Selecting a list of sub elements in XML that may or may not exist

What I have is an XML we receive from a web service which represents a list of questions. The questions are broken down by type which indicates how they should be displayed on the web. For instance:

<step id="109025">
  <stepinf enabled="Yes" errors="0" id="First Name" mandatory="Yes" name="sti_115511" type="FIL">
    <field_label>First Name</field_label>
    <screen_value></screen_value>
  </stepinf>
  <stepinf enabled="Yes" errors="0" id="Last Name" mandatory="Yes" name="sti_115513" type="FIL">
    <field_label>Last Name</field_label>
    <screen_value></screen_value>
  </stepinf>
  <stepinf enabled="Yes" errors="0" id="State" mandatory="Yes" name="sti_109257" type="STE">
    <field_label>State</field_label>
    <screen_value></screen_value>
    <options_list>
      <option label="AK">AK - Alaska</option>
      <option label="AL">AL - Alabama</option>
      <option label="AR">AR - Arkansas</option>
      <option label="AS">AS - American Samoa (Terr.)</option>
      <option label="AZ">AZ 开发者_StackOverflow- Arizona</option>
      ...
    </options_list>
  </stepinf>
</step>

The type "STE" indicates that it will display on the web as a select box.

I am populating a List<> of a custom type I created by doing the following:

var stepinfList = (from stepinf in xdoc.Descendants("stepinf")
    select new Question
    {
       TextID = stepinf.Attribute("id").Value,
       Type = stepinf.Attribute("type").Value,
       Name = stepinf.Attribute("name").Value,
       Label = stepinf.Element("field_label").Value,
       Required = stepinf.Attribute("mandatory").Value,
       ErrorCount = int.Parse(stepinf.Attribute("errors").Value)
    }).ToList();

Where I am getting lost at is I have no idea how to get the option sub-elements into my results. I tried creating a property in the Question type named Options which I defined as an IDictionary and then utilized a sub-select in my LINQ query and the ToDictionary extension.

Options = (from option in xdoc.Element("options_list").Elements("option")
    select option)
    .ToDictionary(x => x.Attribute("label").Value, x => x.Value)

This didn't work as I think it bombs on the stepinf records that do not have child option_list elements. Anyway, I get "Object reference not set to an instance of an object" on the LINQ statement when I run the page.

I'm afraid this is above my current LINQ skill set so any help would be greatly appreciated.

---

This didn't work as I think it bombs on the stepinf records that do not have child option_list elements.

Your assessment is correct, so you need to check whether option_list exists before querying it. Check if it's null and return the dictionary or return null accordingly.

Try this:

Options = stepinf.Element("options_list") == null ? null :
              stepinf.Element("options_list").Elements("option")
                     .ToDictionary(x => x.Attribute("label").Value, x => x.Value)