<configuration>
  <uri>
    <idn enabled="All" />
    <iriParsing enabled="true" />
  </uri>
</configuration>

var client = new WebClient();
client.Headers.Add("user-agent", "Mozilla / 4.0(compatible; MSIE 7.0; Windows NT 6.2; WOW64; Trident / 7.0; .NET4.0C; .NET4.0E; .NET CLR 2.0.50727; .NET CLR 3.0.30729; .NET CLR 3.5.30729; Tablet PC 2.0)");
var download = client.DownloadString(new Uri("https://～"));