Argthtjtr

I'm trying to understand the differences among WebClient.OpenRead, WebClient.OpenReadAsync and WebClient.OpenReadTaskAsync.

It looks like these have differences regarding blocking a thread, but I don't understand it well.

Could you please explain the differences? It would be great if you could give me some example (examples don't have to be sample code, but would be great if you could provide)

As you said, the difference is in thread blocking behavior. First one (OpenRead()) is thread blocking operation, other two - not. For example, let assume, that your network latency to reach google.com is 300ms. When you do var stream = webClient.OpenRead(@"https://www.google.com"); your application is "paused" for this 300ms, so code next to this line is not executed until your webClient return a stream to stream variable. This is calling Thread blocking.

When you do this in UI-thread (in example: in a button click handler) - your application become freezing and not responding to user actions. This is bad user experience, so never-ever call a thread blocking stuff in your UI. Here is example for console application:

var address = @"https://www.google.com/";



Console.WriteLine($"Opening stream from {address}");

using (var stream = webClient.OpenRead(address)) // this will block for 300ms

{

    Console.WriteLine("Stream is open!");

    // some other code

}

Second method (OpenReadAsync()) is asynchronous and return nothing immediately after a call, so your thread is not blocked. After awhile (300ms) OpenReadCompleted event will be raised by your webClient and all attached listeners will handle opened stream one-by-one. Here is an example:

public partial class MainForm : Form

{

    private WebClient _client = new WebClient();



    public MainForm()

    {

        InitializeComponents();

        _client.OpenReadCompleted += OpenReadCompletedHandler;

    }



    private void ButtonClickHandler(object sender, EventArgs e)

    {

        _client.OpenReadAsync(@"https://www.google.com/");

    }



    private void OpenReadCompletedHandler(object sender, OpenReadCompletedEventArgs e)

    {

        // this event will be raiesed 300ms after 'Button' click

        var stream = e.Result; // <- here is your stream

        // some other code

    }

}

The last one (OpenReadTaskAsync()) is all about TPL (Task Parallel Library) and async/await keywords. It runs all stuff in a Task which is returned by this method. Here is an example:

public partial class MainForm : Form

{

    private WebClient _client = new WebClient();



    public MainForm()

    {

        InitializeComponents();

    }



    private async void ButtonClickHandler(object sender, EventArgs e)

    {

        // after 'await' keyword, execution will be returned from this method immediately

        // meanwhile, actual acquiring of 'stream' is running in a background thread

        using (var stream = await _client.OpenReadTaskAsync(@"https://www.google.com/"))

        {

            // after 300ms, this code will be continued in UI thread

            // result will be automaticly unpacked to 'stream' variable



            // some other code

        }

    }

}

Hope this helps.

I'd suggest WebClient is more or less obsolete now, HttpClient is more appropriate for anything targeting .NET framework 4.5+ or .NET core. Just watch out that the latter does not automatically throw exceptions on HTTP error codes (400+).