I like your thinking.. maybe in the next version of VS

For now though, I would split the values into an array. Then you can loop through the results and add them up.
VB Code:
  1. Dim TextboxText As String = "5 4.1 6.23 0 0 7 7.924"
  2.  
  3. Dim ArrayOfValues() As String = TextboxText.Split(" ")
  4. Dim Sum As Double = 0
  5.  
  6. For Each Value As String In ArrayOfValues
  7.     If Double.TryParse(Value, Nothing) Then Sum += CDbl(Value)
  8. Next
  9.  
  10. MsgBox(Sum)