The following example shows how to expose an arbitrary sequence of Event-Based Asynchronous Pattern (EAP) operations as one task by using a System.Threading.Tasks.TaskCompletionSource. The example also shows how to use a System.Threading.CancellationToken to invoke the built-in cancellation methods on the System.Net.WebClient objects.
Example
sample · csharpsamples/snippets/csharp/VS_Snippets_Misc/fromasync/cs/fromasync.cs08
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Net;
namespace specialNamespaceForOneMethodSignature
{
using System;
class Task<TResult>
{
public TResult TValue;
}
class Dummy <TResult>
{
// <snippet01>
public Task<TResult> FromAsync<TArg1, TArg2, TArg3>(
Func<TArg1, TArg2, TArg3, AsyncCallback, object, IAsyncResult> beginMethod, //BeginRead
Func<IAsyncResult, TResult> endMethod, //EndRead
TArg1 arg1, // the byte[] buffer
TArg2 arg2, // the offset in arg1 at which to start writing data
TArg3 arg3, // the maximum number of bytes to read
object state // optional state information
)
//</snippet01>
{ return new Task<TResult>(); }
}
}
namespace APM_Task
{
class TestFileStreamNaive
{
//<snippet02>
public string GetFileData(string path)
{
FileInfo fi = new FileInfo(path);
byte[] data = new byte[fi.Length];
FileStream fs;
// a using statement is ok here because we Wait before returning
// and will be done with the filestream.
using (fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true))
{
//Task<int> returns the number of bytes read
Task<int> task = Task<int>.Factory.FromAsync(
fs.BeginRead, fs.EndRead, data, 0, data.Length, null);
task.Wait();
return new UTF8Encoding().GetString(data);
}
}
//</snippet02>
static void Main()
{
TestFileStreamNaive instance = new TestFileStreamNaive();
string s = instance.GetFileData("testfile.txt");
}
}
class AsyncResult : IAsyncResult
{
public AsyncResult() { }
public object AsyncState
{
get { return "Hello from a parallel universe."; }
}
public WaitHandle AsyncWaitHandle
{
get { throw new NotImplementedException(); }
}
public bool CompletedSynchronously
{
get { throw new NotImplementedException(); }
}
public bool IsCompleted
{
get { throw new NotImplementedException(); }
}
static IAsyncResult DoSomethingAsynchronously()
{
return new AsyncResult();
}
//<snippet07>
static Task<String> ReturnTaskFromAsyncResult()
{
IAsyncResult ar = DoSomethingAsynchronously();
Task<String> t = Task<string>.Factory.FromAsync(ar, _ =>
{
return (string)ar.AsyncState;
});
return t;
}
//</snippet07>
}
//<snippet08>
class WebDataDownloader
{
static void Main()
{
WebDataDownloader downloader = new WebDataDownloader();
string[] addresses = { "http://www.msnbc.com", "http://www.yahoo.com",
"http://www.nytimes.com", "http://www.washingtonpost.com",
"http://www.latimes.com", "http://www.newsday.com" };
CancellationTokenSource cts = new CancellationTokenSource();
// Create a UI thread from which to cancel the entire operation
Task.Factory.StartNew(() =>
{
Console.WriteLine("Press c to cancel");
if (Console.ReadKey().KeyChar == 'c')
cts.Cancel();
});
// Using a neutral search term that is sure to get some hits.
Task<string[]> webTask = downloader.GetWordCounts(addresses, "the", cts.Token);
// Do some other work here unless the method has already completed.
if (!webTask.IsCompleted)
{
// Simulate some work.
