Thread to download files C#

Question

i have a problem with threads, i have this code (example):

private void button_Click(object sender, EventArgs e) {
    ShowMessage("Starting Downloads...");
    <more code>
    StartDownloads();
    RunFileDownload();
    <more code>
}

private void StartDownloads() {
    <more code>
    for (int i=0; i<10; i++) {
        ShowMessage("Downloading file: " + i);
        Download(i);
        <more code>
    }
    <more code>
}

The problem is, when i press the button and the downloading starts, the messages are not displayed... I tried to fix it with threads, like this:

private void button_Click(object sender, EventArgs e) {
    ShowMessage("Starting Downloads...");
    Thread t = new Thread(new ThreadStart(StartDownloads));
    t.Start();
    RunFileDownload();
}

But the RunFileDownload(); function starts before the files are downloaded. I try solve this with "Thread.Join();" but again not displayed messages (The main thread is paused).

I thought solve it with a multi-thread and Thread.Join(); but it isn't efficient and i will have problems with others functions in the main thread.

How can i solve this problem? Thanks.

Edit #2:

Considering this code:

private void Download() {
    ShowMessage("Starting Downloads...");
    Thread t = new Thread(new ThreadStart(StartDownloads));
    ShowMessage("Downloads Finished...");   | not run until
    RunFileDownload();                      | finished
    ShowMessage("Files Executed...");       | thread.
}

How can i expect the thread finish before the rest of the code is executed? I try with Thread.Join(); but it freezes the application.

Answer 1

Assuming you have access to async / await , the simplest solution is this:

private async void button_Click(object sender, EventArgs e)
{
    ShowMessage("Starting Downloads...");
    await StartDownloads(); //Return control until this method completes!
    RunFileDownload();
}

Note that exceptions with await are, suffice it to say, less than kind. Please ensure that you are using proper try/catch blocks, especially from await onwards. Consider using this pattern: Fire-and-forget with async vs "old async delegate" and reading this article .

Note that StartDownloads needs to be async and return a Task for this to work.

Apart from that solution, you need the thread to invoke a callback or raise an event on completion so you can run RunFileDownload . Using a BackgroundWorker can simplify that process.