Create dynamic linq-XML based on XML data

Question

Goal:
To apply the XML data to the List _a

Problem:
When making transaction of Jessica black I retrieve the error message "{"Object reference not set to an instance of an object."}" at the source code "_ab.age = li.Element("age").Value;" because there is no data of Jessicas age in the xml. Same problem can be for Jim West's sex.

What should I do? I started getting crazy!

---

C # below

public class Program
{
private static List<user> _a = new List<user>();
private static user _ab = new user();


static void Main(string[] args)
{
    XDocument xml = XDocument.Load("xml file....");

    xml.Root.Descendants("user").ToList().ForEach(li =>
    {
        _ab = new user();
        _ab.firstname = li.Element("firstname").Value;
        _ab.lastname = li.Element("lastname").Value;
        _ab.age = li.Element("age").Value;
        _ab.sex = li.Element("sex").Value;
        _a.Add(_ab);
    }
}
}

public class user
{
    public String firstname;
    public String lastname;
    public String age;
    public String sex;
}   

---

XML code below

<users>
    <user>
        <firstname>sara</firstname>
        <lastname>brown</lastname>
        <age>20</age>
        <sex>female</sex>
    </user>
    <user>
        <firstname>Jessica</firstname>
        <lastname>black</lastname>
        <sex>Female</sex>
    </user>
    <user>
        <firstname>Jim</firstname>
        <lastname>west</lastname>
        <age>26</age>
    </user>
    <user>
        <firstname>robert</firstname>
        <lastname>lake</lastname>
        <age>41</age>
        <sex>male</sex>
    </user>
    <user>
        <firstname>Britany</firstname>
        <lastname>McLove</lastname>
        <age>21</age>
    </user>
</users>    

Answer 1

Use conversation operator:

_ab.age = (string)li.Element("age");

and so on...

If element age doesn't exist, (string)li.Element("age") will return null and won't throw any exception.

Answer 2

Just test if the element is not null before getting it's Value :

    xml.Root.Descendants("user").ToList().ForEach(li =>
    {
        _ab = new user();
        if (li.Element("firstname") != null) _ab.firstname = li.Element("firstname").Value;
        if (li.Element("lastname") != null) _ab.lastname = li.Element("lastname").Value;
        if (li.Element("age") != null) _ab.age = li.Element("age").Value;
        if (li.Element("sex") != null) _ab.sex = li.Element("sex").Value;
        _a.Add(_ab);
    }