title | ms.date | ms.assetid |
---|---|---|
Handling Reentrancy in Async Apps (C#) |
07/20/2015 |
47c5075e-c448-45ce-9155-ed4e7e98c677 |
When you include asynchronous code in your app, you should consider and possibly prevent reentrancy, which refers to reentering an asynchronous operation before it has completed. If you don't identify and handle possibilities for reentrancy, it can cause unexpected results.
In this topic
Note
To run the example, you must have Visual Studio 2012 or newer and .NET Framework 4.5 or newer installed on your computer.
Note
Transport Layer Security (TLS) version 1.2 is now the minimum version to use in your app development. If your app targets a .NET Framework version earlier than 4.7, refer to the following article for Transport Layer Security (TLS) best practices with the .NET Framework.
In the example in this topic, users choose a Start button to initiate an asynchronous app that downloads a series of websites and calculates the total number of bytes that are downloaded. A synchronous version of the example would respond the same way regardless of how many times a user chooses the button because, after the first time, the UI thread ignores those events until the app finishes running. In an asynchronous app, however, the UI thread continues to respond, and you might reenter the asynchronous operation before it has completed.
The following example shows the expected output if the user chooses the Start button only once. A list of the downloaded websites appears with the size, in bytes, of each site. The total number of bytes appears at the end.
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
3. msdn.microsoft.com/library/jj155761.aspx 29019
4. msdn.microsoft.com/library/hh290140.aspx 117152
5. msdn.microsoft.com/library/hh524395.aspx 68959
6. msdn.microsoft.com/library/ms404677.aspx 197325
7. msdn.microsoft.com 42972
8. msdn.microsoft.com/library/ff730837.aspx 146159
TOTAL bytes returned: 890591
However, if the user chooses the button more than once, the event handler is invoked repeatedly, and the download process is reentered each time. As a result, several asynchronous operations are running at the same time, the output interleaves the results, and the total number of bytes is confusing.
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
3. msdn.microsoft.com/library/jj155761.aspx 29019
4. msdn.microsoft.com/library/hh290140.aspx 117152
5. msdn.microsoft.com/library/hh524395.aspx 68959
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
6. msdn.microsoft.com/library/ms404677.aspx 197325
3. msdn.microsoft.com/library/jj155761.aspx 29019
7. msdn.microsoft.com 42972
4. msdn.microsoft.com/library/hh290140.aspx 117152
8. msdn.microsoft.com/library/ff730837.aspx 146159
TOTAL bytes returned: 890591
5. msdn.microsoft.com/library/hh524395.aspx 68959
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
6. msdn.microsoft.com/library/ms404677.aspx 197325
3. msdn.microsoft.com/library/jj155761.aspx 29019
4. msdn.microsoft.com/library/hh290140.aspx 117152
7. msdn.microsoft.com 42972
5. msdn.microsoft.com/library/hh524395.aspx 68959
8. msdn.microsoft.com/library/ff730837.aspx 146159
TOTAL bytes returned: 890591
6. msdn.microsoft.com/library/ms404677.aspx 197325
7. msdn.microsoft.com 42972
8. msdn.microsoft.com/library/ff730837.aspx 146159
TOTAL bytes returned: 890591
You can review the code that produces this output by scrolling to the end of this topic. You can experiment with the code by downloading the solution to your local computer and then running the WebsiteDownload project or by using the code at the end of this topic to create your own project. For more information and instructions, see Reviewing and Running the Example App.
You can handle reentrancy in a variety of ways, depending on what you want your app to do. This topic presents the following examples:
-
Disable the Start button while the operation is running so that the user can't interrupt it.
-
Cancel and Restart the Operation
Cancel any operation that is still running when the user chooses the Start button again, and then let the most recently requested operation continue.
-
Run Multiple Operations and Queue the Output
Allow all requested operations to run asynchronously, but coordinate the display of output so that the results from each operation appear together and in order.
You can block the Start button while an operation is running by disabling the button at the top of the StartButton_Click
event handler. You can then reenable the button from within a finally
block when the operation finishes so that users can run the app again.
