I'm trying to add a number to a listbox that a user enters into a textbox. The program crashes when I press the button.

Public Class Form1

    Dim numbers() As Integer
    Dim numNumbers As Integer = 0

    Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        e.Handled = NumbersOnly(e.KeyChar, TextBox1)
    End Sub
    Private Function NumbersOnly(ByVal pstrChar As Char, ByVal oTextBox As TextBox) As Boolean
        'validate the entry for a textbox limiting it to only numeric values and the decimal point
        If (Convert.ToString(pstrChar) = "." And InStr(oTextBox.Text, ".")) Then Return True 'accept only one instance of the decimal point
        If Convert.ToString(pstrChar) <> "." And pstrChar <> vbBack Then
            Return IIf(IsNumeric(pstrChar), False, True) 'check if numeric is returned
        End If
        Return False 'for backspace
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If TextBox1.Text <> "" Then
            numNumbers = numNumbers + 1
            numbers(numNumbers) = TextBox1.Text
            ListBox1.Items.Add(numbers(numNumbers).ToString())
        Else
            numNumbers = numNumbers + 0
        End If
    End Sub
End Class

Not quite clear what you are trying to do with the numbers() and numNumbers, but using the ReDim Statement should help.

Private numbers() As Double '// changed to Double since TextBox allows ".".
    Private numNumbers As Integer = 0

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If Not TextBox1.Text = "" Then
            ListBox1.Items.Add(TextBox1.Text)
            ReDim Preserve numbers(numNumbers)
            numbers(numNumbers) = CDbl(TextBox1.Text)
            numNumbers += 1
        End If
    End Sub

    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        '// For testing.
        MsgBox(numbers(ListBox1.SelectedIndex).ToString) '// display the number added to your numbers Array.
    End Sub

If i understood you correctly, all you have to do with the addButton_clicked event is this

Private Sub addButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles addButton.Click

        ListBox1.Items.Add(TextBox1.Text)

    End Sub

Then for the numeric validation for the textbox, simply check with an if statement, example:

If Not IsNumeric(textbox1.Text) Then
             MessageBox.Show("Non-numeric value entered. Please try again.", "User Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
            
             textbox1.Focus()
            
              Exit Sub
         
           End If

I dont know if this is what you're looking for. Hope it helps anyway.