Thread.SpinWait(5000000);
}
string[] results = null;
try
{
results = webTask.Result;
}
catch (AggregateException e)
{
foreach (var ex in e.InnerExceptions)
{
OperationCanceledException oce = ex as OperationCanceledException;
if (oce != null)
{
if (oce.CancellationToken == cts.Token)
{
Console.WriteLine("Operation canceled by user.");
}
}
else
{
Console.WriteLine(ex.Message);
}
}
}
finally
{
cts.Dispose();
}
if (results != null)
{
foreach (var item in results)
Console.WriteLine(item);
}
Console.ReadKey();
}
Task<string[]> GetWordCounts(string[] urls, string name, CancellationToken token)
{
TaskCompletionSource<string[]> tcs = new TaskCompletionSource<string[]>();
WebClient[] webClients = new WebClient[urls.Length];
// If the user cancels the CancellationToken, then we can use the
// WebClient's ability to cancel its own async operations.
token.Register(() =>
{
foreach (var wc in webClients)
{
if (wc != null)
wc.CancelAsync();
}
});
object m_lock = new object();
int count = 0;
List<string> results = new List<string>();
for (int i = 0; i < urls.Length; i++)
{
webClients[i] = new WebClient();
#region callback
// Specify the callback for the DownloadStringCompleted
// event that will be raised by this WebClient instance.
webClients[i].DownloadStringCompleted += (obj, args) =>
{
if (args.Cancelled == true)
{
tcs.TrySetCanceled();
return;
}
else if (args.Error != null)
{
// Pass through to the underlying Task
// any exceptions thrown by the WebClient
// during the asynchronous operation.
tcs.TrySetException(args.Error);
return;
}
else
{
// Split the string into an array of words,
// then count the number of elements that match
// the search term.
string[] words = null;
words = args.Result.Split(' ');
string NAME = name.ToUpper();
int nameCount = (from word in words.AsParallel()
where word.ToUpper().Contains(NAME)
select word)
.Count();
// Associate the results with the url, and add new string to the array that
// the underlying Task object will return in its Result property.
results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, name));
}
// If this is the last async operation to complete,
// then set the Result property on the underlying Task.
lock (m_lock)
{
count++;
if (count == urls.Length)
{
tcs.TrySetResult(results.ToArray());
}
}
};
#endregion
// Call DownloadStringAsync for each URL.
Uri address = null;
try
{
address = new Uri(urls[i]);
// Pass the address, and also use it for the userToken
// to identify the page when the delegate is invoked.
webClients[i].DownloadStringAsync(address, address);
}
catch (UriFormatException ex)
{
// Abandon the entire operation if one url is malformed.
// Other actions are possible here.
tcs.TrySetException(ex);
return tcs.Task;
}
}
// Return the underlying Task. The client code
// waits on the Result property, and handles exceptions
// in the try-catch block there.
return tcs.Task;
}
//</snippet08>
}
#region snippet09
class Calculator
{
public IAsyncResult BeginCalculate(int decimalPlaces, AsyncCallback ac, object state)
{
Console.WriteLine($"Calling BeginCalculate on thread {Thread.CurrentThread.ManagedThreadId}");
Task<string> f = Task<string>.Factory.StartNew(_ => Compute(decimalPlaces), state);
if (ac != null) f.ContinueWith((res) => ac(f));
return f;
}
public string Compute(int numPlaces)
{
Console.WriteLine($"Calling compute on thread {Thread.CurrentThread.ManagedThreadId}");
// Simulating some heavy work.
Thread.SpinWait(500000000);
// Actual implementation left as exercise for the reader.
// Several examples are available on the Web.
return "3.14159265358979323846264338327950288";
}
public string EndCalculate(IAsyncResult ar)
{
Console.WriteLine($"Calling EndCalculate on thread {Thread.CurrentThread.ManagedThreadId}");
return ((Task<string>)ar).Result;
}
}
public class CalculatorClient
{
static int decimalPlaces = 12;
public static void Main()
{
Calculator calc = new Calculator();
int places = 35;
AsyncCallback callBack = new AsyncCallback(PrintResult);
IAsyncResult ar = calc.BeginCalculate(places, callBack, calc);
// Do some work on this thread while the calculator is busy.
