Question

I create an IEnumerable object that just contains the nodes I want from an xml file:

IEnumerable<XElement> rosters = XDocument.Load('roster.xml')                                          .Elements('rosterlist')                                          .Elements('roster')                                          .Where(w => w.Element('division')                                                       .Value                                                       .Equals('SUPER AWESOME DIVISION')); 

So it’s a collection of these:

<rosterlist>     <roster>         <userid>1</userid>         <name></name>         <etc></etc>     </roster>     <roster>         <userid>2</userid>         <name></name>         <etc></etc>     </roster> </rosterlist> 

I want to grab only the users where the userid attribute is also a userid node within the rosters collection.

IEnumerable<XElement> users = XDocument.Load('user.xml')                                        .Elements('userlist')                                        .Elements('user')                                        .Where(w => rosters.Elements('userid')                                                           .Contains(w.Attribute('userid').Value)); 

But it is giving me an error:

The type arguments for method ‘System.Linq.Enumerable.Contains(System.Collections.Generic.IEnumerable, TSource)’ cannot be inferred from the usage. Try specifying the type arguments explicitly.

What is wrong with my approach?

Answer 1

One issue I see is that in the last code snippet, ...Elements('userid') returns a list of XElement objects which cannot contain a String returned by the Value property. This should work…

IEnumerable<XElement> rosters = obRoot.Elements('rosterlist').Elements('roster'); var rosterUserIds = (rosters.Elements('userid').Select(r => r.Value)); IEnumerable<XElement> users = obRoot.Elements('userlist').Elements('user')                 .Where(u => rosterUserIds.Contains(u.Attribute('userid').Value)); 

However I’d do this by using a join query.. Select user join rosters on userid
It’d go something like this

string sXml = @' <root> <rosterlist>     <roster>         <userid>1</userid>         <name>R1</name>         <etc></etc>     </roster>     <roster>         <userid>2</userid>         <name>R2</name>         <etc></etc>     </roster> </rosterlist> <userlist>     <user userid='1'>         <name>User on roster</name>     </user>     <user userid='5'>         <name>User not on roster</name>     </user> </userlist> </root>  ';  XElement obRoot = XElement.Parse( sXml ); var results = from user in obRoot.Elements('userlist').Elements('user')    join roster in obRoot.Elements('rosterlist').Elements('roster')    on user.Attribute('userid').Value equals roster.Element('userid').Value    select new {Name=user.Element('name').Value, RosterName=roster.Element('name').Value} ;  foreach (var v in results) {    Console.WriteLine('{0, -20} on Roster {1, -20}', v.Name, v.RosterName); } 

Outputs:

User on roster       on Roster R1