C# button click EventArgs e information

Question

When I click on a button in e.g. a WinForms application, what information is passed to the EventArgs e of the event method? I'm just wondering because I'm using the as keyword to "convert" e to a mouse event in order to get the coordinates of the point where the button was clicked.

EDIT: In the following example I can convert the variable e to an object of the type MouseEventArgs, and I want to know to what other types of event arguments e can be converted.

private void someEvent(object sender, EventArgs e)
{
  int xCoord = (e as MouseEventArgs).X;
}

Answer 1

This depends on the event, The underlying windows messages for most winforms events have no such concept, so where ever the EventArgs were created will determine the type and the information it contains. It could be something from the framework or you can just make up your own class derived from EventArgs.

After .Net4.5 it doesn't even have to derive from EventArgs

Answer 2

You should use System.Windows.Forms.Cursor.Position: "A Point that represents the cursor's position in screen coordinates."

Answer 3

There are 2 parameters: a sender, and an EventArgs. The sender is the object that initialized the event. The EventArgs contains additional information about the event.

From MSDN

// This example uses the Parent property and the Find method of Control to set 
// properties on the parent control of a Button and its Form. The example assumes 
// that a Button control named button1 is located within a GroupBox control. The  
// example also assumes that the Click event of the Button control is connected to 
// the event handler method defined in the example. 
private void button1_Click(object sender, System.EventArgs e)
{
   // Get the control the Button control is located in. In this case a GroupBox.
   Control control = button1.Parent;
   // Set the text and backcolor of the parent control.
   control.Text = "My Groupbox";
   control.BackColor = Color.Blue;
   // Get the form that the Button control is contained within.
   Form myForm = button1.FindForm();
   // Set the text and color of the form containing the Button.
   myForm.Text = "The Form of My Control";
   myForm.BackColor = Color.Red;
}

Answer 4

I'm using the as keyword to "convert" e to a mouse event

Don't do it. Why?

Because if someone uses keyboard (TABs + Enter) or Button.PerformClick call to trigger your button, the conversion will fail and your:

MouseEventArgs mouseArgs = e as MouseEventArgs ;

if (mouseArgs.MouseButton == MouseButton.Left) ...

will lead to NullReferenceException.

You can't guarantee that EventArgs e in OnClick event will be MouseEventArgs. It is just the most common scenario, but not the only one possible.

P.S.: As it has been already pointed by @terribleProgrammer you could use Cursor.Position static property to get the current cursor position in more independent and robust way.

But if the cursor information makes sense only if the button has been triggered by mouse, then you will have just to be careful and handle possible conversion failure:

MouseEventArgs mouseArgs = e as MouseEventArgs ;

if (mouseArgs == null)
{
    Logger.Log("mouseArgs is null");
    return;
}    

if (mouseArgs.MouseButton == MouseButton.Left) ...

EDIT:

There are three main ways to raise the event and three corresponding eventArgs classes:

1. Using mouse - MouseEventArgs
2. Using keyboard - KeyEventArgs
3. Using PerformClick method - I am not sure, but it will probably be just plain EventArgs(to verify just raise it in such a way and take a look at e.GetType()).
4. Whether there are any other scenarios that will raise it, I don't know. MSDN is of no use for such requests.

P.S.1.: And I want to repeat again - do not assume that e parameter will be of some specific type. Windows Forms API even does not guarantee that e will contain some concrete useful EventArgs-derived class.