How to: Implement a Client of the Event-based Asynchronous Pattern

The following code example demonstrates how to use a component that adheres to the Event-based Asynchronous Pattern Overview. The form for this example uses the PrimeNumberCalculator component described in How to: Implement a Component That Supports the Event-based Asynchronous Pattern.

When you run a project that uses this example, you will see a "Prime Number Calculator" form with a grid and two buttons: Start New Task and Cancel. You can click the Start New Task button several times in succession, and for each click, an asynchronous operation will begin a computation to determine if a randomly generated test number is prime. The form will periodically display progress and incremental results. Each operation is assigned a unique task ID. The result of the computation is displayed in the Result column; if the test number is not prime, it is labeled as Composite, and its first divisor is displayed.

Any pending operation can be canceled with the Cancel button. Multiple selections can be made.

Note

Most numbers will not be prime. If you have not found a prime number after several completed operations, simply start more tasks, and eventually you will find some prime numbers.

Example

using System;
using System.Collections;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Globalization;
using System.Threading;
using System.Windows.Forms;

namespace AsyncPattern
{
 // This form tests the PrimeNumberCalculator component.
 public class PrimeNumberCalculatorMain : System.Windows.Forms.Form
 {
 /////////////////////////////////////////////////////////////
 // Private fields
 //
 #region Private fields

 private PrimeNumberCalculator primeNumberCalculator1;
 private System.Windows.Forms.GroupBox taskGroupBox;
 private System.Windows.Forms.ListView listView1;
 private System.Windows.Forms.ColumnHeader taskIdColHeader;
 private System.Windows.Forms.ColumnHeader progressColHeader;
 private System.Windows.Forms.ColumnHeader currentColHeader;
 private System.Windows.Forms.Panel buttonPanel;
 private System.Windows.Forms.Panel panel2;
 private System.Windows.Forms.Button startAsyncButton;
 private System.Windows.Forms.Button cancelButton;
 private System.Windows.Forms.ColumnHeader testNumberColHeader;
 private System.Windows.Forms.ColumnHeader resultColHeader;
 private System.Windows.Forms.ColumnHeader firstDivisorColHeader;
 private System.ComponentModel.IContainer components;
 private int progressCounter;
 private int progressInterval = 100;

 #endregion // Private fields

 /////////////////////////////////////////////////////////////
 // Construction and destruction
 //
 #region Private fields
 public PrimeNumberCalculatorMain ()
 {
 //
 // Required for Windows Form Designer support
 //
 InitializeComponent();

 // Hook up event handlers.
 this.primeNumberCalculator1.CalculatePrimeCompleted +=
 new CalculatePrimeCompletedEventHandler(
 primeNumberCalculator1_CalculatePrimeCompleted);

 this.primeNumberCalculator1.ProgressChanged +=
 new ProgressChangedEventHandler(
 primeNumberCalculator1_ProgressChanged);

 this.listView1.SelectedIndexChanged +=
 new EventHandler(listView1_SelectedIndexChanged);
 }

 protected override void Dispose( bool disposing )
 {
 if( disposing )
 {
 if (components != null)
 {
 components.Dispose();
 }
 }
 base.Dispose( disposing );
 }

 #endregion // Construction and destruction

 /////////////////////////////////////////////////////////////
 //
 #region Implementation

 // This event handler selects a number randomly to test
 // for primality. It then starts the asynchronous
 // calculation by calling the PrimeNumberCalculator
 // component's CalculatePrimeAsync method.
 private void startAsyncButton_Click (
 System.Object sender, System.EventArgs e)
 {
 // Randomly choose test numbers
 // up to 200,000 for primality.
 Random rand = new Random();
 int testNumber = rand.Next(200000);

 // Task IDs are Guids.
 Guid taskId = Guid.NewGuid();
 this.AddListViewItem(taskId, testNumber);

 // Start the asynchronous task.
 this.primeNumberCalculator1.CalculatePrimeAsync(
 testNumber,
 taskId);
 }

 private void listView1_SelectedIndexChanged(
 object sender,
 EventArgs e)
 {
 this.cancelButton.Enabled = CanCancel();
 }

 // This event handler cancels all pending tasks that are
 // selected in the ListView control.
 private void cancelButton_Click(
 System.Object sender,
 System.EventArgs e)
 {
 Guid taskId = Guid.Empty;

 // Cancel all selected tasks.
 foreach(ListViewItem lvi in this.listView1.SelectedItems)
 {
 // Tasks that have been completed or canceled have
 // their corresponding ListViewItem.Tag property
 // set to null.
 if (lvi.Tag != null)
 {
 taskId = (Guid)lvi.Tag;
 this.primeNumberCalculator1.CancelAsync(taskId);
 lvi.Selected = false;
 }
 }

 cancelButton.Enabled = false;
 }

 // This event handler updates the ListView control when the
 // PrimeNumberCalculator raises the ProgressChanged event.
 //
 // On fast computers, the PrimeNumberCalculator can raise many
 // successive ProgressChanged events, so the user interface
 // may be flooded with messages. To prevent the user interface
 // from hanging, progress is only reported at intervals.
 private void primeNumberCalculator1_ProgressChanged(
 ProgressChangedEventArgs e)
 {
 if (this.progressCounter++ % this.progressInterval == 0)
 {
 Guid taskId = (Guid)e.UserState;

 if (e is CalculatePrimeProgressChangedEventArgs)
 {
 CalculatePrimeProgressChangedEventArgs cppcea =
 e as CalculatePrimeProgressChangedEventArgs;

 this.UpdateListViewItem(
 taskId,
 cppcea.ProgressPercentage,
 cppcea.LatestPrimeNumber);
 }
 else
 {
 this.UpdateListViewItem(
 taskId,
 e.ProgressPercentage);
 }
 }
 else if (this.progressCounter > this.progressInterval)
 {
 this.progressCounter = 0;
 }
 }

 // This event handler updates the ListView control when the
 // PrimeNumberCalculator raises the CalculatePrimeCompleted
 // event. The ListView item is updated with the appropriate
 // outcome of the calculation: Canceled, Error, or result.
 private void primeNumberCalculator1_CalculatePrimeCompleted(
 object sender,
 CalculatePrimeCompletedEventArgs e)
 {
 Guid taskId = (Guid)e.UserState;

 if (e.Cancelled)
 {
 string result = "Canceled";

 ListViewItem lvi = UpdateListViewItem(taskId, result);

 if (lvi != null)
 {
 lvi.BackColor = Color.Pink;
 lvi.Tag = null;
 }
 }
 else if (e.Error != null)
 {
 string result = "Error";

 ListViewItem lvi = UpdateListViewItem(taskId, result);

 if (lvi != null)
 {
 lvi.BackColor = Color.Red;
 lvi.ForeColor = Color.White;
 lvi.Tag = null;
 }
 }
 else
 {
 bool result = e.IsPrime;

