Public Class frmAdjustNull
 
    Dim dt As New DataTable
 
    Private Sub frmAdjustNull_Load(sender As Object, 
        e As EventArgs) Handles MyBase.Load
 
        'Einige Beispieldaten
        dt.Columns.Add("Rechnungsbetrag", GetType(Decimal))
        dt.Columns.Add("Zahlung1", GetType(Decimal))
        dt.Columns.Add("Zahlung2", GetType(Decimal))
 
 
        dt.Rows.Add(1000, 100, 100)
        dt.Rows.Add(2000, 200, Nothing)
        dt.Rows.Add(3000, Nothing, 300)
 
 
        Dim betrag As List(Of Decimal) =
        (From rechnungen In dt.AsEnumerable()
         Let Differenz As Decimal = 
          Adjustnothing(rechnungen.Field(Of Decimal)("Rechnungsbetrag")) - (
          Adjustnothing(rechnungen.Field(Of Nullable(Of Decimal))("Zahlung1")) +
          Adjustnothing(rechnungen.Field(Of Nullable(Of Decimal))("Zahlung2")))
         Select Differenz).ToList
 
    End Sub
 
    Private Function Adjustnothing(vl As Nullable(Of Decimal)) As Decimal
        If vl.HasValue Then
            Return vl.Value
        Else
            Return 0
        End If
    End Function
End Class