I tried this and i want that the source content of the website will be download to a string:
public partial class Form1 : Form
{
WebClient client
No need for async, really:
var result = new System.Net.WebClient().DownloadString(url)
If you don't want to block your UI, you can put the above in a BackgroundWorker. The reason I suggest this rather than the Async methods is because it is dramatically simpler to use, and because I suspect you are just going to stick this string into the UI somewhere anyway (where BackgroundWorker will make your life easier).