从Facebook检索电子邮件在ASP.NET MVC中登录抛出null异常

Question

我正在开发一个ASP.NET MVC应用程序。在我的应用程序中，我需要向用户提供Faccebook Login服务。此外，我需要从Facebook检索用户电子邮件并存储在数据库中。

我正在使用Visual Studio 2013和ASP.NET MVC5。我的Facebook登录方法运行正常。但是当我从Facebook检索电子邮件时，它给了我null异常。我的externalLoginCallback方法如下：

[AllowAnonymous]
        public async Task<ActionResult> ExternalLoginCallback(string returnUrl)
        {

var result = await AuthenticationManager.AuthenticateAsync(DefaultAuthenticationTypes.ExternalCookie);
            if (result == null || result.Identity == null)
            {
                return RedirectToAction("Login");
            }

            var idClaim = result.Identity.FindFirst(ClaimTypes.NameIdentifier);
            if (idClaim == null)
            {
                return RedirectToAction("Login");
            }

            var login = new UserLoginInfo(idClaim.Issuer, idClaim.Value);

            var name = result.Identity.Name == null ? "" : result.Identity.Name.Replace(" ", "");

            //retrieving email from facebook here
            var externalIdentity = HttpContext.GetOwinContext().Authentication.GetExternalIdentityAsync(DefaultAuthenticationTypes.ExternalCookie);
            var emailClaim = externalIdentity.Result.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Email);
            var email = emailClaim.Value;//this line is throwing null exception


.
.
.
}

我评论了我的代码中出错的地方。

这就是我在Startup.Auth.cs中配置Facebook的方式

var facebookAuthenticationOptions = new FacebookAuthenticationOptions()
{
    AppId = "x",
    AppSecret = "y"
};
facebookAuthenticationOptions.Scope.Add("email");
app.UseFacebookAuthentication(facebookAuthenticationOptions);

我也尝试过这样：

app.UseFacebookAuthentication(new FacebookAuthenticationOptions
            {
                AppId = "x",
                AppSecret = "y",
                Scope = { "email" },
                Provider = new FacebookAuthenticationProvider
                {
                    OnAuthenticated = context =>
                    {
                        context.Identity.AddClaim(new System.Security.Claims.Claim("FacebookAccessToken", context.AccessToken));
                        return Task.FromResult(true);
                    }
                }
            });

两者都不起作用。我的代码有什么问题，我该如何解决？

Answer 1

你可以试试这个：

    var facebookAuthenticationOptions = new FacebookAuthenticationOptions()
    {
        AppId = "xxxxxxx",
        AppSecret = "xxxxxx",
        BackchannelHttpHandler = new FacebookBackChannelHandler(),
        UserInformationEndpoint = "https://graph.facebook.com/v2.4/me?fields=id,name,email,first_name,last_name,location"
    };

然后定义这个类：

public class FacebookBackChannelHandler : HttpClientHandler
{
    protected override async System.Threading.Tasks.Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
    {
        if (!request.RequestUri.AbsolutePath.Contains("/oauth"))
        {
            request.RequestUri = new Uri(request.RequestUri.AbsoluteUri.Replace("?access_token", "&access_token"));
        }

        var response = await base.SendAsync(request, cancellationToken);
        return response;
    }
}

最后，您可以通过在帐户控制器中的“ExternalLoginCallback”方法中执行以下操作来获取电子邮件（和first_name，last_name，location）：

 [AllowAnonymous]
        public async Task<ActionResult> ExternalLoginCallback(string returnUrl)
        {
            var loginInfo = await AuthenticationManager.GetExternalLoginInfoAsync();
            if (loginInfo == null)
            {
                return RedirectToAction("LoginRegister");
            }

            // GET MAIL AND NAME
            var email = loginInfo.ExternalIdentity.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Email).Value;
            var name= loginInfo.ExternalIdentity.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Name).Value;