System.Net FW 4 in Silverlight

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Silverlight

In a Asp.Net project, I used System.Net (FW 4.030319). In my Silverlight project, I found the System.Net (also FW 4.030319) contains something different from that used in Asp.Net. I tried to load the .dll used in asp.net but got error warning that prohibits me to use the component out of Silverlight. What I want to do it to use the method below:

System.Net.WebClient client = new System.Net.WebClient();
string downloadedString = client.DownloadString("http://xxxx");

In the SL version does not contain the .DownloadString() method. Then I revised my code like that below:

WebClient client = new WebClient();
client.OpenReadCompleted += new OpenReadCompletedEventHandler(readComplete);
...
private void readComplete(object sender, OpenReadCompletedEventArgs e) {
    byte[] buffer = new byte[e.Result.Length]; //crashes here with exception
}

However, when I run debugging, it never get the break point in the void readComplete().
How should I do in order to get the string result through a URL in SL? Thanks.