Console.WriteLine("Working...");
Thread.SpinWait(500000);
Console.ReadLine();
}
public static void PrintResult(IAsyncResult result)
{
Calculator c = (Calculator)result.AsyncState;
string piString = c.EndCalculate(result);
Console.WriteLine($"Calling PrintResult on thread {Thread.CurrentThread.ManagedThreadId}; result = {piString}");
}
}
#endregion
} //end APM_Task namespace
class FileStreamDemo
{
static void Main()
{
FileStreamDemo fsd = new FileStreamDemo();
// var task = fsd.GetMultiFileData(new string[1]);
fsd.ShowCallingFromAsync();
// fsd.ShowCallingFromAsyncFileData();
// fsd.ProcessFilesAsync();
// task.Wait();
// Console.WriteLine(task.Result);
Console.WriteLine("press any key");
Console.ReadLine();
}
//<snippet03>
const int MAX_FILE_SIZE = 14000000;
public static Task<string> GetFileStringAsync(string path)
{
FileInfo fi = new FileInfo(path);
byte[] data = null;
data = new byte[fi.Length];
FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);
//Task<int> returns the number of bytes read
Task<int> task = Task<int>.Factory.FromAsync(
fs.BeginRead, fs.EndRead, data, 0, data.Length, null);
// It is possible to do other work here while waiting
// for the antecedent task to complete.
// ...
// Add the continuation, which returns a Task<string>.
return task.ContinueWith((antecedent) =>
{
fs.Close();
// Result = "number of bytes read" (if we need it.)
if (antecedent.Result < 100)
{
return "Data is too small to bother with.";
}
else
{
// If we did not receive the entire file, the end of the
// data buffer will contain garbage.
if (antecedent.Result < data.Length)
Array.Resize(ref data, antecedent.Result);
// Will be returned in the Result property of the Task<string>
// at some future point after the asynchronous file I/O operation completes.
return new UTF8Encoding().GetString(data);
}
});
}
//</snippet03>
void ShowCallingFromAsync()
{
string path = @"\\docbuild2\public\Main\Logs\DllDiffReport\DllDiffReport_BetweenInBuildAndInProduct.html";
//<snippet04>
Task<string> t = GetFileStringAsync(path);
// Do some other work:
// ...
try
{
Console.WriteLine(t.Result.Substring(0, 500));
}
catch (AggregateException ae)
{
Console.WriteLine(ae.InnerException.Message);
}
//</snippet04>
Console.ReadLine();
}
//not used???
//<snippet12>
public static Task<int> GetFileDataAsync(string path, ref byte[] data)
{
// Error handling omitted for brevity...
FileInfo fi = new FileInfo(path);
data = new byte[fi.Length];
FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);
// Task<int> returns the number of bytes read
// Pass FileStream in AsyncState parameter in order to close it from the call site.
return Task<int>.Factory.FromAsync(
fs.BeginRead, fs.EndRead, data, 0, data.Length, fs);
}
//</snippet12>
void ShowCallingFromAsyncFileData()
{
//<snippet13>
string path = @"\\remoteComputer\filename.txt";
byte[] buffer = null;
Task<int> t = GetFileDataAsync(path, ref buffer);
// Can do some other work here:
// ...
for(int i = 0; i < 10; i++)
Console.Write("Working... ");
var result =
t.ContinueWith((antecedent) =>
{
// Error handling omitted...
return new UTF8Encoding().GetString(buffer);
});
// Can also do some other work here:
// ...
for (int i = 0; i < 10; i++)
Console.Write("Working more... ");
try
{
t.Wait();
Console.WriteLine(result.Result.Substring(0, 50));
}
catch (AggregateException ae)
{
Console.WriteLine(ae.InnerException.Message);
}
//</snippet13>
Console.ReadLine();
}
// nested dummy class for snippet0 5 below
class MyCustomState
{
public string StateData { get; set; }
public MyCustomState() { StateData = "Hello"; }
}
private MyCustomState GetCustomState()
{
return new MyCustomState();
}
//<snippet05>
public Task<string> GetFileStringAsync2(string path)
{
FileInfo fi = new FileInfo(path);
byte[] data = new byte[fi.Length];
MyCustomState state = GetCustomState();
FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);
// We still pass null for the last parameter because
// the state variable is visible to the continuation delegate.
Task<int> task = Task<int>.Factory.FromAsync(
fs.BeginRead, fs.EndRead, data, 0, data.Length, null);
return task.ContinueWith((antecedent) =>
{
// It is safe to close the filestream now.
fs.Close();
// Capture custom state data directly in the user delegate.
