.:: [Dot].Net bloG ::. Nguyen Thi Bich Thuy

Conversion Operators in to LINQ

1. ToSequence
The ToSequence operator returns its argument typed as IEnumerable.

public static IEnumerable</t><t> ToSequence</t><t>(
this IEnumerable</t><t> source);

The ToSequence operator simply returns the source argument. The operator has no effect other than to change the compile-time type of the source argument to IEnumerable.
The ToSequence operator can be used to choose between query operator implementations in cases where a collection implements IEnumerable but also has a different set of public query operators. For example, given a class Table that implements IEnumerable as well as its own Where, Select, SelectMany, and so on, the query

Table<customer> custTable = GetCustomersTable();
var query = custTable.Where(c => IsGoodCustomer(c));

will invoke the public Where operator of Table. A Table type that represents a database table would likely have a Where operator that takes the predicate argument as an expression tree and converts the tree into SQL for remote execution. If remote execution is not desired, for example because the predicate invokes a local method, the ToSequence operator can be used to hide Table’s operators and instead make the Standard Query Operators available:

Table<customer> custTable = GetCustomersTable();
var query = custTable.ToSequence().Where(c => IsGoodCustomer(c));

This would now cause the query to execute locally.

2. ToArray
The ToArray operator creates an array from a sequence.

public static T[] ToArray<t>(
this IEnumerable</t><t> source);

The ToArray operator enumerates the source sequence and returns an array containing the elements of the sequence. An ArgumentNullException is thrown if the source argument is null.
The following example produces an array of the names of all countries in which there are customers:

string[] customerCountries =
customers.Select(c => c.Country).Distinct().ToArray();

3. ToList
The ToList operator creates a List from a sequence.

public static List</t><t> ToList</t><t>(
this IEnumerable</t><t> source);

The ToList operator enumerates the source sequence and returns a List containing the elements of the sequence. An ArgumentNullException is thrown if the source argument is null.
The following example produces a List containing those customers that placed orders in 2005:

List</customer><customer> customersWithOrdersIn2005 =
customers.
Where(c => c.Orders.Any(o => o.OrderDate.Year == 2005)).
ToList();

4. ToDictionary
The ToDictionary operator creates a Dictionary from a sequence.

public static Dictionary</k><k , T> ToDictionary<t , K>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector);
public static Dictionary<k , T> ToDictionary<t , K>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector,
IEqualityComparer<k> comparer);
public static Dictionary</k><k , E> ToDictionary<t , K, E>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector,
Func</t><t , E> elementSelector);
public static Dictionary<k , E> ToDictionary<t , K, E>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector,
Func</t><t , E> elementSelector,
IEqualityComparer<k> comparer);

The ToDictionary operator enumerates the source sequence and evaluates the keySelector and elementSelector functions for each element to produce that element’s key and value. The resulting key and value pairs are returned in a Dictionary. If no elementSelector was specified, the value for each element is simply the element itself. An ArgumentNullException is thrown if the source, keySelector, or elementSelector argument is null or if a key value produced by keySelector is null. An ArgumentException is thrown if keySelector produces a duplicate key value for two elements. In the resulting dictionary, key values are compared using the given comparer, or, if a null comparer was specified, using the default equality comparer, EqualityComparer.Default.
The following example creates a Dictionary that maps from order ID to order for all orders in 2005:

Dictionary</int><int ,Order> orders =
customers.
SelectMany(c => c.Orders).
Where(o => o.OrderDate.Year == 2005).
ToDictionary(o => o.OrderID);

The following example creates a Dictionary that maps from category name to the maximum product price in that category:

Dictionary</string><string ,decimal> categoryMaxPrice =
products.
GroupBy(p => p.Category).
ToDictionary(g => g.Key, g => g.Group.Max(p => p.UnitPrice));

5. ToLookup
The ToLookup operator creates a Lookup from a sequence.

public static Lookup</k><k , T> ToLookup<t , K>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector);
public static Lookup<k , T> ToLookup<t , K>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector,
IEqualityComparer<k> comparer);
public static Lookup</k><k , E> ToLookup<t , K, E>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector,
Func</t><t , E> elementSelector);
public static Lookup<k , E> ToLookup<t , K, E>(
this IEnumerable</t><t> source,
Func</t><t , K> keySelector,
Func</t><t , E> elementSelector,
IEqualityComparer<k> comparer);
public class Lookup</k><k , T> : IEnumerable<igrouping <K, T>>
{
public int Count { get; }
public IEnumerable<t> this[K key] { get; }
public bool Contains(K key);
public IEnumerator<igrouping <K, T>> GetEnumerator();
}

Lookup implements a one-to-many dictionary that maps keys to sequences of values. This contrasts with Dictionary which implements a one-to-one dictionary that maps keys to single values. The functionality provided by Lookup is used in the implementations of the Join, GroupJoin, and GroupBy operators.
The ToLookup operator enumerates the source sequence and evaluates the keySelector and elementSelector functions for each element to produce that element’s key and value. The resulting key and value pairs are returned in a Lookup. If no elementSelector was specified, the value for each element is simply the element itself. An ArgumentNullException is thrown if the source, keySelector, or elementSelector argument is null. When creating the Lookup, key values are compared using the given comparer, or, if a null comparer was specified, using the default equality comparer, EqualityComparer.Default.
The following example creates a Lookup that maps from category name to the sequence of products in that category:

Lookup</string><string ,Product> productsByCategory =
products.ToLookup(p => p.Category);
IEnumerable<product> beverages = productsByCategory["Beverage"];

6. OfType
The OfType operator filters the elements of a sequence based on a type.

public static IEnumerable<t> OfType</t><t>(
this IEnumerable source);

The OfType operator allocates and returns an enumerable object that captures the source argument. An ArgumentNullException is thrown if the source argument is null.
When the object returned by OfType is enumerated, it enumerates the source sequence and yields those elements that are of type T. Specifically, each element e for which e is T evaluates to true is yielded by evaluating (T)e.
Given a class Employee that inherits from a class Person, the following example returns all employees from a list of persons:

List<person> persons = GetListOfPersons();
IEnumerable<employee> employees = persons.OfType</employee><employee>();

7. Cast
The Cast operator casts the elements of a sequence to a given type.

public static IEnumerable<t> Cast</t><t>(
this IEnumerable source);

The Cast operator allocates and returns an enumerable object that captures the source argument. An ArgumentNullException is thrown if the source argument is null.
When the object returned by Cast is enumerated, it enumerates the source sequence and yields each element cast to type T. An InvalidCastException is thrown if an element in the sequence cannot be cast to type T.
The Cast operator can be used to bridge between non-generic collections and the Standard Query Operators. For example, the non-generic ArrayList doesn’t implement IEnumerable, but the Cast operator can be used to supply the missing type information:

ArrayList objects = GetOrders();
IEnumerable<order> ordersIn2005 =
objects.
Cast</order><order>().
Where(o => o.OrderDate.Year == 2005);

In a C# 3.0 query expression, an explicitly typed iteration variable translates to an invocation of Cast. The example above is equivalent to the translation of

ArrayList objects = GetOrders();
IEnumerable</order><order> ordersIn2005 =
from Order o in objects
where o.OrderDate.Year == 2005
select o;