 ListViewItem lvi = UpdateListViewItem(
 taskId,
 result,
 e.FirstDivisor);

 if (lvi != null)
 {
 lvi.BackColor = Color.LightGray;
 lvi.Tag = null;
 }
 }
 }

 #endregion // Implementation

 /////////////////////////////////////////////////////////////
 //
 #region Private Methods

 private ListViewItem AddListViewItem(
 Guid guid,
 int testNumber )
 {
 ListViewItem lvi = new ListViewItem();
 lvi.Text = testNumber.ToString(
 CultureInfo.CurrentCulture.NumberFormat);

 lvi.SubItems.Add("Not Started");
 lvi.SubItems.Add("1");
 lvi.SubItems.Add(guid.ToString());
 lvi.SubItems.Add("---");
 lvi.SubItems.Add("---");
 lvi.Tag = guid;

 this.listView1.Items.Add( lvi );

 return lvi;
 }

 private ListViewItem UpdateListViewItem(
 Guid guid,
 int percentComplete,
 int current )
 {
 ListViewItem lviRet = null;

 foreach (ListViewItem lvi in this.listView1.Items)
 {
 if (lvi.Tag != null)
 {
 if ((Guid)lvi.Tag == guid)
 {
 lvi.SubItems[1].Text =
 percentComplete.ToString(
 CultureInfo.CurrentCulture.NumberFormat);
 lvi.SubItems[2].Text =
 current.ToString(
 CultureInfo.CurrentCulture.NumberFormat);
 lviRet = lvi;
 break;
 }
 }
 }

 return lviRet;
 }

 private ListViewItem UpdateListViewItem(
 Guid guid,
 int percentComplete,
 int current,
 bool result,
 int firstDivisor )
 {
 ListViewItem lviRet = null;

 foreach (ListViewItem lvi in this.listView1.Items)
 {
 if ((Guid)lvi.Tag == guid)
 {
 lvi.SubItems[1].Text =
 percentComplete.ToString(
 CultureInfo.CurrentCulture.NumberFormat);
 lvi.SubItems[2].Text =
 current.ToString(
 CultureInfo.CurrentCulture.NumberFormat);
 lvi.SubItems[4].Text =
 result ? "Prime" : "Composite";
 lvi.SubItems[5].Text =
 firstDivisor.ToString(
 CultureInfo.CurrentCulture.NumberFormat);

 lviRet = lvi;

 break;
 }
 }

 return lviRet;
 }

 private ListViewItem UpdateListViewItem(
 Guid guid,
 int percentComplete )
 {
 ListViewItem lviRet = null;

 foreach (ListViewItem lvi in this.listView1.Items)
 {
 if (lvi.Tag != null)
 {
 if ((Guid)lvi.Tag == guid)
 {
 lvi.SubItems[1].Text =
 percentComplete.ToString(
 CultureInfo.CurrentCulture.NumberFormat);
 lviRet = lvi;
 break;
 }
 }
 }

 return lviRet;
 }

 private ListViewItem UpdateListViewItem(
 Guid guid,
 bool result,
 int firstDivisor )
 {
 ListViewItem lviRet = null;

 foreach (ListViewItem lvi in this.listView1.Items)
 {
 if (lvi.Tag != null)
 {
 if ((Guid)lvi.Tag == guid)
 {
 lvi.SubItems[4].Text =
 result ? "Prime" : "Composite";
 lvi.SubItems[5].Text =
 firstDivisor.ToString(
 CultureInfo.CurrentCulture.NumberFormat);
 lviRet = lvi;
 break;
 }
 }
 }

 return lviRet;
 }

 private ListViewItem UpdateListViewItem(
 Guid guid,
 string result)
 {
 ListViewItem lviRet = null;

 foreach (ListViewItem lvi in this.listView1.Items)
 {
 if (lvi.Tag != null)
 {
 if ((Guid)lvi.Tag == guid)
 {
 lvi.SubItems[4].Text = result;
 lviRet = lvi;
 break;
 }
 }
 }

 return lviRet;
 }

 private bool CanCancel()
 {
 bool oneIsActive = false;

 foreach(ListViewItem lvi in this.listView1.SelectedItems)
 {
 if (lvi.Tag != null)
 {
 oneIsActive = true;
 break;
 }
 }