// No need to pass it through the FromAsync method.
if (state.StateData.Contains("New York, New York"))
{
return "Start spreading the news!";
}
else
{
// If we did not receive the entire file, the end of the
// data buffer will contain garbage.
if (antecedent.Result < data.Length)
Array.Resize(ref data, antecedent.Result);
// Will be returned in the Result property of the Task<string>
// at some future point after the asynchronous file I/O operation completes.
return new UTF8Encoding().GetString(data);
}
});
}
//</snippet05>
string[] GetFilesToRead()
{
return new string[3] { "test.txt", "test1.txt", "test2.txt" };
}
//<snippet06>
public Task<string> GetMultiFileData(string[] filesToRead)
{
FileStream fs;
Task<string>[] tasks = new Task<string>[filesToRead.Length];
byte[] fileData = null;
for (int i = 0; i < filesToRead.Length; i++)
{
fileData = new byte[0x1000];
fs = new FileStream(filesToRead[i], FileMode.Open, FileAccess.Read, FileShare.Read, fileData.Length, true);
// By adding the continuation here, the
// Result of each task will be a string.
tasks[i] = Task<int>.Factory.FromAsync(
fs.BeginRead, fs.EndRead, fileData, 0, fileData.Length, null)
.ContinueWith((antecedent) =>
{
fs.Close();
// If we did not receive the entire file, the end of the
// data buffer will contain garbage.
if (antecedent.Result < fileData.Length)
Array.Resize(ref fileData, antecedent.Result);
// Will be returned in the Result property of the Task<string>
// at some future point after the asynchronous file I/O operation completes.
return new UTF8Encoding().GetString(fileData);
});
}
// Wait for all tasks to complete.
return Task<string>.Factory.ContinueWhenAll(tasks, (data) =>
{
// Propagate all exceptions and mark all faulted tasks as observed.
Task.WaitAll(data);
// Combine the results from all tasks.
StringBuilder sb = new StringBuilder();
foreach (var t in data)
{
sb.Append(t.Result);
}
// Final result to be returned eventually on the calling thread.
return sb.ToString();
});
}
//</snippet06>
public string ProcessData(Task[] arr)
{
return "processed";
}
//<snippet11>
void ProcessFilesAsync()
{
string[] files = Directory.GetFiles(@"\\remoteComputer\folderName\", "*.*", SearchOption.AllDirectories);
CancellationTokenSource cts = new CancellationTokenSource();
List<Task<string>> tasks = new List<Task<string>>();
foreach (var file in files)
{
Task<string> t = null;
try
{
t = GetFileStringAsync3(file, cts.Token);
}
catch
{
Console.WriteLine($"Argument exception on {file}");
continue;
}
t.ContinueWith((antecedent) => ProcessFileData(antecedent.Result));
tasks.Add(t);
}
Console.WriteLine("All tasks have been added.");
// Do some work here.
try
{
Task.WaitAll(tasks.ToArray());
Console.WriteLine("All tasks have completed.");
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
Console.WriteLine(ex.Message);
}
finally
{
cts.Dispose();
}
// If needed, perform a final step after all tasks have completed.
Task.Factory.ContinueWhenAll(tasks.ToArray(), (arr) =>
{
foreach (var t in arr)
{
CompareFileData(t.Result);
}
});
}
public Task<string> GetFileStringAsync3(string path, CancellationToken token)
{
if (!File.Exists(path))
{
throw new FileNotFoundException(String.Format("Could not find {0}", path));
}
FileInfo fi = new FileInfo(path);
byte[] data = null;
if (fi.Length < MAX_FILE_SIZE && fi.Length > 0)
{
data = new byte[fi.Length];
}
else
{
throw new ArgumentException(String.Format("{0} is too big to load!", path));
}
FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);
// We still pass null for the last parameter because
// the state variable is visible to the continuation delegate.
