clean datagrid rows

dgv.Rows.Count > 0
dgv.DataSource = Nothing
dgv.Rows.Clear

Contar caracteres

Dim x As String = "abcdd"

Dim i As Integer = x.Count(Function(c) c="d")

i = 2

fuente

Recorrer una lista y arreglo

Dim lista As List(Of String)
        For i As Integer = 0 To lista.Count - 1
            ' Código aquí...
        Next

Dim array() As String
        For j As Integer = 0 To array.Length - 1
            ' Código aquí...
        Next