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C# sort object list with start position and loop

I have a strange question :)
I have a object list looking like this:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
I would like to sort the list providing a "start id"
For example, if I provide "start id" 3, the result should look like this:
Id
Name
3
Patric
4
Theodor
1
Marcus
2
Mattias
I have no idea where to start, so I really need some help from you coding gods
The list is from a sql table, but it does not matter for me where the sort take place (in sql query or in c# code)

Try this:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
var start_id = 3;
var max_id = list.Max(y => y.Id);
var result =
from x in list
orderby (x.Id + max_id - start_id) % max_id
select x;
I get:

With LINQ to objects you can do something like that:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
var startId = 3;
var result = list
.GroupBy(i => i.Id >= startId ? 1 : 0) // split in two groups
.OrderByDescending(g => g.Key) // sort to have the group with startId first
.Select(g => g.OrderBy(i => i.Id)) // sort each group
.SelectMany(i => i) // combine result
.ToList();
Console.WriteLine(string.Join(", ", result.Select(i => i.Id))); // prints "3, 4, 1, 2"

You require 2 criteria to apply:
Order ascending by Id.
Return the Ids greater than threshold before the Ids less than threshold.
You can try:
var offset = 3;
var sorted1 = list
.OrderBy(item => item.Id < offset)
.ThenBy(item => item.Id);
The OrderBy condition yields true if Id is less than offset and false otherwise.
true is greater than false and therefore is returned later
A dirty way could also be:
var offset = 3;
var sorted2 = list
.OrderBy(item => unchecked((uint)(item.Id - offset)));
Here the offset is subtracted from Id and the result converted to unsigned int to make the negative values become very large positive ones. A little hacky. Might not work with queries against SQL providers.

Here's a toy Non-Linq Version
object[] ShiftList(int id)
{
var list = new dynamic[]
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
Span<dynamic> listSpan = list;
int indexFound = -1;
for (int i = 0; i < list.Length; i++)
{
if (listSpan[i].Id == id)
{
indexFound = i;
}
}
if (indexFound is -1)
{
return list;
}
var left = listSpan.Slice(0, indexFound);
var right = listSpan[indexFound..];
object[] objs = new object[list.Length];
Span<object> objSpan = objs;
right.CopyTo(objSpan);
left.CopyTo(objSpan[right.Length..]);
return objs;
}

Try using foreach and iterate over each object in your list:
foreach (var item in list)
{
}
from here you should be able to use some of the collection methods for a list to reorder your list.

How to dynamically group a list and select values using linq query in C#?

I have an input list test
class Tracker
{
public string Toolid {get;set;}
public string Description {get;set;}
public int length {get;set;}
public int breadth {get;set;}
public int height {get;set;}
}
List<Tracker> test = new List<Tracker>();
test.Add( new Tracker {Toolid="A.1",Description ="ABC",length = 10, breadth =10,height = 50});
test.Add( new Tracker {Toolid="A.1",Description ="ABC", length = 10, breadth =10,height = 50});
test.Add( new Tracker {Toolid="C.1",Description ="ABCD", length = 10, breadth =10,height = 50});
test.Add( new Tracker {Toolid="D.1",Description ="Admin123", length = 10, breadth =10,height = 50});
This list contain more values like weight, colour etc.
For better understanding I have added only 5 member variables in the class Tracker.
I need to Group the list test based on the values of another list (grpList).
This list (grpList ) is dynamic, hence the number of parameter and values in the list may change.
So I need a dynamic Group By of list using LINQ query.
case 1 : sometimes this list grpList contain 2 values .
List <string> grpList = new List<string>();
grpList.Add(ToolId);
grpList.Add(Description);
if So , I have to group the list test with ToolId and Description,
case 2 : if list grpList contain N values , I have to group the list test with 'N' values .
The number of values in the grpList varies. I have group the main list test using the values in grpList. If grpList contain 2 values ,group the test list with 2 values . if If grpList contain 5 values , group the test list with 5 values.
NB : I need to Group the list test ( Main list).
grpList values only for grouping .

