How do you Raise an Error?

[mendhak]

In a class I'm writing, if a bad value is sent to a property, I'd like to raise an error, so that when the class is being used, a Try...Catch...Finally block can work with it.

Would I use an Err.Raise statement in here:

Property LastName() As String
            Get
                LastName = m_sLastName
            End Get
            Set(ByVal Value As String)
                If Value = "MENDHAK" Then
[b]'Err.Raise ???[/b]
 
                Else
                m_sLastName = Value
                End If
 
            End Set
        End Property

[mendhak]

I did this:

Property LastName() As String
            Get
                LastName = m_sLastName
            End Get
            Set(ByVal Value As String)
                If Value = "MENDHAK" Then
                    Throw (New Exception("Invalid Last Name!"))
 
 
                End If
                m_sLastName = Value
 
            End Set
        End Property

And caught it like this:

Try
            myEmployee.LastName = "MENDHAK"
        Catch myErr As Exception
            MessageBox.Show(myErr.Message)
 
        Finally
        End Try

And it worked.

I'm assuming that's the right way.

Right?

[Edneeis]

Yup that is it. You can also raise a more specific exception type or make your own. They don't do anything different really unless the person is filtering their Try Catch block:

Try
            Throw New ArgumentException("Invalid Last Name!")
 
        Catch exArg As ArgumentException
            MsgBox("An incorrect argument was passed!")
        Catch ex As Exception
            MsgBox("An unknown error occurred!")
        End Try

I also read in some MSDN doc that it is better to use ApplicationException instead of Exception in your own apps but I forget what the difference was.

[Pirate]

Remember , throwing exception error is always resource intensive . So , don't do it unless you have to . Instead you can use your own custom error message in a messagebox .

[mendhak]

Originally posted by Pirate
Remember , throwing exception error is always resource intensive . So , don't do it unless you have to . Instead you can use your own custom error message in a messagebox .

I see. Thanks, both of you.

[Edneeis]

Only the first exception is a hog since it has to initialize the Exception engine. Depending on what kind of component/object you are making a messagebox could be a bad idea. If you use a messagebox then that limits the objects use to a WinForms project where you are allowed to show UI. It also doesn't allow the consumer of the class to handle the error how it wants to. Otherwise your class could be consumed by a windows or web application or things like a service that sometimes can't show UI components.

[PT Exorcist]

Yeah, using a MessageBox is IMO the worst idea possible, because if you put it in anything that might not be used in conjunction with GUI then you just can't find/control whenever you have an error.

[MrPolite]

hmm so can someone explain, why is throwing exceptions resource consuming? I'm kind of confused

also, mendhak, take a look at the WHEN clause that can be used with the catch statement:

Catch ex as exception when [true]

only catches the exception when the statement after WHEN is evaluated as true... I think C# doesnt have this one though

[Edneeis]

The very first exception in an application must initialize the Exception engine which can be slow but after that it is very fast.