SampleWindowsPhoneImplementationOfEvalAPI.cs

using System.Net;

//this method can be called in a button tap event
private void calculate()
{
    string calculationText = txtInput.Text;

    try
    {
        WebClient wc = new WebClient();
        wc.DownloadStringCompleted += HttpCompleted;
        string uri = String.Format("http://gcdc2013-easyapisproject.appspot.com/eval?q={0}", WebUtility.UrlEncode(calculationText));
        wc.DownloadStringAsync(new Uri(uri));
    }
    catch (Exception x)
    {
        MessageBox.Show(x.Message);
    }
}

private void HttpCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    try
    {
        string result = e.Result;
        result = result.Substring(1);
        result = result.Remove(result.Length - 1);
        string[] results = result.Split(new string[] { "}{" }, StringSplitOptions.None);
        result = results[1];

        if (result != "URL Encode/Decode Error, Escape specl chars")
            MessageBox.Show(result);
        else
            MessageBox.Show("Couldn't perform equation");
    }
    catch (Exception err)
    {
        MessageBox.Show("No Internet connectivity, please connect to internet to perform");
    }
}