try reflection:
List<string> grpList = new List<string>();
grpList.Add("Toolid");
grpList.Add("Description");
var groups = new Dictionary<string, IEnumerable>();
var all_properties = typeof(Tracker).GetProperties();
foreach ( var prop_name in grpList )
{
var prop = all_properties.First( x => x.Name == prop_name);
var group = test.GroupBy( x => prop.GetValue( x ) );
groups.Add( prop_name, group );
}
if you want to have an sql-like nested grouping, apply the GroupBy to the resulting groups:
var groups = new List<List<Tracker>>() { test };
foreach ( var prop_name in grpList )
{
var prop = all_properties.First( x => x.Name == prop_name);
var newgroups = new List<List<Tracker>>();
foreach ( var group in groups)
{
var subgroups = group.GroupBy( x => prop.GetValue( x ) );
newgroups.AddRange( subgroups.Select(g => g.ToList()).ToList() );
}
groups = newgroups;
}

I used Enumerable.GroupBy Method selector predicate.
Here's how I generated the predicate and the solution seems to work.
public class Tracker
{
public string Toolid { get; set; }
public string Description { get; set; }
public int length { get; set; }
public int breadth { get; set; }
public int height { get; set; }
}
class Program
{
static void Main(string[] args)
{
List<Tracker> test = new List<Tracker>();
test.Add(new Tracker { Toolid = "A.1", Description = "ABC", length = 50, breadth = 10, height = 50 });
test.Add(new Tracker { Toolid = "A.1", Description = "ABC", length = 20, breadth = 10, height = 50 });
test.Add(new Tracker { Toolid = "C.1", Description = "LMN", length = 10, breadth = 10, height = 50 });
test.Add(new Tracker { Toolid = "D.1", Description = "Admin123", length = 7, breadth = 10, height = 50 });
List<string> grpList = new List<string>();
grpList.Add("length");
grpList.Add("Description");
var sourceParm = Expression.Parameter(typeof(Tracker), "x");
List<Expression> propertyExpressions = new List<Expression>();
foreach (var f in grpList.ToArray())
{
Expression conv = Expression.Convert(Expression.Property(sourceParm, f), typeof(object));
propertyExpressions.Add(conv);
}
var concatMethod = typeof(string).GetMethod(
"Concat",
new[] { typeof(object), typeof(object), typeof(object) });
Expression body = propertyExpressions.Aggregate((x, y) => Expression.Call(concatMethod,
x,
Expression.Constant(","),
y));
var groupSelector = Expression.Lambda<Func<Tracker, string>>(body, sourceParm);
var j = test.GroupBy(groupSelector.Compile());
}

c#: Move element whose ID is in array to top of list

In C#,I have List of Employee object. Employee class is
public class Employee
{
public int ID { get; set; }
public string Name { get; set; }
}
In List objected are sorted based on Employee.ID. I have an array of int which is basically Employee.ID which I want on top of the list and in list,order must remain same as in array.
If I hava input like this
List:
[
{ID:1,Name:A},
{ID:2,Name:B},
{ID:3,Name:AA},
{ID:4,Name:C},
{ID:5,Name:CD},
.
.
{ID:100,Name:Z}
]
and Array: {2,3,1}
Then I want Output List:
[
{ID:2,Name:B},
{ID:3,Name:AA},
{ID:1,Name:A},
{ID:4,Name:C},
{ID:5,Name:CD},
.
.
{ID:100,Name:Z}
]
And I have done this
foreach (int i in a)
{
list = list.OrderBy(x => x.ID != i).ToList();
}
//a is array
//list is List
Any better Solution.Thanks in advance.

After you got your list sorted based on the ID just iterate the array and move the elements. In order to do this you need to first remove and then insert the item at the correct position.
for(int i = 0; i < myArray.Length; i++)
{
var e = myList.Single(x => x.Id == myArray[i]);
myList.Remove(e);
myList.Insert(i, e);
}
You may also want to use SingleOrDefault instead of Single to verify that myList even contains the element with the current id, e.g. when your array contains [2, 3, 101]

To add another version to the mix. The complete sorting can be done in one go:
list = list.OrderBy(e=> {int i =Array.IndexOf(a, e.ID); return i == -1 ? int.MaxValue : i; }).ToList();
where list is the EmployeeList and a the indices array. (NB, the for loop is not needed, the above should do both sortings).
Inside the OrderBy callback, if the id is not inside a, int.MaxValue is returned to place it after the ones inside the array (a.Length would work as well). OrderBy should maintain the original order of the enumeration (list) for those elements that return the same value.
PS, if you want to sort first by index inside a and the rest on the ids (not necessarily the original order), you can use the following (as long as a.Length + largest ID < int.MaxValue) : list = list.OrderBy(e=> {int i =Array.IndexOf(a, e.ID); return i == -1 ? a.Length + e.ID : i; }).ToList();

