how to authenticate boxnet in c#

As you know box uses OAuth2 for authentication and authorization. Therefore to implement it in C# we have use DotNet OpenAuth.

The following illustrates login with authentication for box in ASP.Net MVC using C#:

using DotNetOpenAuth.Messaging;

public ActionResult LogOn()
{
    var openid = new OpenIdRelyingParty();
    IAuthenticationResponse response = openid.GetResponse();

    if (response != null)
    {
        switch (response.Status)
        {
            case AuthenticationStatus.Authenticated:
                FormsAuthentication.RedirectFromLoginPage(
                    response.ClaimedIdentifier, false);
                break;
            case AuthenticationStatus.Canceled:
                ModelState.AddModelError("loginIdentifier",
                    "Login was cancelled at the provider");
                break;
            case AuthenticationStatus.Failed:
                ModelState.AddModelError("loginIdentifier",
                    "Login failed using the provided OpenID identifier");
                break;
        }
    }           

    return View();
}

[System.Web.Mvc.AcceptVerbs(HttpVerbs.Post)]
public ActionResult LogOn(string loginIdentifier)
{
    if (!Identifier.IsValid(loginIdentifier))
    {
        ModelState.AddModelError("loginIdentifier",
                    "The specified login identifier is invalid");
        return View();
    }
    else
    {
        var openid = new OpenIdRelyingParty();
        IAuthenticationRequest request = openid.CreateRequest(
            Identifier.Parse(loginIdentifier));

        // Require some additional data
        request.AddExtension(new ClaimsRequest
        {
            BirthDate = DemandLevel.NoRequest,
            Email = DemandLevel.Require,
            FullName = DemandLevel.Require
        });

        return request.RedirectingResponse.AsActionResult();
    }
}