To set up this scenario, make the following changes to the basic code that is provided in Reviewing and Running the Example App. You also can download the finished app from Async Samples: Reentrancy in .NET Desktop Apps. The name of the project is DisableStartButton.
private async void StartButton_Click(object sender, RoutedEventArgs e)
{
// This line is commented out to make the results clearer in the output.
//ResultsTextBox.Text = "";
// ***Disable the Start button until the downloads are complete.
StartButton.IsEnabled = false;
try
{
await AccessTheWebAsync();
}
catch (Exception)
{
ResultsTextBox.Text += "\r\nDownloads failed.";
}
// ***Enable the Start button in case you want to run the program again.
finally
{
StartButton.IsEnabled = true;
}
}
As a result of the changes, the button doesn't respond while AccessTheWebAsync
is downloading the websites, so the process can’t be reentered.
Instead of disabling the Start button, you can keep the button active but, if the user chooses that button again, cancel the operation that's already running and let the most recently started operation continue.
For more information about cancellation, see Fine-Tuning Your Async Application (C#).
To set up this scenario, make the following changes to the basic code that is provided in Reviewing and Running the Example App. You also can download the finished app from Async Samples: Reentrancy in .NET Desktop Apps. The name of the project is CancelAndRestart.
-
Declare a xref:System.Threading.CancellationTokenSource variable,
cts
, that’s in scope for all methods.public partial class MainWindow : Window // Or class MainPage { // *** Declare a System.Threading.CancellationTokenSource. CancellationTokenSource cts;
-
In
StartButton_Click
, determine whether an operation is already underway. If the value ofcts
is null, no operation is already active. If the value isn't null, the operation that is already running is canceled.// *** If a download process is already underway, cancel it. if (cts != null) { cts.Cancel(); }
-
Set
cts
to a different value that represents the current process.// *** Now set cts to a new value that you can use to cancel the current process // if the button is chosen again. CancellationTokenSource newCTS = new CancellationTokenSource(); cts = newCTS;
-
At the end of
StartButton_Click
, the current process is complete, so set the value ofcts
back to null.// *** When the process is complete, signal that another process can begin. if (cts == newCTS) cts = null;
The following code shows all the changes in StartButton_Click
. The additions are marked with asterisks.
private async void StartButton_Click(object sender, RoutedEventArgs e)
{
// This line is commented out to make the results clearer in the output.
//ResultsTextBox.Clear();
// *** If a download process is already underway, cancel it.
if (cts != null)
{
cts.Cancel();
}
// *** Now set cts to cancel the current process if the button is chosen again.
CancellationTokenSource newCTS = new CancellationTokenSource();
cts = newCTS;
try
{
// ***Send cts.Token to carry the message if there is a cancellation request.
await AccessTheWebAsync(cts.Token);
}
// *** Catch cancellations separately.
catch (OperationCanceledException)
{
ResultsTextBox.Text += "\r\nDownloads canceled.\r\n";
}
catch (Exception)
{
ResultsTextBox.Text += "\r\nDownloads failed.\r\n";
}
// *** When the process is complete, signal that another process can proceed.
if (cts == newCTS)
cts = null;
}
In AccessTheWebAsync
, make the following changes.
-
Add a parameter to accept the cancellation token from
StartButton_Click
. -
Use the xref:System.Net.Http.HttpClient.GetAsync%2A method to download the websites because
GetAsync
accepts a xref:System.Threading.CancellationToken argument. -
Before calling
DisplayResults
to display the results for each downloaded website, checkct
to verify that the current operation hasn’t been canceled.
The following code shows these changes, which are marked with asterisks.
// *** Provide a parameter for the CancellationToken from StartButton_Click.
async Task AccessTheWebAsync(CancellationToken ct)
{
// Declare an HttpClient object.
HttpClient client = new HttpClient();
// Make a list of web addresses.
List<string> urlList = SetUpURLList();
var total = 0;
var position = 0;
foreach (var url in urlList)
{
// *** Use the HttpClient.GetAsync method because it accepts a
// cancellation token.
HttpResponseMessage response = await client.GetAsync(url, ct);
// *** Retrieve the website contents from the HttpResponseMessage.
byte[] urlContents = await response.Content.ReadAsByteArrayAsync();
// *** Check for cancellations before displaying information about the
// latest site.
ct.ThrowIfCancellationRequested();
DisplayResults(url, urlContents, ++position);
// Update the total.
total += urlContents.Length;
}
// Display the total count for all of the websites.
