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How can I get only records with a unique property using entity framework and linq?

public class Person
{
public string firstName;
public string lastName;
}
I want a list of all Persons with a unique first name.
Persons table
Tom Haverford
Tom Baker
Amy Pond
Amy Santiago
Trish Walker
Chidi Anagonye
The query should return
Trish, Chidi
I've tried using Distinct and a combination of GroupBy and Select, but those return Trish, Chidi, Tom, Amy.

Demo on dotnet fiddle
You can Group by then count number of duplicated items. After that, you can get the item with count value equals to 1 like below.
var arr = new []
{
new Person { firstName = "Tom", lastName = "Haverford" },
new Person { firstName = "Tom", lastName = "Baker"},
new Person { firstName = "Amy", lastName = "Pond" },
new Person { firstName = "Amy", lastName = "Santiago"},
new Person { firstName = "Trish", lastName = "Walker"},
new Person { firstName = "Chidi", lastName ="Anagonye" }
};
var result = arr.GroupBy(p => p.firstName).Select(g => new { Name = g.Key, Count = g.Count()});
foreach(var item in result.Where(p => p.Count == 1))
Console.WriteLine(item.Name);
Output
Trish
Chidi

You can use group by and count functionality together for this :
1. Get a list of all persons from DB :
var personList = (from p in db.Person select p).ToList(); // I assumed here that your db obj name is 'db' and table name is 'Person'
2. Now apply group by query to get the count of first names :
var q = from x in personList
group x by x.firstName into g
let count = g.Count()
select new {Count = count, Name = g.First().firstName };
3. Now you can get your final result like this :
var finalResult = (from p in q where p.Count == 1 select p).ToList();
Happy Coding...

Nested group by based on flatten record set

I have the following responses from the API. How can I group them into the following structure?
Student[]
- Name
- Classes[]
- ClassName
- ClassId
- ClassCategories[]
- CategoryName
- CategoryWeight
- Assignments[]
- AssignmentName
- Score
I was managed to group them until the "Classes" level but unable to get the ClassCategories for each of the classes
var data = (from result in results
group result by new { result.StudentId, result.FirstName, result.LastName, result.MiddleInitial }
into StudentGroup
select new GroupedStudent
{
StudentId = StudentGroup.Key.StudentId,
FullName = string.Format("{0} {1} {2}", StudentGroup.Key.FirstName, StudentGroup.Key.MiddleInitial, StudentGroup.Key.LastName).Replace(" ", " "),
Classes = from result in results
group result by new { result.ClassId, result.ClassName } into ClassGroup
select new groupedClass
{
ClassName = ClassGroup.Key.ClassName,
ClassId = ClassGroup.Key.ClassId,
ClassCategories = ...
})
}).ToList();
Can anyone please assists me? Thank you.

First, you have make ClassGroup from StudentGroup not from results.
Classes = from s in StudentGroup group result by new { s.ClassId, s.ClassName } into ClassGroup
The complete linq query is as follows:
var data =
(from result in results
group result by new { result.StudentId, result.FirstName, result.LastName, result.MiddleInitial } into StudentGroup
select new
{
StudentId = StudentGroup.Key.StudentId,
FullName = string.Format("{0} {1} {2}", StudentGroup.Key.FirstName, StudentGroup.Key.MiddleInitial, StudentGroup.Key.LastName).Replace(" ", " "),
Classes = (from s in StudentGroup
group s by new { s.ClassId, s.ClassName } into ClassGroup
select new
{
ClassId = ClassGroup.Key.ClassId,
ClassName = ClassGroup.Key.ClassName,
ClassCategories = (from c in ClassGroup
group c by new { c.CategoryName, c.CategoryWeight } into CategoryGroup
select new
{
CategoryName = CategoryGroup.Key.CategoryName,
CategoryWeight = CategoryGroup.Key.CategoryWeight,
Assignments = (from ct in CategoryGroup
group ct by new { ct.AssignmentName, ct.Score } into AssingnmentGroup
select new
{
AssignmentName = AssingnmentGroup.Key.AssignmentName,
Score = AssingnmentGroup.Key.Score
}).ToList()
}).ToList()
}).ToList()
}).ToList();
For example, if you want to access to the first Assignment's score, you can get it like this:
var student = data.FirstOrDefault();
var score = student.Classes[0].ClassCategories[0].Assignments[0].Score;