Here's a way to do it in pure LINQ, without changing the original sequence.
Broken into steps to see what's going on.
public static void Main()
{
var employeeList = new List<Employee>()
{
new Employee(){ ID= 1,Name= "A"},
new Employee() { ID= 2,Name= "B"},
new Employee() { ID= 3,Name= "AA"},
new Employee() { ID= 4,Name= "C"},
new Employee() { ID= 5,Name= "CD"},
new Employee() { ID= 100,Name= "Z"}
};
var orderByArray = new int[] { 2, 3, 1, 100, 5, 4 };
var sortPos = orderByArray.Select((i, index) => new { ID = i, SortPos = index });
var joinedList = employeeList.Join(sortPos, e => e.ID, sp => sp.ID, (e, sp) => new { ID = e.ID, Name = e.Name, SortPos = sp.SortPos });
var sortedEmployees = joinedList.OrderBy(e => e.SortPos).Select(e => new Employee { ID = e.ID, Name = e.Name });
}

Try this using LINQ:
List<Employee> employees = ...
int[] ids = ...
var orderEmployees = ids.Select(id => employees.Single(employee => employee.ID == id))
.Concat(employees.Where(employee => !ids.Contains(employee.ID)).ToList();
Foreach id in ids array we will grab the matching employee and we will concat to it all the employees that their id does not exist in ids array.

I like to use a special Comparer for that, it seems clearer to me, though a bit more code. It hides the complexity of the sort in the comparer class, and then you can just call it with :
theList.OrderBy(x => x.id, new ListOrderBasedComparer(sortList));
It will sort according to any list passed to the comparer when instantiating, and will put elements not in the "known sort list" at the end.
You can of course adapt it to your special needs.
public class ListOrderBasedComparer: Comparer<int>
{
private List<int> sortList;
public ListOrderBasedComparer(List<int> sortList)
{
// if you want you can make constructor accept arrays and convert it
// (if you find that more convenient)
this.sortList = sortList;
}
public override int Compare(int x, int y)
{
var indexOfX = sortList.FindIndex(a => a == x);
var indexOfY = sortList.FindIndex(a => a == y);
// handle elements not in sortArray : if not in sort array always assume they should be "less than the others" and "equal between them".
if (indexOfX == -1 && indexOfY == -1) return 0;
if (indexOfY == -1) return -1;
if (indexOfX == -1) return 1;
// if elements are in sortArray (FindIndex returned other than -1), use usual comparison of index values
return indexOfX.CompareTo(indexOfY);
}
}
Example on how to use it, with Linq :
public class TestCompare
{
public void test ()
{
var myArray = new MyClass[]
{
new MyClass { id = 1, name = "A" },
new MyClass { id = 2, name = "B" },
new MyClass { id = 3, name = "C" },
new MyClass { id = 4, name = "D" },
new MyClass { id = 5, name = "E" },
new MyClass { id = 6, name = "F" },
};
var myArray2 = new MyClass[]
{
new MyClass { id = 1, name = "A" },
new MyClass { id = 2, name = "B" },
new MyClass { id = 0, name = "X" },
new MyClass { id = 3, name = "C" },
new MyClass { id = 4, name = "D" },
new MyClass { id = 23, name = "Z"},
new MyClass { id = 5, name = "E" },
new MyClass { id = 6, name = "F" },
};
var sortList = new List<int> { 2, 3, 1, 4, 5, 6 };
// good order
var mySortedArray = myArray.OrderBy(x => x.id, new ListOrderBasedComparer(sortList)).ToList();
// good order with elem id 0 and 23 at the end
var mySortedArray2 = myArray2.OrderBy(x => x.id, new ListOrderBasedComparer(sortList)).ToList();
}
}
public class MyClass
{
public int id;
public string name;
}

