UPDATE: I've updated my examples below, and added to some of the text, to take into account tergiver's suggestion, in his comment below, that I should probably use the generic EventHandler
I tend to write introductory topics in my blog. Not always, but typically. And not because I’m a newcomer to .NET (I’ve been using .NET since the beginning of 2002), but because all the complicated topics seem to be covered by everyone else and I think there is still a need to address simpler topics.
Today’s topic is no different. Even though writing custom events isn’t all that complicated, I still see a lot of questions on the Forums asking how to do it. So, let’s get to it.
Say you have a custom UserControl that you need to have raise an event when, for example, a user types "FOO" in a textbox that is on the Control.
Minimally, in your UserControl, you need the following things:
// First you must specify the event that you will be raising:
public event EventHandler MyFooBar;// Then, when you need to fire the event in your UserControl, do this:
if (this.MyTextBox.Text == "FOO")
this.OnMyFooBar(new EventArgs());// Lastly, this raises the MyFooBar event:
protected virtual void OnMyFooBar(EventArgs e)
{
if (MyFooBar != null)
MyFooBar(this, e);
}
Then, in your form, you just set up the usual delegates and EventHandlers:
this.oMyControl.MyFooBar += new System.EventHandler(this.oMyControl_MyFooBarHandler);private void oMyControl_MyFooBarHandler(object sender, System.EventArgs e)
{
// whatever your form code needs to be, such as:
MessageBox.Show("FOO was specified!");
}
Or, the alternative way to do this since anonymous methods were introduced in 2.0:
this.oMyControl.MyFooBar += delegate
{
// whatever your code needs to be, such as
MessageBox.Show("FOO was specified!");
};
You can even get more fancy, creating custom EventArgs and utilizing generic delegates, but the above code is sufficient for simple things, when just the built-in System.EventArgs is all you need. For fancier, custom stuff, try this:
First, custom event args something like this:
public class MyCustomEventArgs : EventArgs
{
public bool IsBarSpecified { get; set; }
}
Your UserControl code then gets changed to this:
// Change the event so that it can be handled by a generic delegate
public event EventHandler<MyCustomEventArgs> MyFooBar;// Firing the event gets changed to something like this:
if (this.MyTextBox.Text.ToUpper().Contains("FOO"))
{
MyCustomEventArgs e = new MyCustomEventArgs();
if (this.MyTextBox.Text.ToUpper().Contains("BAR"))
e.IsBarSpecified = true;
else
e.IsBarSpecified = false;
this.OnMyFooBar(e);
}//And raising the MyFooBar event is the same, other than changing to the custom EventArgs:
protected virtual void OnMyFooBar(MyCustomEventArgs e)
{
if MyFooBar != null)
MyFooBar(this, e);
}
And in your Form, you'd have this instead:
this.oMyControl.MyFooBar += new EventHandler<MyCustomEventArgs>(this.oMyControl_MyFooBarHandler);private void oMyControl_MyFooBarHandler(object sender, MyCustomEventArgs e)
{
// whatever your code needs to be, such as
string message = "FOO was specified!"; if (e.IsBarSpecified == true)
message += " BAR too!"); MessageBox.Show(message);
}
Or this, if you want to use an anonymous delegate (note that since I'm utilizing the custom MyCustomEventArgs, I'll need to specify them here, whereas I didn't need any of the parameters in the first example):
this.oMyControl.MyFooBar += delegate(object sender, MyCustomEventArgs e)
{
// whatever your code needs to be, such as
string message = "FOO was specified!"; if (e.IsBarSpecified == true)
message += " BAR too!"); MessageBox.Show(message);
};
