Dim summe As Integer = 0
For i As Integer = 0 To DataGridView1.Rows.Count - 1 
  If DataGridView1.Rows(i).Cells(5).Value IsNot DbNull.Value Then
    summe += CInt(DataGridView1.Rows(i).Cells(5).Value) 
  End If
Next 
TextBox46.Text = summe