イベントハンドラーの中でさらにイベントを起こすと、再帰的に呼び出されてしまう

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace HogeApp
{
    public partial class Form1 : Form
    {
        private Hoge hoge;

        public Form1()
        {
            InitializeComponent();
            hoge = new Hoge(this);
            hoge.HogeHandler += new HogeEventHandler(hoge_HogeHandler);
        }

        private void hoge_HogeHandler(object sender, HogeEventArgs e)
        {
            Console.WriteLine("hoge_HogeHandler");
            hoge.foo();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            hoge.foo();
        }
    }

    class HogeEventArgs : EventArgs
    {
        public HogeEventArgs()
        {
        }
    }

    delegate void HogeEventHandler(object sender, HogeEventArgs e);

    class Hoge
    {
        public event HogeEventHandler HogeHandler;

        private System.Timers.Timer timer = new System.Timers.Timer();

        public Hoge(ISynchronizeInvoke obj)
        {
            timer.AutoReset = false;
            timer.Interval = double.Epsilon;
            timer.SynchronizingObject = obj;

            timer.Elapsed += new System.Timers.ElapsedEventHandler(timer_Elapsed);
        }

        private void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            if (HogeHandler != null)
            {
                HogeHandler(this, new HogeEventArgs());
            }
        }

        public void foo()
        {
            timer.Start();
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace HogeApp
{
    public partial class Form1 : Form
    {
        private Hoge hoge = new Hoge();

        public Form1()
        {
            InitializeComponent();
            hoge.HogeHandler += new HogeEventHandler(hoge_HogeHandler);
        }

        private void hoge_HogeHandler(object sender, HogeEventArgs e)
        {
            Console.WriteLine("hoge_HogeHandler");
            hoge.foo();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            hoge.foo();
        }
    }

    class HogeEventArgs : EventArgs
    {
        public HogeEventArgs()
        {
        }
    }

    delegate void HogeEventHandler(object sender, HogeEventArgs e);

    class Hoge
    {
        public event HogeEventHandler HogeHandler;
        private ThreadStart ts;

        public Hoge()
        {
            ts = NavigateWork;
        }

        public void foo()
        {
            IAsyncResult ar = ts.BeginInvoke(OnCompleted, ts);
            ar.AsyncWaitHandle.WaitOne();
        }

        private void NavigateWork()
        {
        }

        protected virtual void OnCompleted(IAsyncResult ar)
        {
            ts.EndInvoke(ar);
            if (HogeHandler != null)
            {
                HogeHandler(this, new HogeEventArgs());
            }
        }
    }
}

Public Class Form1

    Private Delegate Sub StartNavigationDelegate(ByVal target As Uri)

    Private Sub StartNavigation(ByVal target As Uri)
        Me.TextBox1.Text = target.ToString()
        Me.WebBrowser1.Navigate(target)
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Me.StartNavigation(New Uri(Me.TextBox1.Text))
    End Sub

    Private Sub WebBrowser1_DocumentCompleted(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted
        Dim target As UriBuilder
        Try
            For Each element As HtmlElement In Me.WebBrowser1.Document.Links
                target = New UriBuilder(Me.WebBrowser1.Document.Links(0).GetAttribute("href"))
                If target.Scheme <> "http" AndAlso target.Scheme <> "https" Then Continue For
                Me.BeginInvoke(New StartNavigationDelegate(AddressOf StartNavigation), target.Uri)
            Next
        Catch ex As Exception
            MessageBox.Show(ex.ToString())
        End Try
    End Sub

End Class

Public Class Form1

    Private nexttarget As Uri

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        AddHandler Application.Idle, New EventHandler(AddressOf Application_Idle)
    End Sub

    Private Sub Application_Idle(ByVal sender As Object, ByVal e As EventArgs)
        If Me.nexttarget IsNot Nothing Then StartNavigation()
    End Sub

    Private Sub StartNavigation()
        Me.TextBox1.Text = nexttarget.ToString()
        Me.WebBrowser1.Navigate(nexttarget)
        nexttarget = Nothing
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Me.nexttarget = New Uri(Me.TextBox1.Text)
    End Sub

    Private Sub WebBrowser1_DocumentCompleted(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted
        Dim target As UriBuilder
        Try
            For Each element As HtmlElement In Me.WebBrowser1.Document.Links
                target = New UriBuilder(Me.WebBrowser1.Document.Links(0).GetAttribute("href"))
                If target.Scheme <> "http" AndAlso target.Scheme <> "https" Then Continue For
                nexttarget = target.Uri
                Exit Sub
            Next
        Catch ex As Exception
            MessageBox.Show(ex.ToString())
        End Try
    End Sub

End Class