Task<int> task = Task<int>.Factory.FromAsync(
fs.BeginRead, fs.EndRead, data, 0, data.Length, null);
return task.ContinueWith((antecedent) =>
{
fs.Close();
// If we did not receive the entire file, the end of the
// data buffer will contain garbage.
if (antecedent.Result < data.Length)
Array.Resize(ref data, antecedent.Result);
// Will be returned in the Result property of the Task<string>
// at some future point after the asynchronous file I/O operation completes.
return new UTF8Encoding().GetString(data);
});
}
string ProcessFileData(string data)
{
string s = data.Substring(0, 80);
Console.WriteLine(s);
return s;
}
string CompareFileData(string s)
{
return "compare results";
}
//</snippet11>
}sample · vbsamples/snippets/visualbasic/VS_Snippets_Misc/fromasync/vb/module1.vb08
Imports System.Collections.Generic
Imports System.IO
Imports System.Net
Imports System.Text
Imports System.Threading
Imports System.Threading.Tasks
Namespace specialNamespaceForOneMethodSignature
Class Task(Of TResult)
Public TValue As TResult
End Class
Class Dummy(Of TResult)
'<snippet01>
Public Function FromAsync(Of TArg1, TArg2, TArg3)(
ByVal beginMethod As Func(Of TArg1, TArg2, TArg3, AsyncCallback, Object, IAsyncResult),
ByVal endMethod As Func(Of IAsyncResult, TResult),
ByVal dataBuffer As TArg1,
ByVal byteOffsetToStartAt As TArg2,
ByVal maxBytesToRead As TArg3,
ByVal stateInfo As Object)
'</snippet01>
Return New Task(Of TResult)
End Function
End Class
End Namespace
Module Module1
Class AsyncResult
Public Sub New()
End Sub
'<snippet07>
Shared Function ReturnTaskFromAsyncResult() As Task(Of String)
Dim ar As IAsyncResult = DoSomethingAsynchronously()
Dim t As Task(Of String) = Task(Of String).Factory.FromAsync(ar, Function(res) CStr(res.AsyncState))
Return t
End Function
'</snippet07>
Private Shared Function DoSomethingAsynchronously() As IAsyncResult
Throw New NotImplementedException
End Function
End Class
'<snippet08>
Class WebDataDownLoader
Dim tcs As New TaskCompletionSource(Of String())
Dim nameToSearch As String
Dim token As CancellationToken
Dim results As New List(Of String)
Dim m_lock As Object
Dim count As Integer
Dim addresses() As String
Shared Sub Main()
Dim downloader As New WebDataDownLoader()
downloader.addresses = {"http://www.msnbc.com", "http://www.yahoo.com", _
"http://www.nytimes.com", "http://www.washingtonpost.com", _
"http://www.latimes.com", "http://www.newsday.com"}
Dim cts As New CancellationTokenSource()
' Create a UI thread from which to cancel the entire operation
Task.Factory.StartNew(Sub()
Console.WriteLine("Press c to cancel")
If Console.ReadKey().KeyChar = "c"c Then
cts.Cancel()
End If
End Sub)
' Using a neutral search term that is sure to get some hits on English web sites.
' Please substitute your favorite search term.
downloader.nameToSearch = "the"
Dim webTask As Task(Of String()) = downloader.GetWordCounts(downloader.addresses, downloader.nameToSearch, cts.Token)
' Do some other work here unless the method has already completed.
If (webTask.IsCompleted = False) Then
' Simulate some work
Thread.SpinWait(5000000)
End If
Dim results As String() = Nothing
Try
results = webTask.Result
Catch ae As AggregateException
For Each ex As Exception In ae.InnerExceptions
If (TypeOf (ex) Is OperationCanceledException) Then
Dim oce As OperationCanceledException = CType(ex, OperationCanceledException)
If oce.CancellationToken = cts.Token Then
Console.WriteLine("Operation canceled by user.")
End If
Else
Console.WriteLine(ex.Message)
End If
Next
Finally
cts.Dispose()
End Try
If (Not results Is Nothing) Then
For Each item As String In results
Console.WriteLine(item)
Next
End If
Console.WriteLine("Press any key to exit")
Console.ReadKey()
End Sub
Public Function GetWordCounts(ByVal urls() As String, ByVal str As String, ByVal token As CancellationToken) As Task(Of String())
Dim webClients() As WebClient
ReDim webClients(urls.Length)
m_lock = New Object()
' If the user cancels the CancellationToken, then we can use the
' WebClient's ability to cancel its own async operations.
token.Register(Sub()
For Each wc As WebClient In webClients
If Not wc Is Nothing Then
wc.CancelAsync()
End If
Next
End Sub)
For i As Integer = 0 To urls.Length - 1
webClients(i) = New WebClient()
' Specify the callback for the DownloadStringCompleted
' event that will be raised by this WebClient instance.
