In String you need to omit ?. String is not a simple type like integer, for example. So, you can pass null without declaring string?, just declare parameter as string.

>>It isn't really nulls - it's a nullable structure
>
>Ok, thanks, BTW, I have this:
>
>

>        ' Avoid to receive a null
>        ' expD1 Value
>        Public Function GetNonNullable(ByVal tdValue As Date?) As Date
>            Dim ldValue As Date = Nothing
>
>            ' If we have a value
>            If tdValue.HasValue Then
>                ldValue = tdValue
>            Else
>                ldValue = oApp.GetEmptyDate()
>            End If
>
>            Return ldValue
>        End Function
>
>        ' Avoid to receive a null
>        ' expN1 Value
>        Public Function GetNonNullable(ByVal tnValue As Double?) As Double
>            Dim lnValue As Double = 0
>
>            ' If we have a value
>            If tnValue.HasValue Then
>                lnValue = tnValue
>            End If
>
>            Return lnValue
>        End Function
>
>        ' Avoid to receive a null
>        ' expL1 Value
>        Public Function GetNonNullable(ByVal tlValue As Boolean?) As Boolean
>            Dim llValue As Double = False
>
>            ' If we have a value
>            If tlValue.HasValue Then
>                llValue = tlValue
>            End If
>
>            Return llValue
>        End Function
>
>        ' Avoid to receive a null
>        ' expN1 Value
>        Public Function GetNonNullable(ByVal tnValue As Integer?) As Integer
>            Dim lnValue As Integer = 0
>
>            ' If we have a value
>            If tnValue.HasValue Then
>                lnValue = tnValue
>            End If
>
>            Return lnValue
>        End Function
>
>        ' Avoid to receive a null
>        ' expC1 Value
>        Public Function GetNonNullable(ByVal tcValue As String?) As String
>            Dim lcValue As Integer = ""
>
>            ' If we have a value
>            If tcValue.HasValue Then
>                lcValue = tcValue
>            End If
>
>            Return lcValue
>        End Function
>

>
>However, for the String type, the designer gives an error. It says "Type 'String' must be a value type or a type argument constrained to 'Structure' in order to be used with 'Nullable' or nullable modifier '?'". I will try with the other approach to see if that would work.