Delegates as parameters in VB.NET

Question

Backstory: I'm using log4net to handle all logging for a project I'm working on. One particular method can be called under several different circumstances -- some that warrant the log messages to be errors and others that warrant the log messages to be warnings.

So, as an example, how could I turn

Public Sub CheckDifference(ByVal A As Integer, ByVal B As Integer)
  If (B - A) > 5 Then
    log.ErrorFormat("Difference ({0}) is outside of acceptable range.", (B - A))
  End If
End Sub

Into something more along the lines of:

Public Sub CheckDifference(ByVal A As Integer, ByVal B As Integer, "Some delegate info here")
  If (B - A) > 5 Then
    **delegateinfo**.Invoke("Difference ({0}) is outside of acceptable range.", (B - A))
  End If
End Sub

So that I could call it and pass either log.ErrorFormat or log.WarnFormat as the delegate?

I'm using VB.NET with VS 2008 and .NET 3.5 SP1. Also, I'm fairly new to delegates in general, so if this question should be worded differently to remove any ambiguities, let me know.

EDIT: Also, how could I initialize the delegate to either the ErrorFormat or the WarnFormat in the class constructor? Would it be as easy as myDelegate = log.ErrorFormat? I would imagine there is more to it than that (pardon my ignorance on the subject -- delegates are really something I want to learn more about, but so far they have eluded my understanding).

Answer 1

You can pass delegate as parameter in VB.NET (and C#). Look here for an example.

Answer 2

Public Delegate errorCall(ByVal error As String, Params objs As Objects())
CheckDifference(10, 0, AddressOf log.ErrorFormat)

Please forgive the formatting :P

Basically though, create the delegate that you want, with the correct signature, and pass the address of it to the method.

Answer 3

You'll first want to declare a delegate at the Class/Module level (all this code is from memory/not tested):

Private Delegate Sub LogErrorDelegate(txt as string, byval paramarray fields() as string)

Then .. you'll want to declare it as a property to your class e.g.

Private _LogError
Public Property LogError as LogErrorDelegate
Get 
return _LogError
End Get
Set(value as LogErrorDelegate)
_LogError = value
End Set
End Property

The way to instantiate the delegate is:

Dim led as New LogErrorDelegate(AddressOf log.ErrorFormat)

Answer 4

Declare your delegate signature:

Public Delegate Sub Format(ByVal value As String)

Define your Test function:

Public Sub CheckDifference(ByVal A As Integer, _
                           ByVal B As Integer, _
                           ByVal format As Format)
    If (B - A) > 5 Then
        format.Invoke(String.Format( _
        "Difference ({0}) is outside of acceptable range.", (B - A)))
    End If
End Sub

Somewhere in your code call your Test function:

CheckDifference(Foo, Bar, AddressOf log.WriteWarn)

Or

CheckDifference(Foo, Bar, AddressOf log.WriteError)

Answer 5

@Nesio,

Instead of creating a new delegate type, why not re-use an existing one? Instead of creating Format you could use Action(Of String) instead.