Не производить вычисления при наличии незаполненных полей

Public Class Form1
    Private Sub TextBox4_TextChanged(sender As Object, e As EventArgs) Handles TextBox4.TextChanged
 
    End Sub
 
    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim l, h, store, cena, stoim, mans, walls, flag As Integer
        Try
            mans = 0
            cena = 1
            flag = 0
            l = TextBox1.Text
            h = TextBox2.Text
            store = TextBox3.Text
            If CheckBox1.Checked = True Then mans = 20
            If ListBox1.SelectedIndex = 0 Then cena = 10 flag = 1
            If ListBox1.SelectedIndex = 1 Then cena = 20 flag = 1
            If ListBox1.SelectedIndex = 2 Then cena = 30 flag = 1
            If ComboBox1.SelectedIndex = 0 Then walls = 20 flag = 1
            If ComboBox1.SelectedIndex = 1 Then walls = 30 flag = 1
            If ComboBox1.SelectedIndex = 2 Then walls = 40 flag = 1
            If ComboBox1.SelectedIndex = 3 Then walls = 10 flag = 1
            Label5.Text = ""
            If flag = 1 Then
                stoim = 4 * l * h * cena * store + mans + walls
                TextBox4.Text = stoim
            ElseIf flag = 0 Then
                TextBox4.Text = "Выберите все параметры"
            End If
        Catch
            If flag = 0 Then
                If TextBox3.Text = "" Then TextBox3.Text = "?"
            If TextBox2.Text = "" Then TextBox2.Text = "?"
            If TextBox1.Text = "" Then TextBox1.Text = "?"
            If ListBox1.SelectedIndex = -1 Then Label5.Text = "Выберите значение" TextBox4.Text = "Выберите все параметры"
            If ComboBox1.SelectedIndex = -1 Then ComboBox1.Text = "Выберите значение" TextBox4.Text = "Выберите все параметры"
                TextBox4.Text = "Выберите все параметры"
            End If
        End Try
 
    End Sub
 
    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
 
    End Sub
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        TextBox1.Text = ""
        TextBox2.Text = ""
        TextBox3.Text = ""
        TextBox4.Text = ""
        CheckBox1.Checked = False
        ListBox1.ClearSelected()
        ComboBox1.Text = ""
        Label5.Text = ""
    End Sub
 
    Private Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ListBox1.SelectedIndexChanged
 
    End Sub
End Class