C# – LINQ Group By and select collection

c++group-bylinq

I have this structure

Customer
 - has many Orders
  - has many OrderItems

I want to generate a list of CustomerItems via LINQ given a subset of OrderItems:

List of new { Customer, List<OrderItem> Items }

which is a grouping of all the items a Customer has ordered from the subset of items

How can i use LINQ to back track through the order and group by Customer to generate this object?

so far I'm on something like

items
 .GroupBy(i => i, i => i.Order.Customer, (i, customer) => new {customer, i})

But thats obviously not a List. I'm guessing I need a SelectMany in there somewhere, but could do with some pointers.

Best Solution

I think you want:

items.GroupBy(item => item.Order.Customer)
     .Select(group => new { Customer = group.Key, Items = group.ToList() })
     .ToList()

If you want to continue use the overload of GroupBy you are currently using, you can do:

items.GroupBy(item => item.Order.Customer, 
              (key, group) =>  new { Customer = key, Items = group.ToList() })
     .ToList()

...but I personally find that less clear.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – LINQ query on a DataTable

You can't query against the DataTable's Rows collection, since DataRowCollection doesn't implement IEnumerable<T>. You need to use the AsEnumerable() extension for DataTable. Like so:

var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field<int>("RowNo") == 1
select myRow;

And as @Keith says, you'll need to add a reference to System.Data.DataSetExtensions

AsEnumerable() returns IEnumerable<DataRow>. If you need to convert IEnumerable<DataRow> to a DataTable, use the CopyToDataTable() extension.

Below is query with Lambda Expression,

var result = myDataTable
    .AsEnumerable()
    .Where(myRow => myRow.Field<int>("RowNo") == 1);

Best Solution

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – LINQ query on a DataTable

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