ResultsTextBox.Text +=
$"\r\n\r\nTOTAL bytes returned: {total}\r\n";
}
If you choose the Start button several times while this app is running, it should produce results that resemble the following output.
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
3. msdn.microsoft.com/library/jj155761.aspx 29019
4. msdn.microsoft.com/library/hh290140.aspx 122505
5. msdn.microsoft.com/library/hh524395.aspx 68959
6. msdn.microsoft.com/library/ms404677.aspx 197325
Download canceled.
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
3. msdn.microsoft.com/library/jj155761.aspx 29019
Download canceled.
1. msdn.microsoft.com/library/hh191443.aspx 83732
2. msdn.microsoft.com/library/aa578028.aspx 205273
3. msdn.microsoft.com/library/jj155761.aspx 29019
4. msdn.microsoft.com/library/hh290140.aspx 117152
5. msdn.microsoft.com/library/hh524395.aspx 68959
6. msdn.microsoft.com/library/ms404677.aspx 197325
7. msdn.microsoft.com 42972
8. msdn.microsoft.com/library/ff730837.aspx 146159
TOTAL bytes returned: 890591
To eliminate the partial lists, uncomment the first line of code in StartButton_Click
to clear the text box each time the user restarts the operation.
This third example is the most complicated in that the app starts another asynchronous operation each time that the user chooses the Start button, and all the operations run to completion. All the requested operations download websites from the list asynchronously, but the output from the operations is presented sequentially. That is, the actual downloading activity is interleaved, as the output in Recognizing Reentrancy shows, but the list of results for each group is presented separately.
The operations share a global xref:System.Threading.Tasks.Task, pendingWork
, which serves as a gatekeeper for the display process.
To set up this scenario, make the following changes to the basic code that is provided in Reviewing and Running the Example App. You also can download the finished app from Async Samples: Reentrancy in .NET Desktop Apps. The name of the project is QueueResults.
The following output shows the result if the user chooses the Start button only once. The letter label, A, indicates that the result is from the first time the Start button is chosen. The numbers show the order of the URLs in the list of download targets.
#Starting group A.
#Task assigned for group A.
A-1. msdn.microsoft.com/library/hh191443.aspx 87389
A-2. msdn.microsoft.com/library/aa578028.aspx 209858
A-3. msdn.microsoft.com/library/jj155761.aspx 30870
A-4. msdn.microsoft.com/library/hh290140.aspx 119027
A-5. msdn.microsoft.com/library/hh524395.aspx 71260
A-6. msdn.microsoft.com/library/ms404677.aspx 199186
A-7. msdn.microsoft.com 53266
A-8. msdn.microsoft.com/library/ff730837.aspx 148020
TOTAL bytes returned: 918876
#Group A is complete.
If the user chooses the Start button three times, the app produces output that resembles the following lines. The information lines that start with a pound sign (#) trace the progress of the application.
#Starting group A.
#Task assigned for group A.
A-1. msdn.microsoft.com/library/hh191443.aspx 87389
A-2. msdn.microsoft.com/library/aa578028.aspx 207089
A-3. msdn.microsoft.com/library/jj155761.aspx 30870
A-4. msdn.microsoft.com/library/hh290140.aspx 119027
A-5. msdn.microsoft.com/library/hh524395.aspx 71259
A-6. msdn.microsoft.com/library/ms404677.aspx 199185
#Starting group B.
#Task assigned for group B.
A-7. msdn.microsoft.com 53266
#Starting group C.
#Task assigned for group C.
A-8. msdn.microsoft.com/library/ff730837.aspx 148010
TOTAL bytes returned: 916095
B-1. msdn.microsoft.com/library/hh191443.aspx 87389
B-2. msdn.microsoft.com/library/aa578028.aspx 207089
B-3. msdn.microsoft.com/library/jj155761.aspx 30870
B-4. msdn.microsoft.com/library/hh290140.aspx 119027
B-5. msdn.microsoft.com/library/hh524395.aspx 71260
B-6. msdn.microsoft.com/library/ms404677.aspx 199186
#Group A is complete.