Find Average of a node in Multiple JSON strings

I have searched through many topics, find some relevant answers too, but I am still not able to reach to a solution, hence I am posting this question.
Problem Description
EmployeeResponse1 = [{"Ques":"1","Rating":"7"},{"Ques":"2","Rating":"1"},{"Ques":"3","Rating":"6"},{"Ques":"4","Rating":"1"},{"Ques":"5","Rating":"1"},{"Ques":"6","Rating":"1"},{"Ques":"7","Rating":"7"},{"Ques":"8","Rating":"1"},{"Ques":"9","Rating":"1"},{"Ques":"10","Rating":"1"},{"Ques":"11","Rating":"1"},{"Ques":"12","Rating":"1"},{"Ques":"13","Rating":"1"},{"Ques":"14","Rating":"1"},{"Ques":"15","Rating":"1"},{"Ques":"16","Rating":"10"}]
EmployeeResponse2 = [{"Ques":"1","Rating":"5"},{"Ques":"2","Rating":"4"},{"Ques":"3","Rating":"7"},{"Ques":"4","Rating":"8"},{"Ques":"5","Rating":"5"},{"Ques":"6","Rating":"9"},{"Ques":"7","Rating":"10"},{"Ques":"8","Rating":"4"},{"Ques":"9","Rating":"9"},{"Ques":"10","Rating":"6"},{"Ques":"11","Rating":"6"},{"Ques":"12","Rating":"6"},{"Ques":"13","Rating":"7"},{"Ques":"14","Rating":"7"},{"Ques":"15","Rating":"9"},{"Ques":"16","Rating":"8"}]
I have these two JSON strings in c# (there can be more).. Now I want to make a final JSON string which will be like:
EmployeeResponseAvg = [{"Ques":"1","Rating":"6"},{"Ques":"2","Rating":"2.5"},{"Ques":"3","Rating":"6.5"},{"Ques":"4","Rating":"4.5"},{"Ques":"5","Rating":"3"},{"Ques":"6","Rating":"5"},{"Ques":"7","Rating":"8.5"},{"Ques":"8","Rating":"2.5"},....,{"Ques":"16", "Rating": "9"}]
like I want rating of Ques = 1 should be the average of Rating (Ques = 1 of string 1) and Rating (Ques = 1 of string 2)... similarly for other questions
i.e like FINAL =[{ QUES = 1, RATING = (Emp1(Rating.WHERE(QUES = 1), Emp2(Rating.WHERE(QUES = 1),).AVERAGE),....................}]
Work So Far
MODEL -> SurveyResponse.cs
public class SurveyResponse
{
public string Ques { get; set; }
public string Rating { get; set; }
}
public class ResponseDataCalls
{
public static SurveyResponse PutData(string t, string v)
{
SurveyResponse s = new SurveyResponse();
s.Ques = t;
s.Rating = v;
return s;
}
}
WebAPI RevGroupChartController.cs
public class RevGroupChartController : ApiController
{
private hr_toolEntities _db = new hr_toolEntities();
public object Get(int cid, int gid)
{
spiderChart obj = new spiderChart();
var group_employees = (from ge in _db.hrt_group_employee
where ge.fk_group_id == gid
select ge.fk_employee_id).ToList();
List<string> EMP = new List<string>();
List<string> SUP = new List<string>();
List<SurveyResponse> EmpResponse = new List<SurveyResponse>();
List<SurveyResponse> SupResponse = new List<SurveyResponse>();
List<List<SurveyResponse>> tmpEMP = new List<List<SurveyResponse>>();
List<List<SurveyResponse>> tmpSUP = new List<List<SurveyResponse>>();
foreach(var emp in group_employees)
{
int eid = Convert.ToInt32(emp);
var Data = (from d in _db.hrt_cycle_response
join g in _db.hrt_cycle_groups on d.hrt_cycle.pk_cycle_id equals g.fk_cycle_id
where d.fk_cycle_id == cid && g.fk_group_id == gid && d.fk_employee_id == eid
select new
{
d.response_employee_answers,
d.response_supervisor_answers
}).First();
EMP.Add(Data.response_employee_answers);
SUP.Add(Data.response_supervisor_answers);
}
foreach(var e in EMP)
{
//tmpEMP = new JavaScriptSerializer().Deserialize<TEMP>(e);
var s = new JavaScriptSerializer();
List<SurveyResponse> em = s.Deserialize<List<SurveyResponse>>(e);
tmpEMP.Add(em);
}
foreach (var s in SUP)
{
//tmpSUP = new JavaScriptSerializer().Deserialize<TEMP>(s);
var e = new JavaScriptSerializer();
List<SurveyResponse> sp = e.Deserialize<List<SurveyResponse>>(s);
tmpSUP.Add(sp);
}
var empl = _db.hrt_questions.Select(x => new { x.question_name }).ToList();
List<int[]> Emprating = new List<int[]>();
//int avgRating;
int cnt = 0;
foreach(var item in tmpSUP)
{
int noofQ = item.Count;
int[] i = new int[noofQ];
for (int y = 0; y > tmpSUP.Count; y++)
{
i[y] = Convert.ToInt32(item[cnt].Rating);
}
Emprating.Add(i);
cnt++;
}
//obj.Employee = Data.response_employee_answers;
//obj.Supervisor = Data.response_supervisor_answers;
obj.ques = new List<object>();
for (int i = 0; i < empl.Count; i++)
{
obj.ques.Add(empl[i].question_name);
}
return obj;
}
public class TEMP
{
public List<SurveyResponse> data { get; set; }
}
}
Explanation of Code
I pass a cycle ID and a group ID...
Each group has more than 1 employee and each employee has a supervisor
so if say group ID 1023 has 2 employees.
Now we have 2 employees and 2 supervisors
we have a json record for each of them
LIKE DB TABLE RESPONSE {fk_emp_id, fk_sup_id, cycle_id, emp_reponse(json), supervisor_response(json)}
so I need to make ONE JSON string for employees (which contains the average of all ratings)
and ONE JSON string for SUPERVISOR (again, average of both the JSONs)
there could be any number of employees, depending on the group size
and each employee will always have a supervisor
In short I want a a string like:
FinalEmployeeResponse = [{'Ques': '1', 'Rating': 'R1'}, {'Ques': '2', 'Rating': 'R2'}, {'Ques': '3', 'Rating': 'R3'}, {'Ques': '4', 'Rating': 'R4'}, ........, {'Ques': '16', 'Rating': 'R16'}]
Here, R1 = AVERAGE(Emp1json.Rating.WHERE('Ques' = 1), Emp2json.Rating.WHERE('Ques' = 1), .....)
and
R2 = AVERAGE(Emp1json.Rating.WHERE('Ques' = 2), Emp2json.Rating.WHERE('Ques' = 2), .....)
... and so on....
Looking forward to your responses.
I am new on stack overflow, please ask for more details if I have missed something.