This is usually how I do It.
Create a class to store your data
Create a list of that class type
In your case instead of string dataRow maybe you can use a sub class
.
// get data from webservice
var json = webClient.DownloadString(url);
var values = JsonConvert.DeserializeObject<JArray>(json);
// create a list to save all the element
List<myClass> classList = new List<myClass>();
// process every row
foreach (string dataRow in values)
{
string[] dataField = dataRow.Split(',');
// have a constructor to assign each value to this element
myClass ctROW = new myClass(dataField);
classList.add(ctROW );

XML parsing with LINQ, has 2 tags with the same name but different level and not always has a value

I have the following XML:
<request>
<book>
<id>1833801</id>
<title>The Yiddish Policemen's Union </title>
<work>
<id>1234</id>
<name/>
</work>
<similar_books>
<book><id>243859</id><title>Stations of Tide</title> <isbn>0380817616</isbn>
<authors><author><id>14454</id><name>Michael Swanwick</name></author> </authors>
</book>
</similar_books>
<authors>
<author>
<id>2715</id><name>Michael Chabon</name>
<ratings_count>215884</ratings_count></author>
</authors>
<popular_shelves>
<shelf name="jewish" count="104"/><shelf name="sci-fi" count="100"/>
</popular_shelves>
</book>
</request>
I want to have all tags with their respective values, and I am using following code:
HttpWebRequest oReq = (HttpWebRequest)WebRequest.Create(uriRoot);
HttpWebResponse resp = (HttpWebResponse)oReq.GetResponse();
log.Info(" (ISBN= " + isbn10 + ") Http request has response.");
if (resp.ContentType.StartsWith("application/xml", StringComparison.InvariantCultureIgnoreCase))
{
Stream resultStreamISBN = resp.GetResponseStream();
Encoding encode = System.Text.Encoding.GetEncoding("utf-8"); //encoding for non-latin chars
StreamReader responseReader = new StreamReader(resultStreamISBN, encode);
XDocument xdoc = XDocument.Parse(responseReader.ReadToEnd());
var books = (from u in xdoc.Descendants().Elements("book")
select new
{
id = (string)u.Element("title"),
title = (string)u.Element("title"),
works = (from i in u.Elements("work")
select new
{
work_best_book_id = (int)i.Element("id"),
work_name = (string)i.Element("name"),
}).ToList(),
authors = (from i in u.Elements("authors").Elements("author")
select new
{
id = (int)i.Element("id"),
name = (string)i.Element("name"),
rating = (int)i.Element("rating_count")
}).ToList(),
popular_shelves = (from i in u.Elements("popular_shelves").Elements("shelf")
select new
{
name = (string)i.Attribute("name"),
count = (int)i.Attribute("count")
}).ToList(),
}).ToList();
The code returns null values and is not working properly. I also should note that different xml files may not have values for all the tags.
Any suggestions on how I can improve my code?

Check for missing elements like such:
name = i.Element("name") == null ? null : i.Element("name")
For value types, you make have to change them to nullable types for this to work correctly.

You can transform
id = (int)i.Element("id"),
to
id = (int?)i.Element("id"),
Whole modified code
// I am inserting the following line to demonstrate how I load the XML in test code
// Basically, I copied & pasted xml in a file OP gave named request.xml
//XDocument xdoc = XDocument.Load( #"d:\Data\request.xml");
EDIT
XDocument xdoc = XDocument.Load("http://www.goodreads.com/book/isbn?isbn=0007295685&key=lbScLXWyNGQ1q0BDoFFSg");
var books = (from u in xdoc.Descendants().Elements("book")
select new
{
id = (string)u.Element("title"),
title = (string)u.Element("title"),
works = (from i in u.Elements("work")
select new
{
work_best_book_id = (int?)i.Element("id"),
work_name = (string)i.Element("name"),
}).ToList(),
authors = (from i in u.Elements("authors").Elements("author")
select new
{
id = (int?)i.Element("id"),
name = (string)i.Element("name"),
rating = (int?)i.Element("rating_count")
}).ToList(),
popular_shelves = (from i in u.Elements("popular_shelves").Elements("shelf")
select new
{
name = (string)i.Attribute("name"),
count = (int?)i.Attribute("count")
}).ToList(),
}).ToList();

