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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

How to: Implement a Component That Supports the Event-based Asynchronous Pattern 

The following code example implements a component with an asynchronous method, according to the Event-based Asynchronous Pattern Overview. The component is a prime number calculator that uses the Sieve of Eratosthenes algorithm to determine if a number is prime or composite.

There is extensive support for this task in Visual Studio. Walkthrough: Implementing a Component That Supports the Event-based Asynchronous Pattern
Walkthrough: Implementing a Component That Supports the Event-based Asynchronous Pattern
Walkthrough: Implementing a Component That Supports the Event-based Asynchronous Pattern
Walkthrough: Implementing a Component That Supports the Event-based Asynchronous Pattern
Walkthrough: Implementing a Component That Supports the Event-based Asynchronous Pattern

For an example client that uses the PrimeNumberCalculator component, see How to: Implement a Client of the Event-based Asynchronous Pattern.

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;
...
    /////////////////////////////////////////////////////////////
    #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


import System.*;
import System.Collections.*;
import System.Collections.Specialized.*;
import System.ComponentModel.*;
import System.Data.*;
import System.Drawing.*;
import System.Threading.*;
import System.Windows.Forms.*;


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