B-7. msdn.microsoft.com 53266
B-8. msdn.microsoft.com/library/ff730837.aspx 148010
TOTAL bytes returned: 916097
C-1. msdn.microsoft.com/library/hh191443.aspx 87389
C-2. msdn.microsoft.com/library/aa578028.aspx 207089
#Group B is complete.
C-3. msdn.microsoft.com/library/jj155761.aspx 30870
C-4. msdn.microsoft.com/library/hh290140.aspx 119027
C-5. msdn.microsoft.com/library/hh524395.aspx 72765
C-6. msdn.microsoft.com/library/ms404677.aspx 199186
C-7. msdn.microsoft.com 56190
C-8. msdn.microsoft.com/library/ff730837.aspx 148010
TOTAL bytes returned: 920526
#Group C is complete.
Groups B and C start before group A has finished, but the output for the each group appears separately. All the output for group A appears first, followed by all the output for group B, and then all the output for group C. The app always displays the groups in order and, for each group, always displays the information about the individual websites in the order that the URLs appear in the list of URLs.
However, you can't predict the order in which the downloads actually happen. After multiple groups have been started, the download tasks that they generate are all active. You can't assume that A-1 will be downloaded before B-1, and you can't assume that A-1 will be downloaded before A-2.
The sample code contains the following two global declarations that are visible from all methods.
public partial class MainWindow : Window // Class MainPage in Windows Store app.
{
// ***Declare the following variables where all methods can access them.
private Task pendingWork = null;
private char group = (char)('A' - 1);
The Task
variable, pendingWork
, oversees the display process and prevents any group from interrupting another group's display operation. The character variable, group
, labels the output from different groups to verify that results appear in the expected order.
The event handler, StartButton_Click
, increments the group letter each time the user chooses the Start button. Then the handler calls AccessTheWebAsync
to run the downloading operation.
private async void StartButton_Click(object sender, RoutedEventArgs e)
{
// ***Verify that each group's results are displayed together, and that
// the groups display in order, by marking each group with a letter.
group = (char)(group + 1);
ResultsTextBox.Text += $"\r\n\r\n#Starting group {group}.";
try
{
// *** Pass the group value to AccessTheWebAsync.
char finishedGroup = await AccessTheWebAsync(group);
// The following line verifies a successful return from the download and
// display procedures.
ResultsTextBox.Text += $"\r\n\r\n#Group {finishedGroup} is complete.\r\n";
}
catch (Exception)
{
ResultsTextBox.Text += "\r\nDownloads failed.";
}
}
This example splits AccessTheWebAsync
into two methods. The first method, AccessTheWebAsync
, starts all the download tasks for a group and sets up pendingWork
to control the display process. The method uses a Language Integrated Query (LINQ query) and xref:System.Linq.Enumerable.ToArray%2A to start all the download tasks at the same time.
AccessTheWebAsync
then calls FinishOneGroupAsync
to await the completion of each download and display its length.
FinishOneGroupAsync
returns a task that's assigned to pendingWork
in AccessTheWebAsync
. That value prevents interruption by another operation before the task is complete.
private async Task<char> AccessTheWebAsync(char grp)
{
HttpClient client = new HttpClient();
// Make a list of the web addresses to download.
List<string> urlList = SetUpURLList();
// ***Kick off the downloads. The application of ToArray activates all the download tasks.
Task<byte[]>[] getContentTasks = urlList.Select(url => client.GetByteArrayAsync(url)).ToArray();
// ***Call the method that awaits the downloads and displays the results.
// Assign the Task that FinishOneGroupAsync returns to the gatekeeper task, pendingWork.
pendingWork = FinishOneGroupAsync(urlList, getContentTasks, grp);
ResultsTextBox.Text += $"\r\n#Task assigned for group {grp}. Download tasks are active.\r\n";
// ***This task is complete when a group has finished downloading and displaying.
await pendingWork;
// You can do other work here or just return.
return grp;
}
This method cycles through the download tasks in a group, awaiting each one, displaying the length of the downloaded website, and adding the length to the total.
