LINQ의 쿼리문법과 람다문법 비교

LINQ의 쿼리문법과 람다문법 비교


 

Filtering
var col = from o in Orders           where o.CustomerID == 84           select o;	var col2 = Orders.Where(o => o.CustomerID == 84);

 

Return Anonymous Type
var col = from o in orders           select new           {               OrderID = o.OrderID,               Cost = o.Cost           };	var col2 = orders.Select(o => new       {           OrderID = o.OrderID,           Cost = o.Cost       }     );

 

Ordering
var col = from o in orders           orderby o.Cost ascending           select o;	var col2 = orders.OrderBy(o => o.Cost);
var col3 = from o in orders            orderby o.Costdescending            select o;	var col4 = orders.OrderByDescending(o => o.Cost);
var col9 = from o in orders   orderby o.CustomerID, o.Costdescending            select o;	var col6 = orders.OrderBy(o => o.CustomerID).           ThenByDescending(o => o.Cost);
//returns same results as above var col5 = from o in orders            orderby o.Costdescending            orderby o.CustomerID            select o; //NOTE the ordering of the orderby’s

 

Joining
var col = from c in customers           join o in orders on           c.CustomerID equalso.CustomerID           select new           {               c.CustomerID,               c.Name,               o.OrderID,               o.Cost           };	var col2 = customers.Join(orders,     c => c.CustomerID,o => o.CustomerID,     (c, o) => new         {             c.CustomerID,             c.Name,             o.OrderID,             o.Cost         }     );
Grouping
var OrderCounts = from o in orders         group o by o.CustomerID intog         select new         {             CustomerID = g.Key,             TotalOrders = g.Count()         };	var OrderCounts1 = orders.GroupBy(          o => o.CustomerID).          Select(g => new          {              CustomerID = g.Key,              TotalOrders = g.Count()          });
NOTE: the grouping’s key is the same type as the grouping value. E.g. in above example grouping key is an int because o.CustomerID is an int.
Paging (using Skip & Take)
//select top 3 var col = (from o in orders            where o.CustomerID == 84            select o).Take(3);	var col2 = orders.Where(            o => o.CustomerID == 84            ).Take(3);
//skip first 2 and return the 2 after   var col3 = (from o in orders            where o.CustomerID == 84            orderby o.Cost            select o).Skip(2).Take(2);	var col3 = (from o in orders             where o.CustomerID == 84             orderby o.Cost             select o).Skip(2).Take(2);

 

Element Operators (Single, Last, First, ElementAt, Defaults)
//throws exception if no elements var cust = (from c in customers            where c.CustomerID == 84            select c).Single();	var cust1 = customers.Single(             c => c.CustomerID == 84);
//returns null if no elements var cust = (from c in customers             where c.CustomerID == 84             selectc).SingleOrDefault();	var cust1 = customers.SingleOrDefault(             c => c.CustomerID == 84);
//returns a new customer instance if no elements var cust = (from c in customers             where c.CustomerID == 85             select c).DefaultIfEmpty(               newCustomer()).Single();	var cust1 = customers.Where(             c => c.CustomerID == 85             ).DefaultIfEmpty(newCustomer()).Single();
//First, Last and ElementAt used in same way var cust4 = (from o in orders              where o.CustomerID == 84              orderby o.Cost              select o).Last();	var cust5 = orders.Where(             o => o.CustomerID == 84).             OrderBy(o => o.Cost).Last();
//returns 0 if no elements var i = (from c in customers          where c.CustomerID == 85          selectc.CustomerID).SingleOrDefault();	var j = customers.Where(         c => c.CustomerID == 85).         Select(o => o.CustomerID).SingleOrDefault();
NOTE: Single, Last, First, ElementAt all throw exceptions if source sequence is empty. SingleOrDefault, LastOrDefault, FirstOrDefault, ElementAtOrDefault all return default(T) if source sequence is empty.  i.e. NULL will be returned if T is a reference type or nullable value type; default(T) will be returned if T is a non-nullable value type (int, bool etc). This can be seen in the last example above.

 

 

Conversions

ToArray

string[] names = (from c in customers                   select c.Name).ToArray();

ToDictionary

Dictionary<int, Customer> col = customers.ToDictionary(c => c.CustomerID); Dictionary<string, double> customerOrdersWithMaxCost = (from oc in   (from o in orders                                    join c incustomers on o.CustomerID equals c.CustomerID        select new { c.Name, o.Cost })   group oc by oc.Name into g       select g).ToDictionary(g => g.Key, g => g.Max(oc => oc.Cost));

ToList

List<Order> ordersOver10 = (from o in orders where o.Cost > 10       orderby o.Cost).ToList();

ToLookup

ILookup<int, string> customerLookup =             customers.ToLookup(c => c.CustomerID, c => c.Name);

 

 

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