Events in
C# are, like in
Delphi (or should I say Object Pascal) just properties of a class. Events are method pointers that delegate a call to the calling class. Just as in Delphi for .NET you can add multiple event handlers to an event.
An event has
three main parts:
1. A method pointer property
2. The raising of the event
3. An event handler
1. A method pointer property.
Delphi and C# have a lot standard method pointer types. The most common Delphi method pointer type is the TNotifyEvent. In C# (or in .NET framework) this is the System.EventHandler.
You can use this methods, but you can also make your own.
Defining a method pointer type:In C#:
public delegate void OnSomething(object Sender, int Value);
In .NET you could, of course, store the parameters in a (derived) System.EventArgs object.
In Delphi:
TOnSomething = procedure(Sender : TObject; Value : Integer) of Object;
They are just types that must be available for the object that uses them.
The class that wants to raise the event must have a public property for it:In C#:public event OnSomething BeforeSomething;
In Delphi:property BeforeSomething : TOnSomething read FBeforeSomething write BeforeSomething;
(Where FBeforeSomething is a private field of the same type)
2. Raising the event
In this example the BeforeSomething event is raised. If the calling class has assigned the property to the (event) class this one can raise it.
In C#:
public virtual void LetItHappen()
{
// The class should add a valid integer value for the event, here 10
if (OnSomething!= null) OnSomething(this, 10);
}
}In Delphi:
procedure TSomeClass.LetItHappen;
begin
if Assigned(FOnSomething) then FOnSomething(Self, 10);
end;
You must check if the property is assigned else you will get an access violation.
The code for C# and Delphi is
amazingly similar. You can make it more similar if you like:
private bool Assigned(object AObject)
{
if (AObject != null)
return true;
else
return false;
}
the C# code then could be:
if Assigned(OnSomething) OnSomething(this, 10);;-)
3. The eventhandlerIn the calling object you should first make an event handler method:
(Suppose the calling is SomeObject : TSomeClass)
In C#:
private void MyBeforeSomething(object Sender, int Value)
{
MessageBox.Show(Value.ToString());
}
In Delphi:
procedure TSomeClass.MyBeforeSomething(Sender : TObject; Value :
Integer);
begin
ShowMessage(IntToStr(Value);
end;
The last thing you should do now is assigning the property:
In C#:
SomeObject.BeforeSomething += new OnSomething(MyBeforeSomething);A new handler is added to list of handlers (or listners). You can remove them by using '-='
In Delphi:
SomeObject.BeforeSomething := MyBeforeSometing;In Delphi for .NET you can also add multiple handlers (listners) for an
event, using the Include en Exclude methods.
Events in C# are basically the same as in Delphi. C# must be Delphi's little brother, no doubt about that.
More information on events in C# can be found on MSDN
here.