 return( oneIsActive == true );
 }

 #endregion

 #region Windows Form Designer generated code
 
 private void InitializeComponent()
 {
 this.components = new System.ComponentModel.Container();
 this.taskGroupBox = new System.Windows.Forms.GroupBox();
 this.buttonPanel = new System.Windows.Forms.Panel();
 this.cancelButton = new System.Windows.Forms.Button();
 this.startAsyncButton = new System.Windows.Forms.Button();
 this.listView1 = new System.Windows.Forms.ListView();
 this.testNumberColHeader = new System.Windows.Forms.ColumnHeader();
 this.progressColHeader = new System.Windows.Forms.ColumnHeader();
 this.currentColHeader = new System.Windows.Forms.ColumnHeader();
 this.taskIdColHeader = new System.Windows.Forms.ColumnHeader();
 this.resultColHeader = new System.Windows.Forms.ColumnHeader();
 this.firstDivisorColHeader = new System.Windows.Forms.ColumnHeader();
 this.panel2 = new System.Windows.Forms.Panel();
 this.primeNumberCalculator1 = new AsyncPattern.PrimeNumberCalculator(this.components);
 this.taskGroupBox.SuspendLayout();
 this.buttonPanel.SuspendLayout();
 this.SuspendLayout();
 //
 // taskGroupBox
 //
 this.taskGroupBox.Controls.Add(this.buttonPanel);
 this.taskGroupBox.Controls.Add(this.listView1);
 this.taskGroupBox.Dock = System.Windows.Forms.DockStyle.Fill;
 this.taskGroupBox.Location = new System.Drawing.Point(0, 0);
 this.taskGroupBox.Name = "taskGroupBox";
 this.taskGroupBox.Size = new System.Drawing.Size(608, 254);
 this.taskGroupBox.TabIndex = 1;
 this.taskGroupBox.TabStop = false;
 this.taskGroupBox.Text = "Tasks";
 //
 // buttonPanel
 //
 this.buttonPanel.Controls.Add(this.cancelButton);
 this.buttonPanel.Controls.Add(this.startAsyncButton);
 this.buttonPanel.Dock = System.Windows.Forms.DockStyle.Bottom;
 this.buttonPanel.Location = new System.Drawing.Point(3, 176);
 this.buttonPanel.Name = "buttonPanel";
 this.buttonPanel.Size = new System.Drawing.Size(602, 75);
 this.buttonPanel.TabIndex = 1;
 //
 // cancelButton
 //
 this.cancelButton.Enabled = false;
 this.cancelButton.Location = new System.Drawing.Point(128, 24);
 this.cancelButton.Name = "cancelButton";
 this.cancelButton.Size = new System.Drawing.Size(88, 23);
 this.cancelButton.TabIndex = 1;
 this.cancelButton.Text = "Cancel";
 this.cancelButton.Click += new System.EventHandler(this.cancelButton_Click);
 //
 // startAsyncButton
 //
 this.startAsyncButton.Location = new System.Drawing.Point(24, 24);
 this.startAsyncButton.Name = "startAsyncButton";
 this.startAsyncButton.Size = new System.Drawing.Size(88, 23);
 this.startAsyncButton.TabIndex = 0;
 this.startAsyncButton.Text = "Start New Task";
 this.startAsyncButton.Click += new System.EventHandler(this.startAsyncButton_Click);
 //
 // listView1
 //
 this.listView1.Columns.AddRange(new System.Windows.Forms.ColumnHeader[] {
 this.testNumberColHeader,
 this.progressColHeader,
 this.currentColHeader,
 this.taskIdColHeader,
 this.resultColHeader,
 this.firstDivisorColHeader});
 this.listView1.Dock = System.Windows.Forms.DockStyle.Fill;
 this.listView1.FullRowSelect = true;
 this.listView1.GridLines = true;
 this.listView1.Location = new System.Drawing.Point(3, 16);
 this.listView1.Name = "listView1";
 this.listView1.Size = new System.Drawing.Size(602, 160);
 this.listView1.TabIndex = 0;
 this.listView1.View = System.Windows.Forms.View.Details;
 //
 // testNumberColHeader
 //
 this.testNumberColHeader.Text = "Test Number";
 this.testNumberColHeader.Width = 80;
 //
 // progressColHeader
 //
 this.progressColHeader.Text = "Progress";
 //
 // currentColHeader
 //
 this.currentColHeader.Text = "Current";
 //
 // taskIdColHeader
 //
 this.taskIdColHeader.Text = "Task ID";
 this.taskIdColHeader.Width = 200;
 //
 // resultColHeader
 //
 this.resultColHeader.Text = "Result";
 this.resultColHeader.Width = 80;
 //
 // firstDivisorColHeader
 //
 this.firstDivisorColHeader.Text = "First Divisor";
 this.firstDivisorColHeader.Width = 80;
 //
 // panel2
 //
 this.panel2.Location = new System.Drawing.Point(200, 128);
 this.panel2.Name = "panel2";
 this.panel2.TabIndex = 2;
 //
 // PrimeNumberCalculatorMain
 //
 this.ClientSize = new System.Drawing.Size(608, 254);
 this.Controls.Add(this.taskGroupBox);
 this.Name = "PrimeNumberCalculatorMain";
 this.Text = "Prime Number Calculator";
 this.taskGroupBox.ResumeLayout(false);
 this.buttonPanel.ResumeLayout(false);
 this.ResumeLayout(false);
 }
 #endregion

 }


 /////////////////////////////////////////////////////////////
 #region PrimeNumberCalculator Implementation

 public delegate void ProgressChangedEventHandler(
 ProgressChangedEventArgs e);

 public delegate void CalculatePrimeCompletedEventHandler(
 object sender,
 CalculatePrimeCompletedEventArgs e);

 // This class implements the Event-based Asynchronous Pattern.
 // It asynchronously computes whether a number is prime or
 // composite (not prime).
 public class PrimeNumberCalculator : Component
 {
 private delegate void WorkerEventHandler(
 int numberToCheck,
 AsyncOperation asyncOp);

 private SendOrPostCallback onProgressReportDelegate;
 private SendOrPostCallback onCompletedDelegate;

 private HybridDictionary userStateToLifetime =
 new HybridDictionary();

 private System.ComponentModel.Container components = null;

 /////////////////////////////////////////////////////////////
 #region Public events

 public event ProgressChangedEventHandler ProgressChanged;
 public event CalculatePrimeCompletedEventHandler CalculatePrimeCompleted;

 #endregion

 /////////////////////////////////////////////////////////////
 #region Construction and destruction

 public PrimeNumberCalculator(IContainer container)
 {
 container.Add(this);
 InitializeComponent();

 InitializeDelegates();
 }

 public PrimeNumberCalculator()
 {
 InitializeComponent();

 InitializeDelegates();
 }

 protected virtual void InitializeDelegates()
 {
 onProgressReportDelegate =
 new SendOrPostCallback(ReportProgress);
 onCompletedDelegate =
 new SendOrPostCallback(CalculateCompleted);
 }

 protected override void Dispose(bool disposing)
 {
 if (disposing)
 {
 if (components != null)
 {
 components.Dispose();
 }
 }
 base.Dispose(disposing);
 }

 #endregion // Construction and destruction

 /////////////////////////////////////////////////////////////
 ///
 #region Implementation

 // This method starts an asynchronous calculation.
 // First, it checks the supplied task ID for uniqueness.
 // If taskId is unique, it creates a new WorkerEventHandler
 // and calls its BeginInvoke method to start the calculation.
 public virtual void CalculatePrimeAsync(
 int numberToTest,
 object taskId)
 {
 // Create an AsyncOperation for taskId.
 AsyncOperation asyncOp =
 AsyncOperationManager.CreateOperation(taskId);

 // Multiple threads will access the task dictionary,
 // so it must be locked to serialize access.
 lock (userStateToLifetime.SyncRoot)
 {
 if (userStateToLifetime.Contains(taskId))
 {
 throw new ArgumentException(
 "Task ID parameter must be unique",
 "taskId");
 }

 userStateToLifetime[taskId] = asyncOp;
 }

 // Start the asynchronous operation.
 WorkerEventHandler workerDelegate = new WorkerEventHandler(CalculateWorker);
 workerDelegate.BeginInvoke(
 numberToTest,
 asyncOp,
 null,
 null);
 }

 // Utility method for determining if a
 // task has been canceled.
 private bool TaskCanceled(object taskId)
 {
 return( userStateToLifetime[taskId] == null );
 }

 // This method cancels a pending asynchronous operation.
 public void CancelAsync(object taskId)
 {
 AsyncOperation asyncOp = userStateToLifetime[taskId] as AsyncOperation;
 if (asyncOp != null)
 {
 lock (userStateToLifetime.SyncRoot)
 {
 userStateToLifetime.Remove(taskId);
 }
 }
 }

 // This method performs the actual prime number computation.
 // It is executed on the worker thread.
 private void CalculateWorker(
 int numberToTest,
 AsyncOperation asyncOp)
 {
 bool isPrime = false;
 int firstDivisor = 1;
 Exception e = null;

 // Check that the task is still active.
 // The operation may have been canceled before
 // the thread was scheduled.
 if (!TaskCanceled(asyncOp.UserSuppliedState))
 {
 try
 {
 // Find all the prime numbers up to
 // the square root of numberToTest.
 ArrayList primes = BuildPrimeNumberList(
 numberToTest,
 asyncOp);

