VB.NET - One routine to handle them all (events)

Ever written about 30 different event handlers that all do roughly the same thing? Not only is it a pain to create it, but it leaves large chunks of asynchronous code where a change in one needs to be copied, and errors are more likely to present themselves as a result.

This code was written in VB.NET 2005 Standard Edition, so no guarentees anywhere else (though I believe it should work anyway in VB.NET).

For this example I am going to parse the input of textboxes 1-10 to make sure that they are all numeric, less than 100, and greater than 0 (such as a percent).

VB Code:

Private Sub TextBox_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
    Handles TextBox1.TextChanged, TextBox2.TextChanged, TextBox3.TextChanged, TextBox4.TextChanged, TextBox5.TextChanged, _
    TextBox6.TextChanged, TextBox7.TextChanged, TextBox8.TextChanged, TextBox9.TextChanged, TextBox10.TextChanged
        ' We can refer to the sender object as a textbox, because we know that the only controls that raise this event are textboxes.
        Dim curTxtBox As TextBox = sender
        ' We could not access sender.Text using Intellisense, but we can through curTxtBox - this can be very useful.
        Dim curText As String = curTxtBox.Text
        If Not (IsNumeric(curText) And Val(curText) < 100 And Val(curText) > 0) Then
            ' The complicated line in the middle is a hack to get the number of the textbox
            ' based on the fact that all the textbox's names start with "TextBox".
            MsgBox("This is not a valid percentage in textbox" & _
            curTxtBox.Name.Substring("TextBox".Length, curTxtBox.Name.Length - "TextBox".Length) & _
            "." & vbNewLine & "Please enter a number between 0 and 100.")
            curTxtBox.Focus()
            ' Since the text is not valid, lets select it all so the user can type away over the top of it.
            ' Again, this method would not appear using Intellisense, but it will now.
            curTxtBox.SelectAll()
            Exit Sub
        End If
    End Sub

That has a few hacks thrown in for good measure, but the important part is the definition of the subroutine.

Note that the name applied is completely arbitrary. It took me a little while to figure out that event handlers are just routines with additional functionality, so I thought I'd pass that on. That this means is you could easy replace my routine name with this:

VB Code:

Private Sub WingDerWongNumber(...)
...

The important part is that you add multiple handlers to the same function.

Also note that I have used _ to break the code into multiple lines at some points. I find this particularly useful when I have a long messagebox or definition to write, and I usually try to group things using this technique.

Hopefully many of you will find this useful at some point. :wave:



Post writing: I decided to compile it to check, and I found 2 typos: a missing 2 and a missing ". Fixed it now, but shows that even when you're paying attention you can make mistakes.

I use a very similar method (one handler, several events) when I have a control that has to respond to event of its parent.

Typically in the OnParentChanged override, I'll use AddHandler and RemoveHandler to add handlers for appropriate events in the parent or the top-level form (i.e. Activate and Deactivate). A hint for you if you use something like this, is to make sure that the control's parent isn't Nothing before adding the handler.

The example you posted, will work, unmoddified, in all versions of vb.net.

Also, for simplicity's sake, all those hack to get the number of the textbox is not needed. Try this instead (it will produce identical output):

VB Code:

MsgBox("This is not a valid percentage in " & _
            curTxtBox.Name.ToLower & "." & vbNewLine & "Please enter a number between 0 and 100.")

I can't believe I found this on the first google search I did, tried it with my code and it worked. You just saved me about 80 modules of repetitive code. (with the "= sender" stuff)