Sample code for using the WebAuthorizationBroker to authenticate with Salesforce.com using the User Agent auth flow.

WebAuthenticationBroker.cs

private const string AuthorizationEndpointUrl = 
    "https://login.salesforce.com/services/oauth2/authorize";
private const string ConsumerKey = "YOURCONSUMERKEY";
private const string CallbackUrl = "sfdc://success";

async private void Page_Loaded(object sender, RoutedEventArgs e)
{
    var startUrl = Common.FormatAuthUrl(AuthorizationEndpointUrl, ResponseTypes.Token, ConsumerKey, 
        WebUtility.UrlEncode(CallbackUrl), DisplayTypes.Popup);
    var startUri = new Uri(startUrl);
    var endUri = new Uri(CallbackUrl);
    string result;

    try
    {
        var webAuthenticationResult =
            await Windows.Security.Authentication.Web.WebAuthenticationBroker.AuthenticateAsync(
                Windows.Security.Authentication.Web.WebAuthenticationOptions.None,
                startUri,
                endUri);

        switch (webAuthenticationResult.ResponseStatus)
        {
            case Windows.Security.Authentication.Web.WebAuthenticationStatus.Success:
                        
                var responseData = webAuthenticationResult.ResponseData;
                var responseUri = new Uri(responseData);
                var decoder = new WwwFormUrlDecoder(responseUri.Fragment.Replace("#", "?"));

                var accessToken = decoder.GetFirstValueByName("access_token");
                var refreshToken = decoder.GetFirstValueByName("refresh_token");
                var instanceUrl = WebUtility.UrlDecode(decoder.GetFirstValueByName("instance_url"));

                result = string.Format("Access Token: {0}\n\nRefresh Token: {1}\n\nInstance URL: {2}", 
                    accessToken, refreshToken, instanceUrl);
                break;
                
            case Windows.Security.Authentication.Web.WebAuthenticationStatus.ErrorHttp:

                result = webAuthenticationResult.ResponseErrorDetail.ToString();
                break;

            default:

                result = webAuthenticationResult.ResponseData;
                break;
        }
    }
    catch (Exception ex)
    {
        result = ex.Message;
    }

    lblOutput.Text = result;
}