The correct way to do this is to parse this as JSON. The quick and dirty way is:
static void Main(string[] args)
{
string json1 = #"[{""Ques"":""1"",""Rating"":""7""},{""Ques"":""2"",""Rating"":""1""},{""Ques"":""3"",""Rating"":""6""},{""Ques"":""4"",""Rating"":""1""},{""Ques"":""5"",""Rating"":""1""},{""Ques"":""6"",""Rating"":""1""},{""Ques"":""7"",""Rating"":""7""},{""Ques"":""8"",""Rating"":""1""},{""Ques"":""9"",""Rating"":""1""},{""Ques"":""10"",""Rating"":""1""},{""Ques"":""11"",""Rating"":""1""},{""Ques"":""12"",""Rating"":""1""},{""Ques"":""13"",""Rating"":""1""},{""Ques"":""14"",""Rating"":""1""},{""Ques"":""15"",""Rating"":""1""},{""Ques"":""16"",""Rating"":""10""}]";
string json2 = #"[{""Ques"":""1"",""Rating"":""5""},{""Ques"":""2"",""Rating"":""4""},{""Ques"":""3"",""Rating"":""7""},{""Ques"":""4"",""Rating"":""8""},{""Ques"":""5"",""Rating"":""5""},{""Ques"":""6"",""Rating"":""9""},{""Ques"":""7"",""Rating"":""10""},{""Ques"":""8"",""Rating"":""4""},{""Ques"":""9"",""Rating"":""9""},{""Ques"":""10"",""Rating"":""6""},{""Ques"":""11"",""Rating"":""6""},{""Ques"":""12"",""Rating"":""6""},{""Ques"":""13"",""Rating"":""7""},{""Ques"":""14"",""Rating"":""7""},{""Ques"":""15"",""Rating"":""9""},{""Ques"":""16"",""Rating"":""8""}]";
string averages = AverageNodes(json1, json2);
Console.WriteLine(averages);
Console.ReadKey();
}
private static string AverageNodes(params string[] json)
{
var regex = new Regex(#"(""Ques"":""(?<question>\d+)"",""Rating"":""(?<rating>\d+)"")", RegexOptions.ExplicitCapture | RegexOptions.IgnoreCase);
var ANUs = regex.Matches(string.Join("", json))
.Cast<Match>()
.Select(m => new { Question = m.Groups["question"].Value, Rating = int.Parse(m.Groups["rating"].Value) })
.GroupBy(a => a.Question, a => a.Rating)
.Select(a => string.Format("{{\"Ques\":\"{0}\",\"Rating\":\"{1}\"}}", a.Key, a.Average()));
return "[" + string.Join(",", ANUs) + "]";
}

