为什么if语句对我不起作用

我试图为我的网站在ASP.NET的登录入口做一个if语句。

一次又一次地

，它只是忽略了这样一个事实:如果某项为空，它不应该进入语句。

你能告诉我我哪里错了吗?

connection.Open ();//FirstName * * * * * 字符串firstName = FirstNameTextBox.Text;字符串sqlquery = ("INSERT INTO Users (FirstName,LastName,Username,Password) VALUES (@FirstName，@LastName，@Username，@Password)");

        SqlCommand command = new SqlCommand(sqlquery , connection);
        command.Parameters.AddWithValue("FirstName", firstName);
        //LastName************
        string lastName = LastNameTextBox.Text;
        command.Parameters.AddWithValue("LastName", lastName);
        //Username*************
        string username = UsernameTextBox.Text;
        command.Parameters.AddWithValue("UserName", username);
        //Password*************
        string password = PasswordTextBox.Text;
        command.Parameters.AddWithValue("Password", password);
        if (lastName != null || username != null || firstName != null || password != null)
        {

            if (PasswordTextBox.Text == ReTypePassword.Text)
            {
                Session["UserEnter"] = FirstNameTextBox.Text;

                command.ExecuteNonQuery();
                Response.Redirect("HomeAfter.aspx");
            }
            else if (PasswordTextBox.Text != ReTypePassword.Text)
            {
                ErrorLabel.Text = "Sorry, You didnt typed your password correctly.  Please type again.";
            }
            else
            {
                ErrorLabel.Text = "Some Error has accured.";
            }
        }
        else
        {
            ErrorLabel.Text = "Please fill all of the fields.";
        }
        connection.Close();
    }

if (lastName != null && username != null && firstName != null && password != null)

if (!string.IsNullorEmpty(lastName) && !string.IsNullorEmpty(username) && !string.IsNullorEmpty(firstName) && !string.IsNullorEmpty(password))
{
      //code here  
}
else
{
ErrorLabel.Text = "Please fill all of the fields.";
}