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How do I put distinct values from a LINQ query into a list?

I've looked at several answers to similar questions, but none of them solve my problem. All I need to do is get distinct values from a LINQ query (that queries a XML file) and put them into a list. Here is what I have tried:
var XmlData = XDocument.Load("PathToFile");
List<string> XmlItems = new List<string>();
var XQuery = from m in XmlData.Root.Elements()
where m.Attribute("Category").Value.ToString().Equals("TheCategory")
select (m.Attribute("TheAttribute").Value).Distinct().ToString();
XmlItems.AddRange(XQuery);
foreach (var item in XmlItems)
{
ComboBoxTeams.Items.Add(item);
}
The Distinct() function call is not giving the expected result. I'm unfamiliar with how to get distinct values from a LINQ query. Any suggestions?

At this point, your Distinct
var XQuery = from m in XmlData.Root.Elements()
where m.Attribute("Category").Value.ToString().Equals("TheCategory")
select (m.Attribute("TheAttribute").Value).Distinct().ToString();
is only for (m.Attribute("TheAttribute").Value), not for the whole statement
You may need to change it to
var XQuery = from m in XmlData.Root.Elements()
where m.Attribute("Category").Value.ToString().Equals("TheCategory")
select (m.Attribute("TheAttribute").Value.ToString()); //get everything first, ToString probably needed
var XQueryDistinct = XQuery.Distinct(); //get distinct among everything you got

You have the .ToString() and .Distinct() in the wrong places.
var XmlData = XDocument.Load("PathToFile");
List<string> XmlItems = new List<string>();
var XQuery = from m in XmlData.Root.Elements()
where m.Attribute("Category").Value.ToString().Equals("TheCategory")
select (m.Attribute("TheAttribute").Value).Distinct().ToString();
XmlItems.AddRange(XQuery);
foreach (var item in XmlItems)
{
ComboBoxTeams.Items.Add(item);
}
becomes:
var XmlData = XDocument.Load("PathToFile");
var XmlItems = (from m in XmlData.Root.Elements()
where m.Attribute("Category").Value.ToString().Equals("TheCategory")
select (m.Attribute("TheAttribute").Value.ToString())).Distinct();
foreach (var item in XmlItems)
{
ComboBoxTeams.Items.Add(item);
}

If you have a list of simple values, you need to remove the use of Distinct in your select and put after.
var XQuery = (from m in XmlData.Root.Elements()
where m.Attribute("Category").Value.ToString().Equals("TheCategory")
select (m.Attribute("TheAttribute").Value.ToString())).Distinct();
If you have complex objects you have two alternatives:
Using morelinq you can use DistinctBy:
XmlItems.DistinctBy(x => x.WhateverProperty);
Otherwise, you can use a group:
XmlItems.GroupBy(x => x.idOrWhateverOtherProperty)
.Select(g => g.First());

You might try
var uniqueList = yourList.Distinct().ToList();
after you got your non-unique list.

Linq query - returning a sum

I am having trouble figuring out how to do something like the following. This is purely pseudocode:
decimal totalActiveCost = (from i in _context.KeyActives
where i.Pk in (active1fk, active2fk, active3fk, active4fk, active5fk, keyActiveFk)
select sum(i.Cost)...`
Then summing the i.Cost. So basically, I need to return the i.Cost for each "Active" - so, for example, say active1fk is 1, active2fk is 2, and so on. I need to get the Cost for each of these and sum them up.

