Linq XML query with nested loop - 1

Question

In this question, if I need to get "date" included in the output, what changes should be made?

When I included

let dt = l.Element("Date").Value

It gives, "Object reference not set to an instance of an object"

var query = from l in doc.Descendants("L1")
            let dt = l.Element("Date").Value
            let id = l.Attribute("id").Value
            from subject in l.Descendants("Subject")
            select new
            {
                Date = dt,
                Id = id,
                SubjectName = (string)subject.Attribute("SubjectName"),
                Score = (string)subject.Attribute("Score")
            };


foreach (var result in query)
{
    Console.WriteLine(result);
}

When you included that, what happened? Note that the XML that you've shown in the other question doesn't have any contents in the <date> elements. — Jon Skeet
– Jon Skeet, Commented May 21, 2012 at 19:14
he completed it with "it gives, "Object reference not set to an instance of an object" it got lumped into the code block — Kevin DiTraglia
– Kevin DiTraglia, Commented May 21, 2012 at 19:16
Note that your xml on the previous question has <date> and you are looking for Date -- the casing of the element name needs to be consistent. — Robaticus
– Robaticus, Commented May 21, 2012 at 19:35
Lets say date is string. Tried D Stanley's and Lucas Reis suggestion but it returns { Date = , Id = 10, SubjectName = subject1, Score = A1 } { Date = , Id = 10, SubjectName = subject2, Score = A2 } { Date = , Id = 10, SubjectName = subject2, Score = A3 } { Date = , Id = 20, SubjectName = subject4, Score = B1 } { Date = , Id = 20, SubjectName = subject5, Score = B2 } { Date = , Id = 20, SubjectName = subject6, Score = B3 } — user219628
– user219628, Commented May 21, 2012 at 19:36

Lucas Reis · Accepted Answer · 2012-05-21 20:04:40Z

2

If l doesn't have an Date element, trying to access l.Element("Date").Value will incur an error. You can use a conditional:

var query = from l in doc.Descendants("L1")
        let dt = l.Elements("date").Any() 
                 ? l.Element("date").Value
                 : AnyDefaultValueIWantForDate
        let id = l.Attribute("id").Value
        from subject in l.Descendants("Subject")
        select new
        {
            Date = dt,
            Id = id,
            SubjectName = subject.Attribute("SubjectName").Value,
            Score = subject.Attribute("Score").Value
        };

(I also added the .Value in SubjectName and Score).

edited May 21, 2012 at 20:04

answered May 21, 2012 at 19:26

Lucas Reis

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3 Comments

Lucas Reis Over a year ago

Is there any data in the Date element in the XML? Because in your link where you show a part of it, there is none...

user219628 Over a year ago

please use this for testing <Root> <L1 id="10"> <date>2012Jan01</date> <Subject SubjectName="subject1" Score="A1"/> <Subject SubjectName="subject2" Score="A2"/> <Subject SubjectName="subject2" Score="A3"/> </L1> <L1 id="20"> <date>2012Jan02</date> <Subject SubjectName="subject4" Score="B1"/> <Subject SubjectName="subject5" Score="B2"/> <Subject SubjectName="subject6" Score="B3"/> </L1> </Root>

Lucas Reis Over a year ago

edited: changed "Date" for "date" (XML is case sensitive).

Robaticus · Accepted Answer · 2012-05-21 19:35:24Z

1

Looking at your other XML, it does not, as Mr. Skeet mentioned, have anything in the <date> elements. You will need to explicitly handle that if you aren't planning on always having data in there.

You can do something like this:

let dt = l.Element("date") == null ? string.Empty : l.Element("date").Value

edited May 21, 2012 at 19:35

answered May 21, 2012 at 19:27

Robaticus

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Comments

D Stanley · Accepted Answer · 2012-05-21 19:20:40Z

1

Assuming l is not null then l.Element("Date") is null which would mean that one or more of your L1 elements does not have a child Date element.

The fix would depend on what you want to do with missing dates. If you want to use default(DateTime) as a "magic" date, you could do:

let dt = (l.Element("Date") == null ? default(DateTime) : l.Element("Date").Value)

answered May 21, 2012 at 19:20

D Stanley

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1 Comment

Robaticus Over a year ago

One problem with that unary is that the default datetime will return a datetime, and the l.Element("Date").Value will return a string. There needs to be a conversion on one side or the other.

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