ErrObject class

The Err object holds information about the most recent run-time error. It is a global, intrinsic singleton — there is no need to declare it or construct one with New, just reference it as Err. The default property is Number, so a bare Err is equivalent to Err.Number.

Inspecting an error

When a run-time error is raised inside a procedure that has an active error handler installed with On Error, execution jumps to the handler with the Err object’s properties populated. The handler reads Number to identify the error, Description for a human-readable message, and Source to learn where it originated.

Sub Demo()
    On Error GoTo Handler
    Err.Raise 6                    ' Generate an Overflow error.
    Exit Sub
Handler:
    Debug.Print Err.Number         ' 6
    Debug.Print Err.Description    ' "Overflow"
End Sub

The properties are reset to their zero values when the handler exits via Resume, Resume Next, or any Exit statement, or when Clear is called explicitly.

Raising a custom error

Code can generate its own run-time error by calling Raise. Custom error numbers should be biased by vbObjectError so that they don’t collide with twinBASIC’s built-in numbers. Setting Source and Description at the call site gives the error handler something useful to inspect or display.

Public Sub WithdrawCash(ByVal Amount As Currency)
    If Amount > Balance Then
        Err.Raise vbObjectError + 1001, _
                  Source:="Account.WithdrawCash", _
                  Description:="Insufficient funds."
    End If
    ' ...
End Sub

Members

• Clear – resets all properties of the Err object to their zero values
• Description – returns or sets a string describing the error
• HelpContext – returns or sets the context ID of a Help topic associated with the error
• HelpFile – returns or sets the path to the Help file associated with the error
• LastDllError – returns the last system error code from a call into a DLL
• LastHresult – returns the last HRESULT returned from a COM object method call
• Number – returns or sets the error number; the default member of Err
• Raise – generates a run-time error
• ReturnHResult – sets a custom HRESULT to be returned from the current method
• Source – returns or sets the name of the object or application that generated the error