The first statement in FinishOneGroupAsync
uses pendingWork
to make sure that entering the method doesn't interfere with an operation that is already in the display process or that's already waiting. If such an operation is in progress, the entering operation must wait its turn.
private async Task FinishOneGroupAsync(List<string> urls, Task<byte[]>[] contentTasks, char grp)
{
// ***Wait for the previous group to finish displaying results.
if (pendingWork != null) await pendingWork;
int total = 0;
// contentTasks is the array of Tasks that was created in AccessTheWebAsync.
for (int i = 0; i < contentTasks.Length; i++)
{
// Await the download of a particular URL, and then display the URL and
// its length.
byte[] content = await contentTasks[i];
DisplayResults(urls[i], content, i, grp);
total += content.Length;
}
// Display the total count for all of the websites.
ResultsTextBox.Text +=
$"\r\n\r\nTOTAL bytes returned: {total}\r\n";
}
The information lines that start with a pound sign (#) in the output clarify how this example works.
The output shows the following patterns.
-
A group can be started while a previous group is displaying its output, but the display of the previous group's output isn't interrupted.
#Starting group A. #Task assigned for group A. Download tasks are active. A-1. msdn.microsoft.com/library/hh191443.aspx 87389 A-2. msdn.microsoft.com/library/aa578028.aspx 207089 A-3. msdn.microsoft.com/library/jj155761.aspx 30870 A-4. msdn.microsoft.com/library/hh290140.aspx 119037 A-5. msdn.microsoft.com/library/hh524395.aspx 71260 #Starting group B. #Task assigned for group B. Download tasks are active. A-6. msdn.microsoft.com/library/ms404677.aspx 199186 A-7. msdn.microsoft.com 53078 A-8. msdn.microsoft.com/library/ff730837.aspx 148010 TOTAL bytes returned: 915919 B-1. msdn.microsoft.com/library/hh191443.aspx 87388 B-2. msdn.microsoft.com/library/aa578028.aspx 207089 B-3. msdn.microsoft.com/library/jj155761.aspx 30870 #Group A is complete. B-4. msdn.microsoft.com/library/hh290140.aspx 119027 B-5. msdn.microsoft.com/library/hh524395.aspx 71260 B-6. msdn.microsoft.com/library/ms404677.aspx 199186 B-7. msdn.microsoft.com 53078 B-8. msdn.microsoft.com/library/ff730837.aspx 148010 TOTAL bytes returned: 915908
-
The
pendingWork
task is null at the start ofFinishOneGroupAsync
only for group A, which started first. Group A hasn’t yet completed an await expression when it reachesFinishOneGroupAsync
. Therefore, control hasn't returned toAccessTheWebAsync
, and the first assignment topendingWork
hasn't occurred. -
The following two lines always appear together in the output. The code is never interrupted between starting a group's operation in
StartButton_Click
and assigning a task for the group topendingWork
.#Starting group B. #Task assigned for group B. Download tasks are active.
After a group enters
StartButton_Click
, the operation doesn't complete an await expression until the operation entersFinishOneGroupAsync
. Therefore, no other operation can gain control during that segment of code.
To better understand the example app, you can download it, build it yourself, or review the code at the end of this topic without implementing the app.
Note
To run the example as a Windows Presentation Foundation (WPF) desktop app, you must have Visual Studio 2012 or newer and the .NET Framework 4.5 or newer installed on your computer.
-
Download the compressed file from Async Samples: Reentrancy in .NET Desktop Apps.
-
Decompress the file that you downloaded, and then start Visual Studio.
-
On the menu bar, choose File, Open, Project/Solution.
-
Navigate to the folder that holds the decompressed sample code, and then open the solution (.sln) file.
-
In Solution Explorer, open the shortcut menu for the project that you want to run, and then choose Set as StartUpProject.
-
Choose the CTRL+F5 keys to build and run the project.
The following section provides the code to build the example as a WPF app.
-
Start Visual Studio.
-
On the menu bar, choose File, New, Project.
The New Project dialog box opens.
-
In the Installed Templates pane, expand Visual C#, and then expand Windows.
-
In the list of project types, choose WPF Application.
