Develop Reference

Use LINQ to get only the most recent JOINed item for each element

I have a LINQ query:
Elements.Join(ElementStates,
element => element.ElementID,
elementState => elementState.ElementID,
(element , elementState ) => new { element, elementState })
OK, so each Element has an ElementState associated to it. However there can be multiple states for each element for historical purposes, marked by a DateModified column. In this query, I would like to return only the most recent ElementState for each Element.
Is such a thing possible, using LINQ?
EDIT:
Credit to Gilad Green for their helpful answer.
I have converted it to Method syntax for anyone else who would like to see this in the future:
Elements.GroupJoin(ElementStates,
element => element.ElementID,
elementState => elementState.ElementID,
(element, elementState) =>
new { element, elementState = elementState.OrderByDescending(y => y.DateModified).FirstOrDefault() });

You can use GroupJoin instead of Join and then retrieve the first record after ordering the group by the DateModified:
var result = from e in Elements
join es in ElementStates on e.ElementID equals es.ElementID into esj
select new {
Element = e,
State = esj.OrderByDescending(i => i.DateModified).FirstOrDefault()
};
The same can be implemented with method syntax instead of query syntax but in my opinion this is more readable
For the difference between simply joining and group joining: Linq to Entities join vs groupjoin

Using a List in a where clause in Entity Framework

I am trying to retrieve document id's over a one-to-many table. I want to use a List in the where clause to find all id's that are connected with every element in the list.
List<int> docIds = (from d in doc
where _tags.Contains(d.Tags)
select d.id).ToList<int>();
I know that the contains must be incorrect but I can't work it out. If I try a foreach I can't work out how to check if the document contains all Tags.

If you want that all d.Tags should be in the included in the _tags list, you can try:
List<int> docIds = (from d in doc
where d.Tags.All(t => _tags.Contains(t))
select d.id).ToList<int>();
If you want that d.Tags should contain all the item from _tags you need:
List<int> docIds = (from d in doc
where _tags.All(t => d.Tags.Contains(t))
select d.id).ToList<int>();
But I don't know how it translates to SQL by EF so maybe you need to evaluate it on the client site.

Use a join:
List<int> docIds = (from d in doc
from t in tags
where d.Tags.Contains(t)
select d.id).ToList<int>();

Linq query on XML to Select multiple elements of subnodes

I want to select all distinct values of child from following xml
<root>
<parent>
<child>value 1</child>
<child>value 2</child>
</parent>
<parent>
<child>value 1</child>
<child>value 4</child>
</parent>
</root>
I tried following:
var vals = (from res in XmlResources.Elements("root").Elements("parent") select res)
.SelectMany(r => r.Elements("child")).Distinct().ToList();
But can't get the value from it, gives me value wrapped in tag and not Distinct
Is it possible to show both ways to get it - query and chaining aka lambda.

yes it is possible both ways
var doc = new XDocument("your xml string");
var values = (from c in doc.Root.Descendants("child") select c.Value).Distinct();
//chaining style
var values = doc.Root.Descendants("child").Select(c=>c.Value).Distinct();

You're selecting the elements, and the elements are all distinct. You need to get the distinct values. For example:
var values = XmlResources.Element("root")
.Elements("parent")
.Elements("child")
.Select(x => x.Value)
.Distinct();
There's really no benefit in using a query expression here - it only adds cruft. I only use a query expression when the query has multiple aspects to it (e.g. a where and a meaningful select, or a join). For just a select or just a where it's pretty pointless. So yes, you can use:
var values = (from x in XmlResources.Element("root")
.Elements("parent")
.Elements("child")
select x.Value).Distinct();
... but why would you? It's a lot less clear IMO.
Note that if you don't care too much about the root/parent/child hierarchy, and are happy to just get all the child descendants, you can use:
var values = XmlResources.Descendants("child")
.Select(x => x.Value)
.Distinct();

Dynamic XML Sorting using LINQ

How can I sort dynamic XML using LINQ having following precedences:
Sort by node-name
Sort by node-value
Sort by attribute-name
Sort by attribute-value

Sorting by Node Name:
var doc = XDocument.Parse("<data><carrot /><apple /><orange /></data>");
var sortedByNames = doc.Root.Elements().OrderBy(e => e.Name.ToString());
foreach(var e in sortedByNames)
Console.WriteLine (e.Name);
Sorted by Node Value:
var doc = XDocument.Parse("<data><thing>carrot</thing><thing>apple</thing><thing>orange</thing></data>");
var sortedByValue = doc.Root.Elements().OrderBy(e => e.Value.ToString());
foreach(var e in sortedByValue)
Console.WriteLine (e.Value);
It all follows the same pattern... You sort based on the criteria you define in the selector function passed into the OrderBy method.

var data = from item in xmldoc.Descendants("content")
orderby (string)item.Element("title") // by node value
//orderby item.Attribute("something") // by attribute value
select new
{
Title = (string)item.Element("title"),
};

Linq query convert to List<string>

I have this code
List<string> IDs = new List<string>();
XDocument doc = XDocument.Parse(xmlFile);
var query = from c in doc.Root.Elements("a").Elements("b")
select new { ID = c.Element("val").Value};
How can I convert query to List without loop foreach ?
{ ID = c.Element("val")}
are strings of course
EDIT
my XML File
<?xml version="1.0" encoding="utf-8"?>
<aBase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<a>
<b>
<val>other data</val>
</b>
<b>
<val>other data</val>
</b>
</a>
</aBase>

IDs = query.Select(a => a.ID).ToList();
or if you'd like to do it in one line
List<string> IDs = (from c in doc.Root.Elements("a").Elements("b")
select c.Element("val").Value).ToList()

The anonymous type isn't really helping you since you only need a sequence of strings, not any sort of tuple. Try:
XDocument doc = XDocument.Parse(xmlFile);
var query = from c in doc.Root.Elements("a").Elements("b")
select c.Element("val").Value;
var IDs = query.ToList();
Personally, I would just use method-syntax all the way:
var IDs = doc.Root.Elements("a")
.Elements("b")
.Select(c => c.Element("val").Value)
.ToList();