ByRef&ByValue Difference

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim first As Double = CDbl(TextBox1.Text)
        Dim second As Double = CDbl(TextBox2.Text)
        SwapValues(first, second)
        TextBox1.Text = CStr(first)
        TextBox2.Text = CStr(second)
    End Sub
    Sub SwapValues(ByRef firstNumber As Double, ByRef secondNumber As Double)
        Dim help As Double
        help = firstNumber
        firstNumber = secondNumber
        secondNumber = help
    End Sub
End Class