-
Name the project
WebsiteDownloadWPF
, choose .NET Framework version of 4.6 or higher and then click the OK button.The new project appears in Solution Explorer.
-
In the Visual Studio Code Editor, choose the MainWindow.xaml tab.
If the tab isn’t visible, open the shortcut menu for MainWindow.xaml in Solution Explorer, and then choose View Code.
-
In the XAML view of MainWindow.xaml, replace the code with the following code.
<Window x:Class="WebsiteDownloadWPF.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="using:WebsiteDownloadWPF" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d"> <Grid Width="517" Height="360"> <Button x:Name="StartButton" Content="Start" HorizontalAlignment="Left" Margin="-1,0,0,0" VerticalAlignment="Top" Click="StartButton_Click" Height="53" Background="#FFA89B9B" FontSize="36" Width="518" /> <TextBox x:Name="ResultsTextBox" HorizontalAlignment="Left" Margin="-1,53,0,-36" TextWrapping="Wrap" VerticalAlignment="Top" Height="343" FontSize="10" ScrollViewer.VerticalScrollBarVisibility="Visible" Width="518" FontFamily="Lucida Console" /> </Grid> </Window>
A simple window that contains a text box and a button appears in the Design view of MainWindow.xaml.
-
In Solution Explorer, right-click on References and select Add Reference.
Add a reference for xref:System.Net.Http, if it is not selected already.
-
In Solution Explorer, open the shortcut menu for MainWindow.xaml.cs, and then choose View Code.
-
In MainWindow.xaml.cs, replace the code with the following code.
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; using System.Windows; using System.Windows.Controls; using System.Windows.Data; using System.Windows.Documents; using System.Windows.Input; using System.Windows.Media; using System.Windows.Media.Imaging; using System.Windows.Navigation; using System.Windows.Shapes; // Add the following using directives, and add a reference for System.Net.Http. using System.Net.Http; using System.Threading; namespace WebsiteDownloadWPF { public partial class MainWindow : Window { public MainWindow() { System.Net.ServicePointManager.SecurityProtocol |= System.Net.SecurityProtocolType.Tls12; InitializeComponent(); } private async void StartButton_Click(object sender, RoutedEventArgs e) { // This line is commented out to make the results clearer in the output. //ResultsTextBox.Text = ""; try { await AccessTheWebAsync(); } catch (Exception) { ResultsTextBox.Text += "\r\nDownloads failed."; } } private async Task AccessTheWebAsync() { // Declare an HttpClient object. HttpClient client = new HttpClient(); // Make a list of web addresses. List<string> urlList = SetUpURLList(); var total = 0; var position = 0; foreach (var url in urlList) { // GetByteArrayAsync returns a task. At completion, the task // produces a byte array. byte[] urlContents = await client.GetByteArrayAsync(url); DisplayResults(url, urlContents, ++position); // Update the total. total += urlContents.Length; } // Display the total count for all of the websites. ResultsTextBox.Text += $"\r\n\r\nTOTAL bytes returned: {total}\r\n"; } private List<string> SetUpURLList() { List<string> urls = new List<string> { "https://msdn.microsoft.com/library/hh191443.aspx", "https://msdn.microsoft.com/library/aa578028.aspx", "https://msdn.microsoft.com/library/jj155761.aspx", "https://msdn.microsoft.com/library/hh290140.aspx", "https://msdn.microsoft.com/library/hh524395.aspx", "https://msdn.microsoft.com/library/ms404677.aspx", "https://msdn.microsoft.com", "https://msdn.microsoft.com/library/ff730837.aspx" }; return urls; } private void DisplayResults(string url, byte[] content, int pos) { // Display the length of each website. The string format is designed // to be used with a monospaced font, such as Lucida Console or // Global Monospace. // Strip off the "https://". var displayURL = url.Replace("https://", ""); // Display position in the URL list, the URL, and the number of bytes. ResultsTextBox.Text += $"\n{pos}. {displayURL,-58} {content.Length,8}"; } } }
-
Choose the CTRL+F5 keys to run the program, and then choose the Start button several times.
-
Make the changes from Disable the Start Button, Cancel and Restart the Operation, or Run Multiple Operations and Queue the Output to handle the reentrancy.