 // Now we have a list of primes less than
 // numberToTest.
 isPrime = IsPrime(
 primes,
 numberToTest,
 out firstDivisor);
 }
 catch (Exception ex)
 {
 e = ex;
 }
 }

 //CalculatePrimeState calcState = new CalculatePrimeState(
 // numberToTest,
 // firstDivisor,
 // isPrime,
 // e,
 // TaskCanceled(asyncOp.UserSuppliedState),
 // asyncOp);

 //this.CompletionMethod(calcState);

 this.CompletionMethod(
 numberToTest,
 firstDivisor,
 isPrime,
 e,
 TaskCanceled(asyncOp.UserSuppliedState),
 asyncOp);

 //completionMethodDelegate(calcState);
 }

 // This method computes the list of prime numbers used by the
 // IsPrime method.
 private ArrayList BuildPrimeNumberList(
 int numberToTest,
 AsyncOperation asyncOp)
 {
 ProgressChangedEventArgs e = null;
 ArrayList primes = new ArrayList();
 int firstDivisor;
 int n = 5;

 // Add the first prime numbers.
 primes.Add(2);
 primes.Add(3);

 // Do the work.
 while (n < numberToTest &&
 !TaskCanceled( asyncOp.UserSuppliedState ) )
 {
 if (IsPrime(primes, n, out firstDivisor))
 {
 // Report to the client that a prime was found.
 e = new CalculatePrimeProgressChangedEventArgs(
 n,
 (int)((float)n / (float)numberToTest * 100),
 asyncOp.UserSuppliedState);

 asyncOp.Post(this.onProgressReportDelegate, e);

 primes.Add(n);

 // Yield the rest of this time slice.
 Thread.Sleep(0);
 }

 // Skip even numbers.
 n += 2;
 }

 return primes;
 }

 // This method tests n for primality against the list of
 // prime numbers contained in the primes parameter.
 private bool IsPrime(
 ArrayList primes,
 int n,
 out int firstDivisor)
 {
 bool foundDivisor = false;
 bool exceedsSquareRoot = false;

 int i = 0;
 int divisor = 0;
 firstDivisor = 1;

 // Stop the search if:
 // there are no more primes in the list,
 // there is a divisor of n in the list, or
 // there is a prime that is larger than
 // the square root of n.
 while (
 (i < primes.Count) &&
 !foundDivisor &&
 !exceedsSquareRoot)
 {
 // The divisor variable will be the smallest
 // prime number not yet tried.
 divisor = (int)primes[i++];

 // Determine whether the divisor is greater
 // than the square root of n.
 if (divisor * divisor > n)
 {
 exceedsSquareRoot = true;
 }
 // Determine whether the divisor is a factor of n.
 else if (n % divisor == 0)
 {
 firstDivisor = divisor;
 foundDivisor = true;
 }
 }

 return !foundDivisor;
 }

 // This method is invoked via the AsyncOperation object,
 // so it is guaranteed to be executed on the correct thread.
 private void CalculateCompleted(object operationState)
 {
 CalculatePrimeCompletedEventArgs e =
 operationState as CalculatePrimeCompletedEventArgs;

 OnCalculatePrimeCompleted(e);
 }

 // This method is invoked via the AsyncOperation object,
 // so it is guaranteed to be executed on the correct thread.
 private void ReportProgress(object state)
 {
 ProgressChangedEventArgs e =
 state as ProgressChangedEventArgs;

 OnProgressChanged(e);
 }

 protected void OnCalculatePrimeCompleted(
 CalculatePrimeCompletedEventArgs e)
 {
 if (CalculatePrimeCompleted != null)
 {
 CalculatePrimeCompleted(this, e);
 }
 }

 protected void OnProgressChanged(ProgressChangedEventArgs e)
 {
 if (ProgressChanged != null)
 {
 ProgressChanged(e);
 }
 }

 // This is the method that the underlying, free-threaded
 // asynchronous behavior will invoke. This will happen on
 // an arbitrary thread.
 private void CompletionMethod(
 int numberToTest,
 int firstDivisor,
 bool isPrime,
 Exception exception,
 bool canceled,
 AsyncOperation asyncOp )

 {
 // If the task was not previously canceled,
 // remove the task from the lifetime collection.
 if (!canceled)
 {
 lock (userStateToLifetime.SyncRoot)
 {
 userStateToLifetime.Remove(asyncOp.UserSuppliedState);
 }
 }

 // Package the results of the operation in a
 // CalculatePrimeCompletedEventArgs.
 CalculatePrimeCompletedEventArgs e =
 new CalculatePrimeCompletedEventArgs(
 numberToTest,
 firstDivisor,
 isPrime,
 exception,
 canceled,
 asyncOp.UserSuppliedState);

 // End the task. The asyncOp object is responsible
 // for marshaling the call.
 asyncOp.PostOperationCompleted(onCompletedDelegate, e);

 // Note that after the call to OperationCompleted,
 // asyncOp is no longer usable, and any attempt to use it
 // will cause an exception to be thrown.
 }

 #endregion

 /////////////////////////////////////////////////////////////
 #region Component Designer generated code

 private void InitializeComponent()
 {
 components = new System.ComponentModel.Container();
 }

 #endregion

 }

 public class CalculatePrimeProgressChangedEventArgs :
 ProgressChangedEventArgs
 {
 private int latestPrimeNumberValue = 1;

 public CalculatePrimeProgressChangedEventArgs(
 int latestPrime,
 int progressPercentage,
 object userToken) : base( progressPercentage, userToken )
 {
 this.latestPrimeNumberValue = latestPrime;
 }

 public int LatestPrimeNumber
 {
 get
 {
 return latestPrimeNumberValue;
 }
 }
 }

 public class CalculatePrimeCompletedEventArgs :
 AsyncCompletedEventArgs
 {
 private int numberToTestValue = 0;
 private int firstDivisorValue = 1;
 private bool isPrimeValue;

 public CalculatePrimeCompletedEventArgs(
 int numberToTest,
 int firstDivisor,
 bool isPrime,
 Exception e,
 bool canceled,
 object state) : base(e, canceled, state)
 {
 this.numberToTestValue = numberToTest;
 this.firstDivisorValue = firstDivisor;
 this.isPrimeValue = isPrime;
 }

 public int NumberToTest
 {
 get
 {
 // Raise an exception if the operation failed or
 // was canceled.
 RaiseExceptionIfNecessary();

 // If the operation was successful, return the
 // property value.
 return numberToTestValue;
 }
 }

 public int FirstDivisor
 {
 get
 {
 // Raise an exception if the operation failed or
 // was canceled.
 RaiseExceptionIfNecessary();

 // If the operation was successful, return the
 // property value.
 return firstDivisorValue;
 }
 }

 public bool IsPrime
 {
 get
 {
 // Raise an exception if the operation failed or
 // was canceled.
 RaiseExceptionIfNecessary();

 // If the operation was successful, return the
 // property value.
 return isPrimeValue;
 }
 }
 }