I found a 1 line answer to this using LINQ.
double _avg1 = tmpEMP.Select(x => Convert.ToInt32(x.ElementAt(i).Rating)).Average();

Merging collections using LINQ while overriding rows with same ID

I got two collections of objects.
For example:
List<Foo> firstFoos = new List<Foo>();
List<Foo> secondFoos = new List<Foo>();
firstFoos.Add(new Foo() { Id = 1, ValueA = 10, ValueB = 15 });
firstFoos.Add(new Foo() { Id = 2, ValueA = 20, ValueB = 25 });
firstFoos.Add(new Foo() { Id = 3, ValueA = 30, ValueB = 35 });
firstFoos.Add(new Foo() { Id = 4, ValueA = 40, ValueB = 45 });
secondFoos.Add(new Foo() { Id = 1, ValueA = 100, ValueB = 150 });
secondFoos.Add(new Foo() { Id = 2, ValueA = 200, ValueB = 250 });
Using LINQ, how can I merge the two collection overriding firstFoos by secondFoos which have the same ID?
Expected result is:
| Id | ValueA | ValueB |
|---------|--------|--------|
| 1 | 100 | 150 |
| 2 | 200 | 250 |
| 3 | 30 | 35 |
| 4 | 40 | 45 |
Please note that this example case has only two value columns (ValueA and ValueB), but an actual case could have many more.

I'd convert it to an Id -> Foo dictionary, and then just update with a regular foreach:
var fooDict = firstFoos.ToDictionary(foo => foo.Id, foo => foo);
foreach (var foo in secondFoos)
fooDict[foo.Id] = foo;
var newFoos = fooDict.Values.OrderBy(foo => foo.Id).ToList();

You can define a custom equality comparer and use Union():
public class FooComparer : IEqualityComparer<Foo>
{
public bool Equals(Foo x, Foo y)
{
return x.Id == y.Id;
}
public int GetHashCode(Foo obj)
{
return obj.Id.GetHashCode();
}
}
And then:
var mergedList = secondFoos.Union(firstFoos, new FooComparer())
.ToList();
This uses the fact that items in secondFoos are added to the resulting enumeration before any item in firstFoo, any item in firstFoo with an already existing Id will hence be filtered out. This assumes of course that Id should be distinct across all items.

This should work for you
var concat = firstFoos.Select(x => new { Foo = x, list=1 })
.Concat(secondFoos.Select(x => new { Foo = x, list= 2 });
var merge = from x in concat
group x by x.Foo.Id into x
select x.Count() == 1 ? x.First().Foo : x.First(y => y.list == 2).Foo;

var result = secondFoos.Concat(
firstFoos.Except(secondFoos,
new LambdaComparer<Foo>((a, b) => a.Id == b.Id)))
.ToList();
Another option, because you can never have too many solutions to the same problem ;)

Another option
var f1NotInF2 = from f1 in firstFoos
where !secondFoos.Exists(f2 => f1.Id == f2.Id)
select f1;
var mixed = f1NotInF2.Concat(secondFoos);

I would use something like this:
List<Foo> newFoos = new List<Foo>();
Foo selected = null;
foreach (Foo foo in firstFoos)
{
selected = secondFoos.FirstOrDefault(x => x.Id == foo.Id);
if (selected != null)
{
newFoos.Add(selected);
}
else
{
newFoos.Add(foo);
}
}

This will work:
var merged = firstFoos.Where(f => !secondFoos.Any(s => s.Id == f.Id))
.Union(secondFoos).OrderBy(c=>c.Id);