Giáng sinh an lành - Năm mới đầm ấm và hạnh phúc

1. Any
The Any operator checks whether any element of a sequence satisfies a condition.

public static bool Any<T>(
this IEnumerable<T> source);
public static bool Any<T>(
this IEnumerable<T> source,
Func<T, bool> predicate);

The Any operator enumerates the source sequence and returns true if any element satisfies the test given by the predicate. If no predicate function is specified the Any operator simply returns true if the source sequence contains any elements.
The enumeration of the source sequence is terminated as soon as the result is known.
An ArgumentNullException is thrown if any argument is null.
The following example checks whether any products with a price of 100 or more are out of stock.

bool b = products.Any(p => p.UnitPrice >= 100 && p.UnitsInStock == 0);

2. All
The All operator checks whether all elements of a sequence satisfy a condition.

public static bool All<T>(
this IEnumerable<T> source,
Func<T, bool> predicate);

The All operator enumerates the source sequence and returns true if no element fails the test given by the predicate.
The enumeration of the source sequence is terminated as soon as the result is known.
An ArgumentNullException is thrown if any argument is null.
The All operator returns true for an empty sequence. This is consistent with established predicate logic and other query languages such as SQL.
The following example produces the names of the product categories for which all products are in stock:

IEnumerable<string> fullyStockedCategories =
products.
GroupBy(p => p.Category).
Where(g => g.Group.All(p => p.UnitsInStock > 0)).
Select(g => g.Key);

3. Contains
The Contains operator checks whether a sequence contains a given element.

public static bool Contains<T>(
this IEnumerable<T> source,
T value);

The Contains operator first checks whether the source sequence implements ICollection<T>. If so, the Contains method in sequence’s implementation of ICollection<T> is invoked to obtain the result. Otherwise, the source sequence is enumerated to determine if it contains an element with the given value. If a matching element is found, the enumeration of the source sequence is terminated at that point. The elements and the given value are compared using the default equality comparer, EqualityComparer<K>.Default.
An ArgumentNullException is thrown if the source argument is null.

1. First
The First operator returns the first element of a sequence.

public static T First<T>(
this IEnumerable<T> source);
public static T First<T>(
this IEnumerable<T> source,
Func<T, bool> predicate);

The First operator enumerates the source sequence and returns the first element for which the predicate function returns true. If no predicate function is specified, the First operator simply returns the first element of the sequence.
An ArgumentNullException is thrown if any argument is null. An InvalidOperationException is thrown if no element matches the predicate or if the source sequence is empty.
The following example returns the first customer with a given phone number:

string phone = "206-555-1212";
Customer c = customers.First(c => c.Phone == phone);

In the example above, an exception is thrown if no customer with the given phone number exists. To instead return null when no element is found, use the FirstOrDefault operator.

2. FirstOrDefault
The FirstOrDefault operator returns the first element of a sequence, or a default value if no element is found.

public static T FirstOrDefault<T>(
this IEnumerable<T> source);
public static T FirstOrDefault<T>(
this IEnumerable<T> source,
Func<T, bool> predicate);

The FirstOrDefault operator enumerates the source sequence and returns the first element for which the predicate function returns true. If no predicate function is specified, the FirstOrDefault operator simply returns the first element of the sequence.
An ArgumentNullException is thrown if any argument is null. If no element matches the predicate or if the source sequence is empty, default(T) is returned. The default value for reference and nullable types is null.
3. Last
The Last operator returns the last element of a sequence.

public static T Last<T>(
this IEnumerable<T> source);
public static T Last<T>(
this IEnumerable<T> source,
Func<T, bool> predicate);

The Last operator enumerates the source sequence and returns the last element for which the predicate function returned true. If no predicate function is specified, the Last operator simply returns the last element of the sequence.
An ArgumentNullException is thrown if any argument is null. An InvalidOperationException is thrown if no element matches the predicate or if the source sequence is empty.

4. LastOrDefault
The LastOrDefault operator returns the last element of a sequence, or a default value if no element is found.

public static T LastOrDefault<T>(
this IEnumerable<T> source);
public static T LastOrDefault<T>(
this IEnumerable<T> source,
Func<T, bool> predicate);

The LastOrDefault operator enumerates the source sequence and returns the last element for which the predicate function returned true. If no predicate function is specified, the LastOrDefault operator simply returns the last element of the sequence.
An ArgumentNullException is thrown if any argument is null. If no element matches the predicate or if the source sequence is empty, default(T) is returned. The default value for reference and nullable types is null.

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1. ToSequence
The ToSequence operator returns its argument typed as IEnumerable<T>.

public static IEnumerable<T> ToSequence<T>(
this IEnumerable<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<T>.
The ToSequence operator can be used to choose between query operator implementations in cases where a collection implements IEnumerable<T> but also has a different set of public query operators. For example, given a class Table<T> that implements IEnumerable<T> 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<T>. A Table<T> 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<T>’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> 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<T> from a sequence.

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

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

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

4. ToDictionary
The ToDictionary operator creates a Dictionary<K,E> from a sequence.

public static Dictionary<K, T> ToDictionary<T, K>(
this IEnumerable<T> source,
Func<T, K> keySelector);
public static Dictionary<K, T> ToDictionary<T, K>(
this IEnumerable<T> source,
Func<T, K> keySelector,
IEqualityComparer<K> comparer);
public static Dictionary<K, E> ToDictionary<T, K, E>(
this IEnumerable<T> source,
Func<T, K> keySelector,
Func<T, E> elementSelector);
public static Dictionary<K, E> ToDictionary<T, K, E>(
this IEnumerable<T> source,
Func<T, K> keySelector,
Func<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<K,E>. 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<K>.Default.
The following example creates a Dictionary<int,Order> that maps from order ID to order for all orders in 2005:

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

The following example creates a Dictionary<string,decimal> that maps from category name to the maximum product price in that category:

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

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