 #endregion

}

Imports System.Collections
Imports System.Collections.Specialized
Imports System.ComponentModel
Imports System.Drawing
Imports System.Globalization
Imports System.Threading
Imports System.Windows.Forms

' This form tests the PrimeNumberCalculator component.
Public Class PrimeNumberCalculatorMain
 Inherits System.Windows.Forms.Form

 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
 ' Private fields
 '
 #Region "Private fields"
 
 Private WithEvents primeNumberCalculator1 As PrimeNumberCalculator
 Private taskGroupBox As System.Windows.Forms.GroupBox
 Private WithEvents listView1 As System.Windows.Forms.ListView
 Private taskIdColHeader As System.Windows.Forms.ColumnHeader
 Private progressColHeader As System.Windows.Forms.ColumnHeader
 Private currentColHeader As System.Windows.Forms.ColumnHeader
 Private buttonPanel As System.Windows.Forms.Panel
 Private panel2 As System.Windows.Forms.Panel
 Private WithEvents startAsyncButton As System.Windows.Forms.Button
 Private WithEvents cancelAsyncButton As System.Windows.Forms.Button
 Private testNumberColHeader As System.Windows.Forms.ColumnHeader
 Private resultColHeader As System.Windows.Forms.ColumnHeader
 Private firstDivisorColHeader As System.Windows.Forms.ColumnHeader
 Private components As System.ComponentModel.IContainer
 Private progressCounter As Integer
 Private progressInterval As Integer = 100


#End Region

 
 Public Sub New()

 InitializeComponent()

 End Sub

 Protected Overrides Sub Dispose(ByVal disposing As Boolean)
 If disposing Then
 If (components IsNot Nothing) Then
 components.Dispose()
 End If
 End If
 MyBase.Dispose(disposing)

 End Sub


 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
#Region "Implementation"

 ' This event handler selects a number randomly to test
 ' for primality. It then starts the asynchronous 
 ' calculation by calling the PrimeNumberCalculator
 ' component's CalculatePrimeAsync method.
 Private Sub startAsyncButton_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) _
 Handles startAsyncButton.Click

 ' Randomly choose test numbers 
 ' up to 200,000 for primality.
 Dim rand As New Random
 Dim testNumber As Integer = rand.Next(200000)

 ' Task IDs are Guids.
 Dim taskId As Guid = Guid.NewGuid()
 Me.AddListViewItem(taskId, testNumber)

 ' Start the asynchronous task.
 Me.primeNumberCalculator1.CalculatePrimeAsync( _
 testNumber, _
 taskId)

 End Sub


 Private Sub listView1_SelectedIndexChanged( _
 ByVal sender As Object, ByVal e As EventArgs) _
 Handles listView1.SelectedIndexChanged

 Me.cancelAsyncButton.Enabled = CanCancel()

 End Sub


 ' This event handler cancels all pending tasks that are
 ' selected in the ListView control.
 Private Sub cancelAsyncButton_Click( _
 ByVal sender As System.Object, _
 ByVal e As System.EventArgs) _
 Handles cancelAsyncButton.Click

 Dim taskId As Guid = Guid.Empty

 ' Cancel all selected tasks.
 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.SelectedItems
 ' Tasks that have been completed or canceled have 
 ' their corresponding ListViewItem.Tag property
 ' set to Nothing.
 If (lvi.Tag IsNot Nothing) Then
 taskId = CType(lvi.Tag, Guid)
 Me.primeNumberCalculator1.CancelAsync(taskId)
 lvi.Selected = False
 End If
 Next lvi

 cancelAsyncButton.Enabled = False

 End Sub

 ' This event handler updates the ListView control when the
 ' PrimeNumberCalculator raises the ProgressChanged event.
 '
 ' On fast computers, the PrimeNumberCalculator can raise many
 ' successive ProgressChanged events, so the user interface 
 ' may be flooded with messages. To prevent the user interface
 ' from hanging, progress is only reported at intervals. 
 Private Sub primeNumberCalculator1_ProgressChanged( _
 ByVal e As ProgressChangedEventArgs) _
 Handles primeNumberCalculator1.ProgressChanged

 Me.progressCounter += 1

 If Me.progressCounter Mod Me.progressInterval = 0 Then

 Dim taskId As Guid = CType(e.UserState, Guid)

 If TypeOf e Is CalculatePrimeProgressChangedEventArgs Then
 Dim cppcea As CalculatePrimeProgressChangedEventArgs = e
 Me.UpdateListViewItem( _
 taskId, _
 cppcea.ProgressPercentage, _
 cppcea.LatestPrimeNumber)
 Else
 Me.UpdateListViewItem( _
 taskId, e.ProgressPercentage)
 End If
 ElseIf Me.progressCounter > Me.progressInterval Then
 Me.progressCounter = 0
 End If

 End Sub

 ' This event handler updates the ListView control when the
 ' PrimeNumberCalculator raises the CalculatePrimeCompleted
 ' event. The ListView item is updated with the appropriate
 ' outcome of the calculation: Canceled, Error, or result.
 Private Sub primeNumberCalculator1_CalculatePrimeCompleted( _
 ByVal sender As Object, _
 ByVal e As CalculatePrimeCompletedEventArgs) _
 Handles primeNumberCalculator1.CalculatePrimeCompleted

 Dim taskId As Guid = CType(e.UserState, Guid)

 If e.Cancelled Then
 Dim result As String = "Canceled"

 Dim lvi As ListViewItem = UpdateListViewItem( _
 taskId, _
 result)

 If (lvi IsNot Nothing) Then
 lvi.BackColor = Color.Pink
 lvi.Tag = Nothing
 End If

 ElseIf e.Error IsNot Nothing Then

 Dim result As String = "Error"

 Dim lvi As ListViewItem = UpdateListViewItem( _
 taskId, result)

 If (lvi IsNot Nothing) Then
 lvi.BackColor = Color.Red
 lvi.ForeColor = Color.White
 lvi.Tag = Nothing
 End If
 Else
 Dim result As Boolean = e.IsPrime

 Dim lvi As ListViewItem = UpdateListViewItem( _
 taskId, _
 result, _
 e.FirstDivisor)

 If (lvi IsNot Nothing) Then
 lvi.BackColor = Color.LightGray
 lvi.Tag = Nothing
 End If
 End If

 End Sub

#End Region

 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
#Region "Private Methods"

 Private Function AddListViewItem( _
 ByVal guid As Guid, _
 ByVal testNumber As Integer) As ListViewItem

 Dim lvi As New ListViewItem
 lvi.Text = testNumber.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)

 lvi.SubItems.Add("Not Started")
 lvi.SubItems.Add("1")
 lvi.SubItems.Add(guid.ToString())
 lvi.SubItems.Add("---")
 lvi.SubItems.Add("---")
 lvi.Tag = guid

 Me.listView1.Items.Add(lvi)