AddHandler webClients(i).DownloadStringCompleted, AddressOf WebEventHandler
Dim address As Uri = Nothing
Try
address = New Uri(urls(i))
' Pass the address, and also use it for the userToken
' to identify the page when the delegate is invoked.
webClients(i).DownloadStringAsync(address, address)
Catch ex As UriFormatException
tcs.TrySetException(ex)
Return tcs.Task
End Try
Next
' Return the underlying Task. The client code
' waits on the Result property, and handles exceptions
' in the try-catch block there.
Return tcs.Task
End Function
Public Sub WebEventHandler(ByVal sender As Object, ByVal args As DownloadStringCompletedEventArgs)
If args.Cancelled = True Then
tcs.TrySetCanceled()
Return
ElseIf Not args.Error Is Nothing Then
tcs.TrySetException(args.Error)
Return
Else
' Split the string into an array of words,
' then count the number of elements that match
' the search term.
Dim words() As String = args.Result.Split(" "c)
Dim NAME As String = nameToSearch.ToUpper()
Dim nameCount = (From word In words.AsParallel()
Where word.ToUpper().Contains(NAME)
Select word).Count()
' Associate the results with the url, and add new string to the array that
' the underlying Task object will return in its Result property.
results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, nameToSearch))
End If
SyncLock (m_lock)
count = count + 1
If (count = addresses.Length) Then
tcs.TrySetResult(results.ToArray())
End If
End SyncLock
End Sub
'</snippet08>
End Class
'<snippet09>
Class Calculator
Public Function BeginCalculate(ByVal decimalPlaces As Integer, ByVal ac As AsyncCallback, ByVal state As Object) As IAsyncResult
Console.WriteLine("Calling BeginCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId)
Dim myTask = Task(Of String).Factory.StartNew(Function(obj) Compute(decimalPlaces), state)
myTask.ContinueWith(Sub(antecedent) ac(myTask))
End Function
Private Function Compute(ByVal decimalPlaces As Integer)
Console.WriteLine("Calling compute on thread {0}", Thread.CurrentThread.ManagedThreadId)
' Simulating some heavy work.
Thread.SpinWait(500000000)
' Actual implementation left as exercise for the reader.
' Several examples are available on the Web.
Return "3.14159265358979323846264338327950288"
End Function
Public Function EndCalculate(ByVal ar As IAsyncResult) As String
Console.WriteLine("Calling EndCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId)
Return CType(ar, Task(Of String)).Result
End Function
End Class
Class CalculatorClient
Shared decimalPlaces As Integer
Shared Sub Main()
Dim calc As New Calculator
Dim places As Integer = 35
Dim callback As New AsyncCallback(AddressOf PrintResult)
Dim ar As IAsyncResult = calc.BeginCalculate(places, callback, calc)
' Do some work on this thread while the calculator is busy.
Console.WriteLine("Working...")
Thread.SpinWait(500000)
Console.ReadLine()
End Sub
Public Shared Sub PrintResult(ByVal result As IAsyncResult)
Dim c As Calculator = CType(result.AsyncState, Calculator)
Dim piString As String = c.EndCalculate(result)
Console.WriteLine("Calling PrintResult on thread {0}; result = {1}",
Thread.CurrentThread.ManagedThreadId, piString)
End Sub
End Class
'</snippet09>
Class FileStreamDemo
Shared Sub Main()
Dim fsd As New FileStreamDemo()
fsd.ShowCallingFromAsync()
End Sub
'<snippet03>
Const MAX_FILE_SIZE As Integer = 14000000
Shared Function GetFileStringAsync(ByVal path As String) As Task(Of String)
Dim fi As New FileInfo(path)
Dim data(fi.Length - 1) As Byte
Dim fs As FileStream = New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, True)
' Task(Of Integer) returns the number of bytes read
Dim myTask As Task(Of Integer) = Task(Of Integer).Factory.FromAsync(
AddressOf fs.BeginRead, AddressOf fs.EndRead, data, 0, data.Length, Nothing)
' It is possible to do other work here while waiting
' for the antecedent task to complete.
