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hi all please give me the solution for this problem

i have two generic list having Data from two classes using GetAll Functions

i extracted two fields by using LINQ from both Class type Generic List.
LINQ Resultset have two fields in both Lists from them i need to data .

same columns .
monthId ,

first list have main records for a student month wise fee
while second have concessional record for that student month wise Concessional fee


List1 contains

MonthId | Amount
1 | 1000
4 | 500
7 | 1000
10 | 500

And List 2 Contains records like

MonthId | Amount
1 | 100
4 | 50

So i need to get Subtracted results like

MonthId | Amount
1 | 900
4 | 450
7 | 1000
10 | 500

Please give solution how i perform this ..

What should i do to get Result Set .

LINQ can help i tried but did not get right result in another var list
please give me a sloution

Updated 25-Apr-12 6:15am
Sergey Alexandrovich Kryukov 25-Apr-12 10:43am
What is the exact type of your "generic list"? I never heard that a list could have columns... :-)
gauravupadhyay 25-Apr-12 11:49am
@ SAKryukov --
did you understand question ??

The following code can be used to obtain the result set specified in the question

void Main()
    List<fee> list1 = new List<fee> {
    	new Fee(1,1000),new Fee(4,500),
    	new Fee(7,1000),new Fee(10,500)
    List<fee> list2 = new List<fee> {
    	new Fee(1,100),new Fee(4,50)
    var resultSet = list1.Select (fee => 
    	new Fee(fee.MonthId, fee.Amount - 
    	list2.FirstOrDefault (f => f.MonthId==fee.MonthId).Amount)
public struct Fee{
    public int MonthId;
    public decimal Amount;
    public Fee(int monthId, decimal amount){
    	MonthId = monthId;
    	Amount = amount;
//resultSet is List<fee>
//MonthId Amount
// 1 	  900 
// 4 	  450 
// 7	 1000 
//10	  500

The FirstOrDefault extension method of IEnumerable interface returns the First found value satisfying the predicate or the default value of the item. In this case the Fee struct is Value type, hence we get Amount = 0 for the default item, when the MonthId is not present in list2.

If the Fee is a class, then when the MonthId is not present in list2, null will be returned, as the default value for reference type is null. In such case we have check for null, before deducting the amount from list2 from the amount of list1.

In addition to VJ Reddy's solution, I think an extension method along the following lines might be an option for you as well. The extension method is called using a LINQ query for each of the object references that has an update pending inside a foreach() loop:

(Note: The following code fragments were written in LINQPad

class MonthlyAmount
    public int MonthID {get; set;}
    public decimal Amount {get; set;}

    public MonthlyAmount(int Month, decimal Amount)
        this.MonthID = Month;
        this.Amount = Amount;

static class Extension
    public static void Update<t>(this T Item, Action<t> UpdateAction)

void Main()
    List<monthlyamount> monthlyAmountsList = new List<monthlyamount>();

    if (monthlyAmountsList == null)

    monthlyAmountsList.Add(new MonthlyAmount (1, 1000));
    monthlyAmountsList.Add(new MonthlyAmount (4, 500));
    monthlyAmountsList.Add(new MonthlyAmount (7, 1000));
    monthlyAmountsList.Add(new MonthlyAmount (10, 500));


    List<monthlyamount> monthlyUpdatesList = new List<monthlyamount>();

    if (monthlyUpdatesList == null)

    monthlyUpdatesList.Add(new MonthlyAmount (1, 100));
    monthlyUpdatesList.Add(new MonthlyAmount (4, 50));


    foreach(MonthlyAmount monthlyUpdate in monthlyUpdatesList)
        (from entry in monthlyAmountsList
         where entry.MonthID == monthlyUpdate.MonthID
         select entry).First().Update(updatedEntry => updatedEntry.Amount -= monthlyUpdate.Amount);



And the Lists<> before the Update Extension is executed contain:

monthlyAmountsList List<MonthlyAmount> (4 items)

MonthID Amount
1 1000
4 500
7 1000
10 500

monthlyUpdatesList List<MonthlyAmount> (2 items)

MonthID Amount
1 100
4 50

The contents of the Updated List<> after the foreach() loop would be as follows:

monthlyAmountsList List<MonthlyAmount> (4 items)

MonthID Amount
1 900
4 450
7 1000
10 500

I hope this was of help and interest to you.

Best Regards...
list1.forech(item => {
for (int i=0; i<=list2.count;i++)
if(item.monthid == list2[i].monthid)
item.amont -= list2[i].monthid;
dear VJ Sir

You are right i am getting null reference problem .

my code is :

for first List

var lisFeeStrdetail = listditinct.GroupBy(x => new { x.MonthId }).Select(group => new { monthId = group.Key.MonthId, Feesum = group.Sum(x => x.Amt) });

and for Second List

    var concessionAmount = from c in lFeeConcessionDetails2
                           orderby c.MonthId ascending
                           select new { MonthId = c.MonthId, Feesum = c.Amount };

and for result Set

if (concessionAmount.Count() != 0) 

 var resultSet = lisFeeStrdetail.Select(a => new { monthId = a.monthId, Feesum = a.Feesum -  concessionAmount.FirstOrDefault(f => f.MonthId == a.monthId).Feesum  } ).ToList();


getting object reference not set to an instance of an Object exception

what are possible mistakes i am doing Please help .

VJ Reddy 26-Apr-12 23:27pm
The reason is, in the statement concessionAmount.FirstOrDefault(f => f.MonthId == a.monthId).Feesum

FirstOrDefault returns default value when the required item is not found. In this case the default is null as the default value for reference type is null. So replace

concessionAmount.FirstOrDefault(f => f.MonthId == a.monthId).Feesum with

concessionAmount.FirstOrDefault(f => f.MonthId == a.monthId).Feesum == null ? 0 : concessionAmount.FirstOrDefault(f => f.MonthId == a.monthId).Feesum
gauravupadhyay 27-Apr-12 1:30am
hi sir
thanks for your valuable reply .

getting exception due to ---

the result of this expression is alway 'false' since a value of type 'decimal' is never equal to 'null' type of 'decimal'

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