You can have your active foreign keys in a List<T> like:
List<int> activeFks = new List<int> {1,2,3,4,5,};
var sum = (from i in _context.KeyActives
where activeFks.Contains(i.PK)
select i.Cost).Sum();
Or with a method syntax:
var sum = _context.KeyActives
.Where(r=> activeFks.Contains(r.PK))
.Sum(r=> r.Cost);

Something like this will work:
List<Int> ids = new List<int>();
ids.Add(1);
ids.Add(2);
var result = _context.KeyActives.
Where(c => ids.Contains(c.id))
.Sum(c => c.Cost);

var ids = new List<int> {active1fk, active2fk, active3fk, active4fk, active5fk, keyActiveFk};
var sum = (from i in _context.KeyActives
where ids.Contains(i.Pk)
select i).Sum(a=> a.Cost);

Why I'm still getting System.NotSupportedException

My code is:
using( var ctxA = new AEntities())
using( var ctxB = new BEntities())
{
var listOfA = (from A in AEntities.AEntity select A).ToList();
var listOfB = (from B in BEntities.BEntity
where listOfA.Select( A = A.Id)contains(B.Id)
select B).ToList();
}
I'm getting the error:
The specified LINQ expression contains references to queries that are associated with different contexts.
However, because of 'ToList' I already did a fetch in AEntity thats makes second query only about one of the contexts, did not?
How can a separate the two queries still using one list to query the other?

Try to store just IDs into listOfA
var listOfA = (from A in AEntities.AEntity select A.Id).ToList();
var listOfB = (from B in BEntities.BEntity
where listOfA.Contains(B.Id)
select B).ToList();

If anyone wants to know, the answer to my question was:
var listOfA = (from A in AEntities.AEntity select A.Id).ToList();
List<long> listOfIdsOfA = listOfA.Select( A=>A.id).ToList();
var listOfB = (from B in BEntities.BEntity
where listOfIdsOfA.Contains(B.Id)
select B).ToList();
The only difference to Selman's answer was the creation of Id's list after the first query. That's why I need the whole entity too.

What is the LINQ query to get a Cartesian Product even when one set is EMPTY?

Imagine I have 2 list and one is empty:
List<string> foo = new List<string>(){ "Ali","wall-e","Ellie" };
List<string> bar = new List<string>();
And I obtain the Cartesian Product of 2:
var q = from f in foo
from b in bar
select new {f,b};
As bar is empty LINQ returns an empty result set.
Question:
How can I write the above query so that I can receive this result set:
Ali,NULL
Wall-e,NULL
Ellie,NULL

Maybe this is what you want:
var q = from f in foo.DefaultIfEmpty()
from b in bar.DefaultIfEmpty()
select new {f,b};

Get records which do not start with an alphabetical character in linq

I need to get a list of records that do not start with an alphabetical character, i.e. which either starts with a numerical character or any special character.
Whats the simple LINQ query to get this list?

List<string> Entries = new List<string>();
Entries.Add("foo");
Entries.Add("bar");
Entries.Add("#foo");
Entries.Add("1bar");
var NonAlphas = (from n in Entries
where !char.IsLetter(n.ToCharArray().First())
select n);
For Linq-to-sql you could hydrate your retrieval from the database by by enumerating the query (call ToList). From that point on, your operations will be against in-memory objects and those operations will not be translated into SQL.
List<string> Entries = dbContext.Entry.Where(n => n.EntryName).ToList();
var NonAlphas = Entries.Where(n => !char.IsLetter(n.First()));

Something like this?
List<string> lst = new List<string>();
lst.Add("first");
lst.Add("second");
lst.Add("third");
lst.Add("2abc");
var result = from i in lst where !char.IsLetter(i[0]) select i;
List<string> output = result.ToList();
Edit: I realized that using Regex here was overkill and my solution wasn't perfect anyway.

string[] x = new string[3];
x[0] = "avb";
x[1] = "31df";
x[2] = "%dfg";
var linq = from s in x where !char.IsLetter(s.ToString().First()) select s;
List<string> simplelist = new List<string>(linq);
/* in simple list you have only "31df" & "dfg" */

One thing to note is that you don't need to convert the string to a chararray to use linq on it.
The more consise version would be:
var list = new List<string> {"first","third","second","2abc"};
var result = list.Where(word => !char.IsLetter(word.First()));