这可能有效，

  string  userId = Membership.GetUser（  userName）。ProviderOserKey.ToString（）;  //  在GetUser中传递您的姓名。 

-KR

GetUser 返回 null ，或 ProviderUserKey 。您没有测试其中任何一个，看看它们是否返回 null 。最后，您尝试在 null 上调用 .ToString（）。这就是你收到错误信息的原因。

After customer validation i want to retrieve his user id.. I am using below code for that but it is giving error : Object reference not set to an instance of an object.

if (Membership.ValidateUser(txt_email.Text, txt_password.Text))
            {

                string userId = Membership.GetUser().ProviderUserKey.ToString();
            }

Any possible reasons????

This might work,

string userId = Membership.GetUser("userName").ProviderUserKey.ToString(); // pass your name in GetUser.

-KR

Either GetUser returned null, or ProviderUserKey did. You didn't test either of these to see if they returned null. In the end, you tried to call .ToString() on null. That's why you got the error message you did.