Set the default value of an optional 'List(Of t)' parameter to an empty list?

Question

In this example, I can't figure out how to set the optional parameter c to an empty List(Of thing):

Sub abcd(a as something, b as something, optional c as List(Of thing) = ?? )
    ' *stuff*
End Sub

I considered setting c to null, but that seems like a bad thing to do.

Answer 1

You can't. Optional values have to be compile-time constants. The only compile-time constant you can assign to List(Of T) is Nothing.

What you can do is overload that method with one that omits the List(Of T)parameter. This overload can then pass an empty List(Of T) to the original method:

Sub abcd(a as something, b as something)
    abcd(a, b, New List(Of T)())
End Sub

Sub abcd(a as something, b as something, c as list(of thing))
    doStuff()
End Sub

Answer 2

I appreciate this is an old question (and shame on me for breaching etiquette in replying) but...

I had exactly the same issue today. It was resolved by passing an object...

Sub abcd(a as something, b as something, optional c as Object = Nothing )
    Dim lstC as new List(Of thing)
    If Not IsNothing(c) then
        lstC = c
    End IF
    ' Then in your code you just have to see if lstC.Count > 0
    ' *stuff*
End Sub