Nested function does not have same signature as delegate

Question

I'm trying to achieve a currying-like effect in VB. I want to be able to do this:

Dim wrap = WrapNumber("-")
wrap(5) 'returns "-5-"

Here's my function:

Private Function WrapNumber(ByVal separator As String) As Func(Of Integer, String)

    Return Function(ByRef number As Integer) As String
               Return separator + number + separator
           End Function

End Function

I get an error on the Return line with this message:

Nested function does not have the same signature as delegate Func(Of Integer, String)

I'm not sure why I'm getting this error. The function WrapNumber returns a function which takes an Integer and returns a String, so from what I can see it has the same type as the Func given in the WrapNumber signature.

Answer 1

If you turn Option Strict On, it will show you a lot of what's going wrong. In addition to ByRef not being required as stated in the comments, the concatenation is trying to treat separator as a double. If you make those changes then it works fine:

Dim wrap = WrapNumber("-")
Dim test As String = wrap(5)
'test is equal to "-5-"

Private Function WrapNumber(ByVal separator As String) As Func(Of Integer, String)
    Return Function(number As Integer) As String
               Return separator & number.ToString & separator
           End Function
End Function