 Return lvi

 End Function

 Private Overloads Function UpdateListViewItem( _
 ByVal guid As Guid, _
 ByVal percentComplete As Integer, _
 ByVal current As Integer) As ListViewItem

 Dim lviRet As ListViewItem = Nothing

 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.Items
 If (lvi.Tag IsNot Nothing) Then
 If guid.CompareTo(CType(lvi.Tag, Guid)) = 0 Then
 lvi.SubItems(1).Text = percentComplete.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)
 lvi.SubItems(2).Text = current.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)
 lviRet = lvi
 Exit For
 End If
 End If
 Next lvi

 Return lviRet

 End Function


 Private Overloads Function UpdateListViewItem( _
 ByVal guid As Guid, _
 ByVal percentComplete As Integer, _
 ByVal current As Integer, _
 ByVal result As Boolean, _
 ByVal firstDivisor As Integer) As ListViewItem

 Dim lviRet As ListViewItem = Nothing

 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.Items
 If guid.CompareTo(CType(lvi.Tag, Guid)) = 0 Then
 lvi.SubItems(1).Text = percentComplete.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)
 lvi.SubItems(2).Text = current.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)
 lvi.SubItems(4).Text = _
 IIf(result, "Prime", "Composite")
 lvi.SubItems(5).Text = firstDivisor.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)

 lviRet = lvi

 Exit For
 End If
 Next lvi

 Return lviRet

 End Function


 Private Overloads Function UpdateListViewItem( _
 ByVal guid As Guid, _
 ByVal percentComplete As Integer) As ListViewItem

 Dim lviRet As ListViewItem = Nothing

 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.Items
 If (lvi.Tag IsNot Nothing) Then
 If guid.CompareTo(CType(lvi.Tag, Guid)) = 0 Then
 lvi.SubItems(1).Text = percentComplete.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)
 lviRet = lvi
 Exit For
 End If
 End If
 Next lvi

 Return lviRet

 End Function


 Private Overloads Function UpdateListViewItem( _
 ByVal guid As Guid, _
 ByVal result As Boolean, _
 ByVal firstDivisor As Integer) As ListViewItem

 Dim lviRet As ListViewItem = Nothing

 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.Items
 If (lvi.Tag IsNot Nothing) Then
 If guid.CompareTo(CType(lvi.Tag, Guid)) = 0 Then
 lvi.SubItems(4).Text = _
 IIf(result, "Prime", "Composite")
 lvi.SubItems(5).Text = firstDivisor.ToString( _
 CultureInfo.CurrentCulture.NumberFormat)
 lviRet = lvi
 Exit For
 End If
 End If
 Next lvi

 Return lviRet

 End Function


 Private Overloads Function UpdateListViewItem( _
 ByVal guid As Guid, _
 ByVal result As String) As ListViewItem

 Dim lviRet As ListViewItem = Nothing

 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.Items
 If (lvi.Tag IsNot Nothing) Then
 If guid.CompareTo(CType(lvi.Tag, Guid)) = 0 Then
 lvi.SubItems(4).Text = result
 lviRet = lvi
 Exit For
 End If
 End If
 Next lvi

 Return lviRet

 End Function


 Private Function CanCancel() As Boolean
 Dim oneIsActive As Boolean = False

 Dim lvi As ListViewItem
 For Each lvi In Me.listView1.SelectedItems
 If (lvi.Tag IsNot Nothing) Then
 oneIsActive = True
 Exit For
 End If
 Next lvi

 Return oneIsActive = True

 End Function

#End Region


 Private Sub InitializeComponent()
 Me.components = New System.ComponentModel.Container
 Me.taskGroupBox = New System.Windows.Forms.GroupBox
 Me.buttonPanel = New System.Windows.Forms.Panel
 Me.cancelAsyncButton = New System.Windows.Forms.Button
 Me.startAsyncButton = New System.Windows.Forms.Button
 Me.listView1 = New System.Windows.Forms.ListView
 Me.testNumberColHeader = New System.Windows.Forms.ColumnHeader
 Me.progressColHeader = New System.Windows.Forms.ColumnHeader
 Me.currentColHeader = New System.Windows.Forms.ColumnHeader
 Me.taskIdColHeader = New System.Windows.Forms.ColumnHeader
 Me.resultColHeader = New System.Windows.Forms.ColumnHeader
 Me.firstDivisorColHeader = New System.Windows.Forms.ColumnHeader
 Me.panel2 = New System.Windows.Forms.Panel
 Me.primeNumberCalculator1 = New PrimeNumberCalculator(Me.components)
 Me.taskGroupBox.SuspendLayout()
 Me.buttonPanel.SuspendLayout()
 Me.SuspendLayout()
 ' 
 ' taskGroupBox
 ' 
 Me.taskGroupBox.Controls.Add(Me.buttonPanel)
 Me.taskGroupBox.Controls.Add(Me.listView1)
 Me.taskGroupBox.Dock = System.Windows.Forms.DockStyle.Fill
 Me.taskGroupBox.Location = New System.Drawing.Point(0, 0)
 Me.taskGroupBox.Name = "taskGroupBox"
 Me.taskGroupBox.Size = New System.Drawing.Size(608, 254)
 Me.taskGroupBox.TabIndex = 1
 Me.taskGroupBox.TabStop = False
 Me.taskGroupBox.Text = "Tasks"
 ' 
 ' buttonPanel
 ' 
 Me.buttonPanel.Controls.Add(Me.cancelAsyncButton)
 Me.buttonPanel.Controls.Add(Me.startAsyncButton)
 Me.buttonPanel.Dock = System.Windows.Forms.DockStyle.Bottom
 Me.buttonPanel.Location = New System.Drawing.Point(3, 176)
 Me.buttonPanel.Name = "buttonPanel"
 Me.buttonPanel.Size = New System.Drawing.Size(602, 75)
 Me.buttonPanel.TabIndex = 1
 ' 
 ' cancelAsyncButton
 ' 
 Me.cancelAsyncButton.Enabled = False
 Me.cancelAsyncButton.Location = New System.Drawing.Point(128, 24)
 Me.cancelAsyncButton.Name = "cancelAsyncButton"
 Me.cancelAsyncButton.Size = New System.Drawing.Size(88, 23)
 Me.cancelAsyncButton.TabIndex = 1
 Me.cancelAsyncButton.Text = "Cancel"
 ' 
 ' startAsyncButton
 ' 
 Me.startAsyncButton.Location = New System.Drawing.Point(24, 24)
 Me.startAsyncButton.Name = "startAsyncButton"
 Me.startAsyncButton.Size = New System.Drawing.Size(88, 23)
 Me.startAsyncButton.TabIndex = 0
 Me.startAsyncButton.Text = "Start New Task"
 ' 
 ' listView1
 ' 
 Me.listView1.Columns.AddRange(New System.Windows.Forms.ColumnHeader() {Me.testNumberColHeader, Me.progressColHeader, Me.currentColHeader, Me.taskIdColHeader, Me.resultColHeader, Me.firstDivisorColHeader})
 Me.listView1.Dock = System.Windows.Forms.DockStyle.Fill
 Me.listView1.FullRowSelect = True
 Me.listView1.GridLines = True
 Me.listView1.Location = New System.Drawing.Point(3, 16)
 Me.listView1.Name = "listView1"
 Me.listView1.Size = New System.Drawing.Size(602, 160)
 Me.listView1.TabIndex = 0
 Me.listView1.View = System.Windows.Forms.View.Details
 ' 
 ' testNumberColHeader
 ' 
 Me.testNumberColHeader.Text = "Test Number"
 Me.testNumberColHeader.Width = 80
 ' 
 ' progressColHeader
 ' 
 Me.progressColHeader.Text = "Progress"
 ' 
 ' currentColHeader
 ' 
 Me.currentColHeader.Text = "Current"
 ' 
 ' taskIdColHeader
 ' 
 Me.taskIdColHeader.Text = "Task ID"
 Me.taskIdColHeader.Width = 200
 ' 
 ' resultColHeader
 ' 
 Me.resultColHeader.Text = "Result"
 Me.resultColHeader.Width = 80
 ' 
 ' firstDivisorColHeader
 ' 
 Me.firstDivisorColHeader.Text = "First Divisor"
 Me.firstDivisorColHeader.Width = 80
 ' 
 ' panel2
 ' 
 Me.panel2.Location = New System.Drawing.Point(200, 128)
 Me.panel2.Name = "panel2"
 Me.panel2.TabIndex = 2
 ' 
 ' PrimeNumberCalculatorMain
 '
 Me.ClientSize = New System.Drawing.Size(608, 254)
 Me.Controls.Add(taskGroupBox)
 Me.Name = "PrimeNumberCalculatorMain"
 Me.Text = "Prime Number Calculator"
 Me.taskGroupBox.ResumeLayout(False)
 Me.buttonPanel.ResumeLayout(False)
 Me.ResumeLayout(False)

