下载文件时如何显示进度栏？

Question

I'm brand new to coding with c# so can someone tell me how I can include code into this to show the progress bar of file downloading?

    private void button4_Click(object sender, EventArgs e)
    {
        using (var client = new WebClient())
        {
            MessageBox.Show("File will start downloading");

            var path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), "SOMEFILENAME.exe");

            client.DownloadFile("GOOGLE DRIVE LINK", path);

            MessageBox.Show("File has been downloaded!");

            System.Diagnostics.Process.Start(path);
        }
    }

Answer 1

To update a progress bar while your WebClient downloads data you have to use a function that does this task in the background . WebClient has a useful function called DownloadFileAsync . This function does exactly that: It downloads in the background.

The code so with this change:

private void button4_Click(object sender, EventArgs e)
{
    using (var client = new WebClient())
    {
        MessageBox.Show("File will start downloading");

        var path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), "SOMEFILENAME.exe");

        client.DownloadFileAsync(new Uri("GOOGLE DRIVE LINK"), path);

        MessageBox.Show("File has been downloaded!");

        System.Diagnostics.Process.Start(path);
    }
}

Unfortunately we have a problem now. The method starts the download in the background and your code immediately continues. This means you press your button, the first MessageBox pops up, the second MessageBox pops up right after the first one and if your download isn't completed when you close the second one your file is executed too early.

To avoid this WebClient has events . The one we need is called DownloadFileCompleted . As the name suggests it executes whatever you want when the download is completed. So let's look at the new code:

string path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), "SOMEFILENAME.exe"); // We need our path to be global

private void button4_Click(object sender, EventArgs e)
{
    using (var client = new WebClient())
    {
        client.DownloadFileCompleted += client_DownloadFileCompleted; // Add our new event handler
        MessageBox.Show("File will start downloading");

        client.DownloadFileAsync(new Uri("GOOGLE DRIVE LINK"), path);
    }
}

private void client_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e) // This is our new method!
{
    MessageBox.Show("File has been downloaded!");

    System.Diagnostics.Process.Start(path);
}

Our next problem: client is inside a using block. This is great for foreground downloads but if we do it asynchronously (that's what doing it in the background is called) your client will be dead as soon as the block is left which is immediately after the download has been started . So let's make our client global to be able to destroy it later on.

string path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), "SOMEFILENAME.exe");
WebClient client; // Here it is!

private void button4_Click(object sender, EventArgs e)
{
    client = new WebClient(); // Create a new client here
    client.DownloadFileCompleted += client_DownloadFileCompleted; // Add our new event handler
    MessageBox.Show("File will start downloading");

    client.DownloadFileAsync(new Uri("GOOGLE DRIVE LINK"), path);
}

private void client_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e) // This is our new method!
{
    MessageBox.Show("File has been downloaded!");

    System.Diagnostics.Process.Start(path);
}

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    if (client != null)
        client.Dispose(); // We have to delete our client manually when we close the window or whenever you want
}

Now let's assume the user can press the button a second time before the download is completed. Our client would be overwritten then and the download would be canceled. So let's just ignore the button press if we're already downloading something and only create the new client if we don't have one. New code:

string path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), "SOMEFILENAME.exe");
WebClient client;

private void button4_Click(object sender, EventArgs e)
{
    if (client != null && client.IsBusy) // If the client is already downloading something we don't start a new download
            return;

    if (client == null) // We only create a new client if we don't already have one
    {
        client = new WebClient(); // Create a new client here
        client.DownloadFileCompleted += client_DownloadFileCompleted; // Add our new event handler
    }
    MessageBox.Show("File will start downloading");

    client.DownloadFileAsync(new Uri("GOOGLE DRIVE LINK"), path);
}

private void client_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e) // This is our new method!
{
    MessageBox.Show("File has been downloaded!");

    System.Diagnostics.Process.Start(path);
}

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    if (client != null)
        client.Dispose(); // We have to delete our client manually when we close the window or whenever you want
}

Now that the boring part is done let's come to your problem: Viewing the progress in a progress bar. WebClient has got another event called DownloadProgressChanged . We can use it to update our progress bar.

Talking about progress bars: In Windows Forms you can create one by searching for ProgressBar in the tool bow window in Visual Studio. Then place it somewhere in your window. The ProgressBar component has a few properties which are important for its range. We're lucky, the default values are exactly what we need.

Our updated code (assuming your progress bar is called progressBar1 :

string path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), "SOMEFILENAME.exe");
WebClient client;

private void button4_Click(object sender, EventArgs e)
{
    if (client != null && client.IsBusy) // If the client is already downloading something we don't start a new download
            return;

    if (client == null) // We only create a new client if we don't already have one
    {
        client = new WebClient(); // Create a new client here
        client.DownloadFileCompleted += client_DownloadFileCompleted;
        client.DownloadProgressChanged += client_DownloadProgressChanged; // Add new event handler for updating the progress bar
    }
    MessageBox.Show("File will start downloading");

    client.DownloadFileAsync(new Uri("GOOGLE DRIVE LINK"), path);
}

private void client_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e) // This is our new method!
{
    MessageBox.Show("File has been downloaded!");

    System.Diagnostics.Process.Start(path);
}

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    if (client != null)
        client.Dispose(); // We have to delete our client manually when we close the window or whenever you want
}

private void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e) // NEW
{
    progressBar1.Value = e.ProgressPercentage;
}

Notes:

1. You can create the FormClosing method by double clicking the FormClosing event in your window's property box.
2. Calling client.Dispose() is only necessary when your program doesn't exit after closing the window. In any other case you could get rid of the FormClosing stuff completely.

That's all finally. I hope this wasn't too long for you and I could help you. Feel free to ask for clarification. That's what comments are there for.