Detailed explanation of FTP file download and timeout control in C#

Preface

If your function is not async, you can still implement the same timeout function, but you will have to rely on more synchronization code or use .Result or .GetAwaiter().GetResult() to block waiting for the task to complete, which can lead to the risk of deadlocks, especially in UI threads or contexts. However, this approach may be feasible in some simple background tasks or console applications.

Here is an example of how to implement FTP request and timeout control in non-asynchronous functions:

using System;
using ;
using ;

public bool DownloadFileWithTimeoutSync(string uri, string localPath)
{
    var ftpRequest = (FtpWebRequest)(uri);
     = ;
    // Set other properties of FtpWebRequest, such as Credentials, etc.
    Task<bool> ftpTask = (() =>
    {
        try
        {
            using (var response = (FtpWebResponse)())
            using (var responseStream = ())
            using (var fileStream = (localPath))
            {
                (fileStream);
            }
            return true; // Or return success/failed according to the response status        }
        catch
        {
            return false;
        }
    });

    // Create a timeout task with a delay of 10 seconds    Task delayTask = ((10));

    // Wait for the FTP task to complete or time out    var completedTask = (ftpTask, delayTask).GetAwaiter().GetResult();

    if (completedTask == ftpTask)
    {
        // FTP task is completed, check the results        return ().GetResult();
    }
    else
    {
        // Timeout occurs        // Here you can cancel the FTP request as needed        return false;
    }
}

Note that using .Result or .GetAwaiter().GetResult() causes the current thread to block until the task is completed. This may be acceptable in background threads or console applications, but may result in the application being unresponsive when used in UI threads. If possible, the best practice is to use async and await as they provide a clearer and safer way to handle asynchronous operations and concurrency.

Additionally, when you call .GetAwaiter().GetResult() or .Result, if an exception is thrown in the task, these exceptions will be encapsulated in the AggregateException. If you need to handle a specific exception type, you may need to check the InnerExceptions property of AggregateException.

Scene description

When downloading FTP files, we may encounter network delays or service instability. It is necessary to set a timeout limit for the download task. If the scheduled time limit is exceeded, the program should be able to automatically give up the download task to avoid waiting indefinitely, affecting the user experience.

Implementation steps

Our goal is to create a synchronous method DownloadFileWithTimeoutSync, which encapsulates asynchronous operations to download files from the FTP server and automatically cancels if the operation exceeds the specified time (such as 10 seconds).

1. Create an FTP request

First, we need to create an FtpWebRequest object and set necessary properties such as request method, credentials, etc.

var ftpRequest = (FtpWebRequest)(uri);
 = ;

2. Enable asynchronous download task

We perform the download operation by starting an asynchronous task. This keeps the UI responsive, or avoids blocking the main thread.

Task<bool> ftpTask = (() => {
    // This contains the logic for downloading files});

3. Implement timeout control

To implement timeout control, we create a delay task as a timer for timeout. We then complete first using either of the waiting task and the timeout task.

Task delayTask = ((10));
var completedTask = (ftpTask, delayTask).GetAwaiter().GetResult();

4. Process download results and timeouts

Finally, we check whether the download task is completed first or the timeout task. If the download task is completed, we check whether the download is successful; if the timeout task is completed first, it is considered that the download operation timed out and the return failed.

if (completedTask == ftpTask)
{
    // Check the download results    return ().GetResult();
}
else
{
    // Processing timeout    return false;
}

Summarize

Through the above steps, we implement a FTP file download method with timeout control. This method not only takes advantage of the advantages of asynchronous programming to improve the performance and responsiveness of the application, but also avoids long waits caused by network problems through the timeout mechanism.

Asynchronous programming is particularly important when dealing with I/O-intensive tasks, it can effectively improve the concurrency and user experience of the application. I hope that the content of this article can help you better use asynchronous programming technology in actual development.

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