Stop Button

Question

I need to use a "Run" button and a "Stop" button to control the execution of an internal subroutine. The following code works, but you have to click the Stop button twice to stop the routine. What do I have to do to get it to stop in a single click?

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' RunTest.exe
' Version 1.0.0
' MDJ 2012/06/27

' Label1 initially displays "STOPPED"

Option Strict On

Imports System.Windows

Public Class Form1

    ' Global Variable
    Public runControl As Boolean

    ' Run Button
    Private Sub RunButton_Click(sender As System.Object, e As System.EventArgs) Handles RunButton.Click
        runControl = True
        Label1.Text = "RUNNING"
        While runControl = True
            RunStep()
        End While
    End Sub

    ' Stop Button
    Private Sub StopButton_Click(sender As System.Object, e As System.EventArgs) Handles StopButton.Click
        ' With this configuration, the Stop button must be clicked twice before the While Loop will exit ???
        runControl = False
        Label1.Text = "STOPPED"
    End Sub

    Public Sub RunStep()
        Dim ValueHolder As Integer

        ' Process some data
        For i = 1 To 79
            For j = 1 To 79
                ValueHolder = i * j + 23
            Next
        Next

        ' Allow outside events to be processed
        Application.DoEvents()

    End Sub
End Class