Blog.Web.Api.Controllers.AccountController.GetExternalLogin C# (CSharp) Method

        public async Task<IHttpActionResult> GetExternalLogin(string provider, string error = null)
        {
            if (error != null)
            {
                return Redirect(Url.Content("~/") + "#error=" + Uri.EscapeDataString(error));
            }

            if (!User.Identity.IsAuthenticated)
            {
                return new ChallengeResult(provider, this);
            }

            var externalLogin = ExternalLoginData.FromIdentity(User.Identity as ClaimsIdentity);

            if (externalLogin == null)
            {
                return InternalServerError();
            }

            if (externalLogin.LoginProvider != provider)
            {
                Authentication.SignOut(DefaultAuthenticationTypes.ExternalCookie);
                return new ChallengeResult(provider, this);
            }

            var user = await UserManager.FindAsync(new UserLoginInfo(externalLogin.LoginProvider, externalLogin.ProviderKey));

            var hasRegistered = user != null;

            if (hasRegistered)
            {
                Authentication.SignOut(DefaultAuthenticationTypes.ExternalCookie);

                var oAuthIdentity = await user.GenerateUserIdentityAsync(UserManager);
                var cookieIdentity = await user.GenerateUserIdentityAsync(UserManager);

                var properties = BlogOAuthAuthorizationServerProvider.CreateProperties(user.UserName);
                Authentication.SignIn(properties, oAuthIdentity, cookieIdentity);
            }
            else
            {
                var claims = externalLogin.GetClaims();
                var identity = new ClaimsIdentity(claims, OAuthDefaults.AuthenticationType);
                Authentication.SignIn(identity);
            }

            return Ok();
        }