如何读取 dgv vb net 中的行？

问：

Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
    TextBox2.Text = DataGridView1.Item(0, DataGridView1.CurrentRow.Index).Value
    Call connect()
    Dim str As String

    For Each rows As DataGridViewRow In DataGridView1.Rows
        str = "select *from Pr_MasterItem where item_id= '" & TextBox2.Text & "'"
        ListBox1.Items.Add("ada")
        Cmd = New SqlCommand(str, conn)
        Rd = Cmd.ExecuteReader
        Rd.Read()

        If Rd.HasRows Then
            TextBox4.Text = Rd.Item("item_id")
        Else
            ListBox1.Items.Add("Kosong")
        End If
    Next
    conn.Close()
End Sub

此代码必须在数据网格中逐行读取，在数据库中也是如此。