public partial class Form1 : Form {
	private Form2 childForm = null;
	public Form1() {
		InitializeComponent();
		ButtonStateInit();
	}
	
	private void ButtonStateInit() {
		this.bInvoke.Enabled = true;
		// we have no start and stop when no child form
		this.bStart.Enabled = false;
		this.bStop.Enabled = false;
	}

	private void bInvoke_Click(object sender, EventArgs e) {
		// dont ivoke twice.
		this.bInvoke.Enabled = false;
		childForm = new Form2();
		// listen to standard close event, for button reset
		childForm.FormClosed += new FormClosedEventHandler(childForm_FormClosed);
		
		// listen to custom event, for state change
		childForm.RunningStateChanged += new Form2.RunningStateChangedHandler(childForm_RunningStateChanged);
		
		// show the form
		childForm.Show();
	}

	void childForm_FormClosed(object sender, FormClosedEventArgs e) {
		// closed, but the buttons back
		ButtonStateInit();
	}

	void childForm_RunningStateChanged(object sender, EventArgs e) {
		// not using my object var (childForm) for this, we often don't have one
		// so this is how it's usually done
		Form2 frm = (Form2)sender;
		// look at the form state, update my buttons
		this.bStop.Enabled = frm.RunningState;
		this.bStart.Enabled = !this.bStop.Enabled;
	}

	private void bStop_Click(object sender, EventArgs e) {
		// tell the child, the child with then tell me and I'll update the buttons
		childForm.RunningState = false;
	}
	private void bStart_Click(object sender, EventArgs e) {
		// tell the child, the child with then tell me and I'll update the buttons
		childForm.RunningState = true;
	}
}

public partial class Form2 : Form {
	public Form2() {
		InitializeComponent();
	}

	// set up an event, we need a delegate
	public delegate void RunningStateChangedHandler(object sender, EventArgs e);
	// an the event itself
	public event RunningStateChangedHandler RunningStateChanged;

	// a nice public property, so outside forces can mess with our state
	public bool RunningState {
		get {
			return this.bStop.Enabled;
		}
		set {
			// nothing to do
			if (this.bStop.Enabled && value) { return; }
			this.bStop.Enabled = value;
			this.bStart.Enabled = !this.bStop.Enabled;
			// do something
			//...
			// tell listeners
			if (this.RunningStateChanged != null) {
				// we have listeners, fire an event
				RunningStateChanged(this, new EventArgs());
			}
		}
	}

	// buttons call our own property
	private void bStop_Click(object sender, EventArgs e) {
		RunningState = false;
	}
	private void bStart_Click(object sender, EventArgs e) {
		RunningState = true;
	}

	// on load, set a false state
	private void Form2_Load(object sender, EventArgs e) {
		this.RunningState = false;
	}
}