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Add two objects in the same list

I have a list of objects. Each object contains an ID and a value. There are several objects which have the same ID but different values. How would I go about adding the values together based on the matching ID? Additionally, how would I go about removing one entry after the addition is complete?
The object in question:
public class MyObject
{
public int ID { get; set; }
public int Value { get; set; }
}
Below is where I am getting the duplicate objects from a list of all objects. I'm simply getting all the duplicate IDs into a list of strings and then grabbing the entire duplicate object in the duplicateObjects list.
List<MyObject> myObjects = GetMyObjectsList();
List<string> duplicateIds = myObjects.GroupBy(x => x.ID)
.Where(group => group.Count() > 1)
.Select(group => group.Key).ToList();
List<MyObject> duplicateObjects = myObjects.Where(x => duplicateIds.Contains(x.ID)).ToList();
I'm stuck on the last steps which are adding the duplicate values and then removing one of the duplicates. How would I accomplish this with Linq?

Assume the list look like this
var list = new MyObject[]
{
new MyObject {ID = 1, Value = 2},
new MyObject {ID = 2, Value = 2},
new MyObject {ID = 1, Value = 3},
new MyObject {ID = 4, Value = 4},
new MyObject {ID = 2, Value = 4},
};
Then just select from list group by ID and sum value like this
var result = (from tm in list
group tm by tm.ID into Test
select new
{
ID = Test.Key,
Value = Test.Sum(x => x.Value)
});
Output
ID = 1, Value = 5
ID = 2, Value = 6
ID = 4, Value = 4
working fiddle here

I hope I understood your question correctly.As clarified in the comment,you want to "sum" all the values grouped by ID and then remove one of the duplicates. Please verify if the following is how you could like it to be behave.
var duplicates = myObjects.GroupBy(x => x.ID)
.Select(group => new { Group = group.Count() > 1 ? group.ToList().Take(group.Count()-1) : group.ToList(),Sum=group.Count()});
For Input
Output
If you need to exclude non-duplicates, then you would need to include an additional Where Condition
var duplicates = myObjects.GroupBy(x => x.ID)
.Where(x=>x.Count()>1)
.Select(group => new { Group = group.Count() > 1 ? group.ToList().Take(group.Count()-1) : group.ToList(),Sum=group.Count()});

Updating property values in one list with a property value average of matching items in another list

I have two Lists and need to update a property value of all the items in the 1st list with a property value average of all the matching items in another list.
class transaction
{
public string orderId;
public string parentOrderId;
public int quantity;
public decimal marketPrice;
public decimal fillPrice;
}
List<transaction> makerTransactions = new List<transaction>()
{
new transaction(){
orderId = "1",
parentOrderId = "1",
quantity = 100,
marketPrice = 75.87M,
fillPrice = 75.87M
}
};
List<transaction> takerTransactions = new List<transaction>()
{
new transaction(){
orderId = "2",
parentOrderId = "1",
quantity = 50,
marketPrice = 75.97M,
fillPrice = 75.97M
},
new transaction(){
orderId = "3",
parentOrderId = "1",
quantity = 50,
marketPrice = 75.85M,
fillPrice = 75.85M
}
};
Trying to make this work with LINQ extension methods but cant figure out the correct way.
makerTransactions.All(mt => mt.fillPrice = takerTransactions
.Where(tt => tt.parentOrderId == mt.orderId)
.Average(ta => ta.fillPrice));

try this:
makerTransactions.ForEach(mt => mt.fillPrice = takerTransactions
.Where(tt => tt.parentOrderId == mt.orderId)
.Average(ta => ta.fillPrice));
All is an extension method. It tells you if all the elements in a collection match a certain condition and, apparently, it's not what you need.
To make it more efficient, first create a dictionary and use that to take the averages from:
var priceDictionary = takerTransactions
.GroupBy(tt => tt.parentOrderId)
.ToDictionary(grp => gr.Key, grp => grp.Average(ta => ta.fillPrice));
makerTransactions.ForEach(mt => mt.fillPrice = priceDictionary[mt.orderId]);