 End Sub

End Class


Public Delegate Sub ProgressChangedEventHandler( _
 ByVal e As ProgressChangedEventArgs)

Public Delegate Sub CalculatePrimeCompletedEventHandler( _
 ByVal sender As Object, _
 ByVal e As CalculatePrimeCompletedEventArgs)

' This class implements the Event-based Asynchronous Pattern.
' It asynchronously computes whether a number is prime or
' composite (not prime).
Public Class PrimeNumberCalculator
 Inherits System.ComponentModel.Component

 Private Delegate Sub WorkerEventHandler( _
 ByVal numberToCheck As Integer, _
 ByVal asyncOp As AsyncOperation)

 Private onProgressReportDelegate As SendOrPostCallback
 Private onCompletedDelegate As SendOrPostCallback

 Private userStateToLifetime As New HybridDictionary()

 Private components As System.ComponentModel.Container = Nothing

 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
#Region "Public events"

 Public Event ProgressChanged _
 As ProgressChangedEventHandler
 Public Event CalculatePrimeCompleted _
 As CalculatePrimeCompletedEventHandler

#End Region

 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
#Region "Construction and destruction"


 Public Sub New(ByVal container As System.ComponentModel.IContainer)

 container.Add(Me)
 InitializeComponent()

 InitializeDelegates()

 End Sub


 Public Sub New()

 InitializeComponent()

 InitializeDelegates()

 End Sub


 Protected Overridable Sub InitializeDelegates()
 onProgressReportDelegate = _
 New SendOrPostCallback(AddressOf ReportProgress)
 onCompletedDelegate = _
 New SendOrPostCallback(AddressOf CalculateCompleted)
 End Sub


 Protected Overrides Sub Dispose(ByVal disposing As Boolean)
 If disposing Then
 If (components IsNot Nothing) Then
 components.Dispose()
 End If
 End If
 MyBase.Dispose(disposing)

 End Sub

#End Region

 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
#Region "Implementation"


 ' This method starts an asynchronous calculation. 
 ' First, it checks the supplied task ID for uniqueness.
 ' If taskId is unique, it creates a new WorkerEventHandler 
 ' and calls its BeginInvoke method to start the calculation.
 Public Overridable Sub CalculatePrimeAsync( _
 ByVal numberToTest As Integer, _
 ByVal taskId As Object)

 ' Create an AsyncOperation for taskId.
 Dim asyncOp As AsyncOperation = _
 AsyncOperationManager.CreateOperation(taskId)

 ' Multiple threads will access the task dictionary,
 ' so it must be locked to serialize access.
 SyncLock userStateToLifetime.SyncRoot
 If userStateToLifetime.Contains(taskId) Then
 Throw New ArgumentException( _
 "Task ID parameter must be unique", _
 "taskId")
 End If

 userStateToLifetime(taskId) = asyncOp
 End SyncLock

 ' Start the asynchronous operation.
 Dim workerDelegate As New WorkerEventHandler( _
 AddressOf CalculateWorker)

 workerDelegate.BeginInvoke( _
 numberToTest, _
 asyncOp, _
 Nothing, _
 Nothing)

 End Sub

 ' Utility method for determining if a 
 ' task has been canceled.
 Private Function TaskCanceled(ByVal taskId As Object) As Boolean
 Return (userStateToLifetime(taskId) Is Nothing)
 End Function

 ' This method cancels a pending asynchronous operation.
 Public Sub CancelAsync(ByVal taskId As Object)

 Dim obj As Object = userStateToLifetime(taskId)
 If (obj IsNot Nothing) Then

 SyncLock userStateToLifetime.SyncRoot

 userStateToLifetime.Remove(taskId)

 End SyncLock

 End If

 End Sub

 ' This method performs the actual prime number computation.
 ' It is executed on the worker thread.
 Private Sub CalculateWorker( _
 ByVal numberToTest As Integer, _
 ByVal asyncOp As AsyncOperation)

 Dim prime As Boolean = False
 Dim firstDivisor As Integer = 1
 Dim exc As Exception = Nothing

 ' Check that the task is still active.
 ' The operation may have been canceled before
 ' the thread was scheduled.
 If Not Me.TaskCanceled(asyncOp.UserSuppliedState) Then

 Try
 ' Find all the prime numbers up to the
 ' square root of numberToTest.
 Dim primes As ArrayList = BuildPrimeNumberList( _
 numberToTest, asyncOp)

 ' Now we have a list of primes less than 
 'numberToTest.
 prime = IsPrime( _
 primes, _
 numberToTest, _
 firstDivisor)

 Catch ex As Exception
 exc = ex
 End Try

 End If

 Me.CompletionMethod( _
 numberToTest, _
 firstDivisor, _
 prime, _
 exc, _
 TaskCanceled(asyncOp.UserSuppliedState), _
 asyncOp)