Reading the XML values using LINQ

what is the best way of reading xml file using linq and the below code you will see that, I have three different loops and I feel like its not elegant or do I have options to retrofit the below code?
public static void readXMLOutput(Stream stream)
{
XDocument xml = new XDocument();
xml = LoadFromStream(stream);
var header = from p in xml.Elements("App").Elements("Application")
select p;
foreach (var record in header)
{
string noym = record.Element("nomy").Value;
string Description = record.Element("Description").Value;
string Name = record.Element("Name").Value;
string Code = record.Element("Code").Value;
}
var appRoles = from q in xml.Elements("App").Elements("Application").Elements("AppRoles").Elements("Role")
select q;
foreach (var record1 in appRoles)
{
string Name = record1.Element("Name").Value;
string modifiedName = record1.Element("ModifiedName").Value;
}
var memeber = from r in xml.Elements("App").Elements("Application").Elements("AppRoles").Elements("Role").Elements("Members")
select r;
foreach (var record2 in memeber)
{
string ExpirationDate = record2.Element("ExpirationDate").Value;
string FullName = record2.Element("FullName").Value;
}
}
UPDATED:
foreach (var record in headers)
{
..............
string Name1 = record.Attribute("Name").Value;
string UnmodifiedName = record.Attribute("UnmodifiedName").Value;
string ExpirationDate = record.Attribute("ExpirationDate").Value;
string FullName = record.Attribute("FullName").Value;
...............
}

Is that your actual code ? All those string variables you are assigning in the foreach loops only have a scope of one iteration of the loop. They are created and destroyed each time.

This may not work precisely in your case depending on the xml structure. Play around with it. Try it using LinqPad
var applications = from p in xml.Descendants("Application")
select new { Nomy = p.Element("nomy").Value
, Description = p.Element("Description").Value
, Name = p.Element("Name").Value
, Code = p.Element("Code").Value
};
var appRoles = from r in xml.Descendants("Role")
select new { Name = r.Element("Name").Value
, ModifiedName = r.Element("ModifiedName").Value
};

This answer is a hierarchical query.
var headers =
from header in xml.Elements("App").Elements("Application")
select new XElement("Header",
new XAttribute("noym", header.Element("nomy").Value),
new XAttribute("Description", header.Element("Description").Value),
new XAttribute("Name", header.Element("Name").Value),
new XAttribute("Code", header.Element("Code").Value),
from role in header.Elements("AppRoles").Elements("Role")
select new XElement("Role",
new XAttribute("Name", role.Element("Name").Value),
new XAttribute("ModifiedName", role.Element("ModifiedName").Value),
from member in role.Elements("Members")
select new XElement("Member",
new XAttribute("ExpirationDate", member.Element("ExpirationDate").Value),
new XAttribute("FullName", member.Element("FullName").Value)
)
)
);

Generate string from Grouping Linq Query

Given the following code:
var people = new List<person>(){ new person { Name = "John", FamilyName = "Pendray" },
new person { FamilyName = "Emery", Name = "Jake"},
new person { FamilyName = "Pendray", Name = "Richard" } };
var q = from p in people
orderby p.Name
group p by p.FamilyName into fam
orderby fam.Key
select new { fam.Key, members = from p in fam select p };
Is it possible to replace the last line with a select that will output a IEnumerable<string> that contains these two strings:
"Pendray John Richard"
"Emery Jake"? Is it possible to project a linq query into strings like this?
Edit: I know this is possible with further code but I'm interested in whether this can be done from within the linq query itself in a similar way to VB being able to project xml out of a query as in http://www.thinqlinq.com/default/Projecting-XML-from-LINQ-to-SQL.aspx (particularly the last code block on this page)

var q = from p in people
orderby p.Name
group p by p.FamilyName into fam
orderby fam.Key
select fam.Key + " " + string.Join(" ", (from fm in fam select fm.Name).ToArray());
Returns
Emery Jake
Pendray John Richard

Definitely.
You would have to change the select part. The easiest way would be to define a function that would take the IEnumerable and generate that string, then call that function
people = new List<person>(){ new person { Name = "John", FamilyName = "Pendray" },
new person { FamilyName = "Emery", Name = "Jake"},
new person { FamilyName = "Pendray", Name = "Richard" } };
var q = from p in people
orderby p.Name
group p by p.FamilyName into fam
orderby fam.Key
select new { Key = fam.Key, Text = GetText(fam) };
// and elsewhere...
private string GetText(IEnumerable<person> family) {
string result = "Whatever"; // build the result string here
return result;
}
Also, if you only want the text, you can change the last line of the query to simply
select GetText(fam);