Add duplicates together in List

First question :)
I have a List<Materiau> (where Materiau implements IComparable<Materiau>), and I would like to remove all duplicates and add them together
(if two Materiau is the same (using the comparator), merge it to the first and remove the second from the list)
A Materiau contains an ID and a quantity, when I merge two Materiau using += or +, it keeps the same ID, and the quantity is added
I cannot control the input of the list.
I would like something like this:
List<Materiau> materiaux = getList().mergeDuplicates();
Thank you for your time :)

Check out Linq! Specifically the GroupBy method.
I don't know how familiar you are with sql, but Linq lets you query collections similarly to how sql works.
It's a bit in depth to explain of you are totally unfamiliar, but Code Project has a wonderful example
To sum it up:
Imagine we have this
List<Product> prodList = new List<Product>
{
new Product
{
ID = 1,
Quantity = 1
},
new Product
{
ID = 2,
Quantity = 2
},
new Product
{
ID = 3,
Quantity = 7
},
new Product
{
ID = 4,
Quantity = 3
}
};
and we wanted to group all the duplicate products, and sum their quantities.
We can do this:
var groupedProducts = prodList.GroupBy(item => item.ID)
and then select the values out of the grouping, with the aggregates as needed
var results = groupedProducts.Select( i => new Product
{
ID = i.Key, // this is what we Grouped By above
Quantity = i.Sum(prod => prod.Quantity) // we want to sum up all the quantities in this grouping
});
and boom! we have a list of aggregated products

Lets say you have a class
class Foo
{
public int Id { get; set; }
public int Value { get; set; }
}
and a bunch of them inside a list
var foocollection = new List<Foo> {
new Foo { Id = 1, Value = 1, },
new Foo { Id = 2, Value = 1, },
new Foo { Id = 2, Value = 1, },
};
then you can group them and build the aggregate on each group
var foogrouped = foocollection
.GroupBy( f => f.Id )
.Select( g => new Foo { Id = g.Key, Value = g.Aggregate( 0, ( a, f ) => a + f.Value ) } )
.ToList();

List<Materiau> distinctList = getList().Distinct(EqualityComparer<Materiau>.Default).ToList();

how to filter dictionary values if its key exists in a string[] using linq

string[] strLst = { "One", "Three" };
Dictionary<string, Customer> dicDocTypes = new Dictionary<string, Customer>();
dicDocTypes.Add("One", new Customer { Id = 1, Name = "Rabi" });
dicDocTypes.Add("Two", new Customer { Id = 2, Name = "Shuvankar" });
dicDocTypes.Add("Three", new Customer { Id = 3, Name = "Krishna" });
dicDocTypes.Add("Four", new Customer { Id = 4, Name = "Suresh" });
var rootNodes = from node in dicDocTypes
where node.Key.Contains(**strLst**)
select new KeyValuePair<string, Customer>(node.Key, node.Value);
Question: how to filter dictionary items if the keys matches with a string[]

Instead of trying to ask the key if it belongs to an array, you can ask the array if it contains the key:
var rootNodes = from node in dicDocTypes
where strLst.Contains(node.Key)
select new KeyValuePair<string, Customer>(node.Key, node.Value);
If you have a lot of items, it will be faster to use a HashSet instead of an array (O(1) lookups instead of O(n) lookups):
var strLst = new HashSet<string> { "One", "Three" };
Also, you can simplify the select to this:
var rootNodes = dicDocTypes.Where(n => strLst.Contains(n.Key));
Additionally, there's a cleaner way to initialize dictionaries:
var dicDocTypes = new Dictionary<string, Customer> {
{ "One", new Customer { Id = 1, Name = "Rabi" } },
{ "Two", new Customer { Id = 2, Name = "Shuvankar" } },
{ "Three", new Customer { Id = 3, Name = "Krishna" } },
{ "Four", new Customer { Id = 4, Name = "Suresh" } }
};