 End Sub

 ' This method computes the list of prime numbers used by the
 ' IsPrime method.
 Private Function BuildPrimeNumberList( _
 ByVal numberToTest As Integer, _
 ByVal asyncOp As AsyncOperation) As ArrayList

 Dim e As ProgressChangedEventArgs = Nothing
 Dim primes As New ArrayList
 Dim firstDivisor As Integer
 Dim n As Integer = 5

 ' Add the first prime numbers.
 primes.Add(2)
 primes.Add(3)

 ' Do the work.
 While n < numberToTest And _
 Not Me.TaskCanceled(asyncOp.UserSuppliedState)

 If IsPrime(primes, n, firstDivisor) Then
 ' Report to the client that you found a prime.
 e = New CalculatePrimeProgressChangedEventArgs( _
 n, _
 CSng(n) / CSng(numberToTest) * 100, _
 asyncOp.UserSuppliedState)

 asyncOp.Post(Me.onProgressReportDelegate, e)

 primes.Add(n)

 ' Yield the rest of this time slice.
 Thread.Sleep(0)
 End If

 ' Skip even numbers.
 n += 2

 End While

 Return primes

 End Function


 ' This method tests n for primality against the list of 
 ' prime numbers contained in the primes parameter.
 Private Function IsPrime( _
 ByVal primes As ArrayList, _
 ByVal n As Integer, _
 ByRef firstDivisor As Integer) As Boolean

 Dim foundDivisor As Boolean = False
 Dim exceedsSquareRoot As Boolean = False

 Dim i As Integer = 0
 Dim divisor As Integer = 0
 firstDivisor = 1

 ' Stop the search if:
 ' there are no more primes in the list,
 ' there is a divisor of n in the list, or
 ' there is a prime that is larger than 
 ' the square root of n.
 While i < primes.Count AndAlso _
 Not foundDivisor AndAlso _
 Not exceedsSquareRoot

 ' The divisor variable will be the smallest prime number 
 ' not yet tried.
 divisor = primes(i)
 i = i + 1

 ' Determine whether the divisor is greater than the 
 ' square root of n.
 If divisor * divisor > n Then
 exceedsSquareRoot = True
 ' Determine whether the divisor is a factor of n.
 ElseIf n Mod divisor = 0 Then
 firstDivisor = divisor
 foundDivisor = True
 End If
 End While

 Return Not foundDivisor

 End Function


 ' This method is invoked via the AsyncOperation object,
 ' so it is guaranteed to be executed on the correct thread.
 Private Sub CalculateCompleted(ByVal operationState As Object)
 Dim e As CalculatePrimeCompletedEventArgs = operationState

 OnCalculatePrimeCompleted(e)

 End Sub


 ' This method is invoked via the AsyncOperation object,
 ' so it is guaranteed to be executed on the correct thread.
 Private Sub ReportProgress(ByVal state As Object)
 Dim e As ProgressChangedEventArgs = state

 OnProgressChanged(e)

 End Sub

 Protected Sub OnCalculatePrimeCompleted( _
 ByVal e As CalculatePrimeCompletedEventArgs)

 RaiseEvent CalculatePrimeCompleted(Me, e)

 End Sub


 Protected Sub OnProgressChanged( _
 ByVal e As ProgressChangedEventArgs)

 RaiseEvent ProgressChanged(e)

 End Sub


 ' This is the method that the underlying, free-threaded 
 ' asynchronous behavior will invoke. This will happen on
 ' an arbitrary thread.
 Private Sub CompletionMethod( _
 ByVal numberToTest As Integer, _
 ByVal firstDivisor As Integer, _
 ByVal prime As Boolean, _
 ByVal exc As Exception, _
 ByVal canceled As Boolean, _
 ByVal asyncOp As AsyncOperation)

 ' If the task was not previously canceled,
 ' remove the task from the lifetime collection.
 If Not canceled Then
 SyncLock userStateToLifetime.SyncRoot
 userStateToLifetime.Remove(asyncOp.UserSuppliedState)
 End SyncLock
 End If

 ' Package the results of the operation in a 
 ' CalculatePrimeCompletedEventArgs.
 Dim e As New CalculatePrimeCompletedEventArgs( _
 numberToTest, _
 firstDivisor, _
 prime, _
 exc, _
 canceled, _
 asyncOp.UserSuppliedState)

 ' End the task. The asyncOp object is responsible 
 ' for marshaling the call.
 asyncOp.PostOperationCompleted(onCompletedDelegate, e)

 ' Note that after the call to PostOperationCompleted, asyncOp
 ' is no longer usable, and any attempt to use it will cause.
 ' an exception to be thrown.

 End Sub

#End Region


 Private Sub InitializeComponent()

 End Sub


End Class

Public Class CalculatePrimeProgressChangedEventArgs
 Inherits ProgressChangedEventArgs
 Private latestPrimeNumberValue As Integer = 1


 Public Sub New( _
 ByVal latestPrime As Integer, _
 ByVal progressPercentage As Integer, _
 ByVal UserState As Object)

 MyBase.New(progressPercentage, UserState)
 Me.latestPrimeNumberValue = latestPrime

 End Sub

 Public ReadOnly Property LatestPrimeNumber() As Integer
 Get
 Return latestPrimeNumberValue
 End Get
 End Property
End Class

Public Class CalculatePrimeCompletedEventArgs
 Inherits AsyncCompletedEventArgs
 Private numberToTestValue As Integer = 0
 Private firstDivisorValue As Integer = 1
 Private isPrimeValue As Boolean


 Public Sub New( _
 ByVal numberToTest As Integer, _
 ByVal firstDivisor As Integer, _
 ByVal isPrime As Boolean, _
 ByVal e As Exception, _
 ByVal canceled As Boolean, _
 ByVal state As Object)

 MyBase.New(e, canceled, state)
 Me.numberToTestValue = numberToTest
 Me.firstDivisorValue = firstDivisor
 Me.isPrimeValue = isPrime

 End Sub


 Public ReadOnly Property NumberToTest() As Integer
 Get
 ' Raise an exception if the operation failed 
 ' or was canceled.
 RaiseExceptionIfNecessary()

 ' If the operation was successful, return 
 ' the property value.
 Return numberToTestValue
 End Get
 End Property


 Public ReadOnly Property FirstDivisor() As Integer
 Get
 ' Raise an exception if the operation failed 
 ' or was canceled.
 RaiseExceptionIfNecessary()

 ' If the operation was successful, return 
 ' the property value.
 Return firstDivisorValue
 End Get
 End Property


 Public ReadOnly Property IsPrime() As Boolean
 Get
 ' Raise an exception if the operation failed 
 ' or was canceled.
 RaiseExceptionIfNecessary()

 ' If the operation was successful, return 
 ' the property value.
 Return isPrimeValue
 End Get
 End Property
End Class

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URL: https://learn.microsoft.com/en-us/dotnet/standard/asynchronous-programming-patterns/how-to-implement-a-client-of-the-event-based-asynchronous-pattern