Произведение положительных элементов первой строки матрицы - VB

Public Class Form1
 
    Dim n As Integer
 
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Application.Exit()
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'заполняем произвольными числами
        Randomize()
        For i = 0 To n - 1
            DataGridView1.Item(i, 0).Value = Int(15 * Rnd() - 7)
        Next
    End Sub
 
    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        n = Val(TextBox1.Text)
        DataGridView1.ColumnCount = n
        For n = 0 To n - 1
            DataGridView1.Columns(n).Width = 30
        Next n
        DataGridView1.Width = 30 * n + 45
        Width = IIf(DataGridView1.Width < 200, 200, DataGridView1.Width + 20)
    End Sub
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        TextBox1.Text = 15
    End Sub
 
       Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim p As Single
        Dim i As Integer
        Dim a(,) As Single
        ReDim a(n, 0)
        Dim S
        For i = 0 To n - 1
            a(i, 0) = DataGridView1.Item(i, 0).Value
        Next
        p = 1
        For i = 0 To n - 1
            If a(i, 0) > 0 Then
                p = p * a(i, 0)
                S = IIf(S = Nothing, a(i, 0), S & vbCrLf & p / a(i, 0) & " * " & a(i, 0) & " = " & p)
            End If
        Next i
        TextBox2.Text = p
        MsgBox("Были перемножены числа" & vbCrLf & S, 64, "Проверь на калькуляторе")
    End SubEnd Class