As Cameron has said, you're just asking the question the wrong way round. However, you can also make your code simpler - the "node" in a dictionary is already a KeyValuePair, so all you need is:
var rootNodes = dicDocTypes.Where(pair => strLst.Contains(pair.Key));
If you need a new dictionary at the end, you can use the ToDictionary method:
var rootNodes = dicDocTypes.Where(pair => strLst.Contains(pair.Key))
.ToDictionary(pair => pair.Key, pair => pair.Value);
If your list of strings to filter by becomes longer, you might want to consider making it a HashSet<string> instead of an array.

var rootNodes = dicDocTypes.Join(strList, kvp => kvp.Key, s => s, (kvp, str) => kvp);

"in" operator in linq c#?

I have a generic list which contains member details and I have a string array of memberIds..I need to filter the list and get the results which contains all the memberIds..How can I achieve this using LINQ.
I tried the following
string[] memberList = hdnSelectedMemberList.Value.Split(',');
_lstFilteredMembers = lstMainMembers.Where(p =>memberList.Contains(p.MemberId))
.ToList();
But the above query is giving me only the results that match the first member ID..so lets say if I have memberIds 1,2,3,4 in the memberList array..the result it returns after the query contains only the members with member ID 1..even though the actual list has 1,2,3,4,5 in it..
Can you please guide me what I am doing wrong.
Thanks and appreciate your feedback.

Strings make terrible primary keys. Try trimming the list:
string[] memberList = hdnSelectedMemberList.Value
.Split(',')
.Select(p => p.Trim())
.ToList();
_lstFilteredMembers = lstMainMembers.Where(p => memberList.Contains(p.MemberId)).ToList();
Because I have a feeling hdnSelectedMemberList may be "1, 2, 3, 4".

Use a join:
var memquery = from member in lstMainMembers
join memberid in memberList
on member.MemberId equals memberid
select member;

With jmh, I'd use a join
var members = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
var ids = new[] { 1, 3, 6, 14 };
var result = members.Join(ids, m => m, id => id, (m, id) => m);
foreach (var r in result)
Console.WriteLine(r); //prints 1, 3, 6

The code you are showing is correct, and works in a Unit Test:
public class Data
{
public string MemberId { get; set; }
}
[TestMethod]
public void Your_Code_Works()
{
// Arrange fake data.
var hdnSelectedMemberList = "1,2,3,4";
var lstMainMembers = new[]
{
new Data { MemberId = "1" },
new Data { MemberId = "2" },
new Data { MemberId = "3" },
new Data { MemberId = "4" },
new Data { MemberId = "5" }
};
// Act - copy/pasted from StackOverflow
string[] memberList = hdnSelectedMemberList.Split(',');
var _lstFilteredMembers = lstMainMembers.Where(p => memberList.Contains(p.MemberId)).ToList();
// Assert - All pass.
Assert.AreEqual(4, _lstFilteredMembers.Count);
Assert.AreEqual("1", _lstFilteredMembers[0].MemberId);
Assert.AreEqual("2", _lstFilteredMembers[1].MemberId);
Assert.AreEqual("3", _lstFilteredMembers[2].MemberId);
Assert.AreEqual("4", _lstFilteredMembers[3].MemberId);
}
There must be something wrong with your code outside what you have shown.

Try Enumerable.Intersect to get the intersection of two collections:
http://msdn.microsoft.com/en-us/library/system.linq.enumerable.intersect.aspx
_lstFilteredMembers = lstMainMembers.Intersect(memberList.Select(p => p.MemberID.ToString())).ToList()

Why not just project the IDs list into a list of members?
var result = memberList.Select(m => lstMainMembers.SingleOrDefault(mm => mm.MemberId == m))
Of course, that will give you a list that contains null entries for items that don't match.
You could filter those out, if you wanted to...
result = result.Where(r => r != null)
Or you could filter it before the initial select...
memberList.Where(m => lstMainMembers.Any(mm => mm.MemberId == m)).Select(m => lstMainMembers.Single(mm => mm.MemberId == m))
That's pretty ugly, though.