how to login into a website programatically using asp.net

Here is working example:

This class extends default .NET WebClient with ability to store cookies:

public class CookieAwareWebClient : WebClient
{
    public void Login(string loginPageAddress, NameValueCollection loginData)
    {
        var parameters = new StringBuilder();
        foreach (string key in loginData.Keys)
        {
            parameters.AppendFormat("{0}={1}&",
                HttpUtility.UrlEncode(key),
                HttpUtility.UrlEncode(loginData[key]));
        }
        parameters.Length -= 1;

        var request = (HttpWebRequest)WebRequest.Create(loginPageAddress);

        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";
        var buffer = Encoding.ASCII.GetBytes(parameters.ToString());
        request.ContentLength = buffer.Length;
        using (var requestStream = request.GetRequestStream())
        {
            requestStream.Write(buffer, 0, buffer.Length);
        }

        var container = request.CookieContainer = new CookieContainer();

        using (var response = request.GetResponse())
        {
            CookieContainer = container;
        }
    }

    public CookieAwareWebClient(CookieContainer container)
    {
        CookieContainer = container;
    }

    public CookieAwareWebClient()
        : this(new CookieContainer())
    { }

    public CookieContainer CookieContainer { get; private set; }

    protected override WebRequest GetWebRequest(Uri address)
    {
        var request = (HttpWebRequest)base.GetWebRequest(address);
        request.CookieContainer = CookieContainer;
        return request;
    }
}

You can use it like this:

    protected CookieAwareWebClient GetAuthenticatedClient()
    {
        var client = new CookieAwareWebClient();

        var loginData = new NameValueCollection
                {
                      { "Email", "[email protected]" },
                      { "Password", "testPassword" }
                };

        client.Login("https://myurl.com/login", loginData);

        return client;
    }

and then navigate to some internal page using:

        using (var client = GetAuthenticatedClient())
        {
            var html = client.DownloadString("https://myurl.com/internalPage");
            //do your stuff with received HTML here
        }

I'm successfully using that logic in my ASP.NET projects.

Are you trying to emulate user activity of input a username and password?

Why don't you try to use a POST or GET method (usualy for authentication web sites use POST) to achieve your goal.

You can use HTTPClient methode for that purpose