Error Handling

There are various debugging tools to help to locate the source of errors that are detected.
There are various types of errors that can occur when running and writing procedures.
They can be separated into three different types. The most common one is the logical error.
The term inline error handling refers to checking for errors after each line of code is executed.
An error handling routine is not a sub or function but is just a section of code marked by a line label.

Sub DoSomeThing(var1 as Variant) 
   On Error GoTo ErrorHandler 

   Exit Sub 
ErrorHandler: 

End Sub 

To prevent error handling code from running when no error has occurred, place an Exit Sub or Exit Function statement immediately before the error handling routine.

Handling Errors

The best way to handle errors is to have a centralized error handling procedure or procedures.
Error dialog boxes offer a few options: end the procedure, get help or enter break mode to debug the code.
Execution will always stop at the offending line of code and it will be highlighted in yellow.
If the project is password protected then to enter debug mode is not an option.

A run-time error will normally cause execution to stop and a message box will be displayed displaying the error number and a short description the error.
A good application does not let the user see these error messages and rather incorporates error handling to trap errors and take the appropriate action.
At the very least your error handling should display a more meaningful error message.

When you get an error a dialog box will be displayed. Click on the Debug button to find out which line of code is causing the error. If the VBE project is password protected then this button will be disabled.
Trappable errors can occur while an application is running. Some trappable errors can also occur during development or compile time. You can test and respond to trappable errors using the On Error statement and the Err object. Unused error numbers in the range 1 - 1000 are reserved for future use by Visual Basic.

Your error handler procedures will not work on users PC's if they have the "Break on all Errors" option set in their (Tools > Options)(General tab) of the VBE.

Error Handling Keywords

You can use the "On Error" statement to specify what happens when an error occurs:

What happens here ??

On Error Resume Next 
Set rngRange = Range("DoesNotExist") 
lerrorno = Err.Number 
On Error GoTo 0 
Call MsgBox(lerrorno) 

Shortcut Keys

(Alt + F5) - Resume execution (through an error handler).
(Alt + F8) - Steps through an error handler.

Important

If you do not use an On Error statement then any run time errors are fatal and execution will be stopped and a message box displayed.
Always add an Error Handler to every procedure and function.
Avoid using On Error Resume Next
All objects must be set to Nothing before exiting
Error handling should be displayed if the gbDEBUG_ERRORMESSAGES constant is used and is set to True.
Make sure any screenupdating, cursor or statusbar is always reset including in any error handling.
If an run time occurs inside the actual error handler then control is passed to the calling procedure until an active error handler is found. If none can be found then this is a fatal error.
An error handler is automatically disabled when a procedure is exited.
You can always split the Code window horizontally just drag the split bar
You can use the Immediate window to display the message associated with a particular error. Just type "? Error" followed by the error number