' ...
' Add the continuation, which returns a Task<string>.
Return myTask.ContinueWith(Function(antecedent)
fs.Close()
If (antecedent.Result < 100) Then
Return "Data is too small to bother with."
End If
' If we did not receive the entire file, the end of the
' data buffer will contain garbage.
If (antecedent.Result < data.Length) Then
Array.Resize(data, antecedent.Result)
End If
' Will be returned in the Result property of the Task<string>
' at some future point after the asynchronous file I/O operation completes.
Return New UTF8Encoding().GetString(data)
End Function)
End Function
'</snippet03>
Public Sub ShowCallingFromAsync()
Dim path As String = "\\docbuild2\public\Main\Logs\DllDiffReport\DllDiffReport_BetweenInBuildAndInPublicShare.html"
'<snippet04>
Dim myTask As Task(Of String) = GetFileStringAsync(path)
' Do some other work
' ...
Try
Console.WriteLine(myTask.Result.Substring(0, 500))
Catch ex As AggregateException
Console.WriteLine(ex.InnerException.Message)
End Try
'</snippet04>
Console.WriteLine("Press Enter to exit.")
Console.ReadLine()
End Sub
'<snippet05>
Public Function GetFileStringAsync2(ByVal path As String) As Task(Of String)
Dim fi = New FileInfo(path)
Dim data(fi.Length - 1) As Byte
Dim state As New MyCustomState()
Dim fs As New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, True)
' We still pass null for the last parameter because
' the state variable is visible to the continuation delegate.
Dim myTask As Task(Of Integer) = Task(Of Integer).Factory.FromAsync(
AddressOf fs.BeginRead, AddressOf fs.EndRead, data, 0, data.Length, Nothing)
Return myTask.ContinueWith(Function(antecedent)
fs.Close()
' Capture custom state data directly in the user delegate.
' No need to pass it through the FromAsync method.
If (state.StateData.Contains("New York, New York")) Then
Return "Start spreading the news!"
End If
' If we did not receive the entire file, the end of the
' data buffer will contain garbage.
If (antecedent.Result < data.Length) Then
Array.Resize(data, antecedent.Result)
End If
'/ Will be returned in the Result property of the Task<string>
'/ at some future point after the asynchronous file I/O operation completes.
Return New UTF8Encoding().GetString(data)
End Function)
End Function
'</snippet05>
'<snippet06>
Public Function GetMultiFileData(ByVal filesToRead As String()) As Task(Of String)
Dim fs As FileStream
Dim tasks(filesToRead.Length - 1) As Task(Of String)
Dim fileData() As Byte = Nothing
For i As Integer = 0 To filesToRead.Length
fileData(&H1000) = New Byte()
fs = New FileStream(filesToRead(i), FileMode.Open, FileAccess.Read, FileShare.Read, fileData.Length, True)
' By adding the continuation here, the
' Result of each task will be a string.
tasks(i) = Task(Of Integer).Factory.FromAsync(AddressOf fs.BeginRead,
AddressOf fs.EndRead,
fileData,
0,
fileData.Length,
Nothing).
ContinueWith(Function(antecedent)
fs.Close()
'If we did not receive the entire file, the end of the
' data buffer will contain garbage.
If (antecedent.Result < fileData.Length) Then
ReDim Preserve fileData(antecedent.Result)
End If
'Will be returned in the Result property of the Task<string>
' at some future point after the asynchronous file I/O operation completes.
Return New UTF8Encoding().GetString(fileData)
End Function)
Next
Return Task(Of String).Factory.ContinueWhenAll(tasks, Function(data)
' Propagate all exceptions and mark all faulted tasks as observed.
Task.WaitAll(data)
' Combine the results from all tasks.
Dim sb As New StringBuilder()
For Each t As Task(Of String) In data
sb.Append(t.Result)
Next
' Final result to be returned eventually on the calling thread.
Return sb.ToString()
End Function)
End Function
'</snippet06>
Class MyCustomState
Public StateData As String
Public Sub New()
StateData = "Hello"
End Sub
End Class
End Class
End Module