I'm using C# on Framework 3.5. I'm looking to quickly group a Generic List<> by two properties. For the sake of this example lets say I have a List of an Order type with properties of CustomerId, ProductId, and ProductCount. How would I get the sum of ProductCounts grouped by CustomerId and ProductId using a lambda expression?
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17
I realize this thread is very old but since I just struggled through this syntax I thought I'd post my additional findings - you can return the sum and the IDs (w/o foreach) in one query like so:
var sums = Orders
.GroupBy(x => new { x.CustomerID, x.ProductID })
.Select(group =>new {group.Key, ProductCount = group.Sum(x => x.ProductCount)});
The tricky part for me to get it working is that the sum must be aliased, apparently...

Todd Langdon
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I realize this answer is very old, but if anyone was wondering why the alias is necessary, properties generate implicit field names but methods do not. – Jimmy Nov 20 '12 at 23:26
7
Alternatively, if you want to get the IDs for each sum, you could do this
var customerAndProductGroups =
from order in Orders
orderby order.CustomerID, order.ProductID // orderby not necessary, but neater
group order by new { order.CustomerID, order.ProductID };
foreach (var customerAndProductGroup in customerAndProductGroups)
{
Console.WriteLine("Customer {0} has ordered product {1} for a total count of {2}",
customerAndProductGroup.Key.CustomerID,
customerAndProductGroup.Key.ProductID,
customerAndProductGroup.Sum(item => item.ProductCount));
}

Klas Mellbourn
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var New1 = EmpList.GroupBy(z => z.Age).OrderBy(Employee => Employee.Key);
dataGridView1.DataSource = New1;

AnshuMan SrivAstav
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