button1_Click);

with the interesting twist that you could actually subscribe an event handling method of another object or class. Not that this gets a lot of use.

Upvotes: 2

Lasse V. Karlsen
Lasse V. Karlsen

Reputation: 391456

The second one is syntactic sugar that expands to the first one, through compiler magic.

The first one might be preferred because the event type isn't hidden from the code, and thus might be easier to read later on, but functionally (and IL-wise) they're the same.

Upvotes: 2

Jon Skeet
Jon Skeet

Reputation: 1502036

The second form (implicit conversion from a method group to a delegate type) wasn't supported before C# 2, so any tutorials etc written before 2005 would have used the first form.

Also, IIRC Visual Studio auto-completes the first form. Personally I prefer the second.

Upvotes: 5

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