§7.15.3 The query expression pattern, C# Language Specification, Version 3.0
Copyright © Microsoft Corporation 1999-2007. All Rights Reserved.
The Query expression pattern establishes a pattern of methods that types can implement to support query expressions. Because query expressions are translated to method invocations by means of a syntactic mapping, types have considerable flexibility in how they implement the query expression pattern. For example, the methods of the pattern can be implemented as instance methods or as extension methods because the two have the same invocation syntax, and the methods can request delegates or expression trees because anonymous functions are convertible to both.
The recommended shape of a generic type C<T> that supports the query
expression pattern is shown below. A generic type is used in order to
illustrate the proper relationships between parameter and result types, but it
is possible to implement the pattern for non-generic types as well.
delegate R Func<T1,R>(T1 arg1);
delegate R Func<T1,T2,R>(T1 arg1, T2 arg2);
class C
{
public C<T> Cast<T>();
}
class C<T> : C
{
public C<T> Where(Func<T,bool> predicate);
public C<U> Select<U>(Func<T,U> selector);
public C<V> SelectMany<U,V>(Func<T,C<U>> selector,
Func<T,U,V> resultSelector);
public C<V> Join<U,K,V>(C<U> inner, Func<T,K> outerKeySelector,
Func<U,K> innerKeySelector, Func<T,U,V> resultSelector);
public C<V> GroupJoin<U,K,V>(C<U> inner, Func<T,K> outerKeySelector,
Func<U,K> innerKeySelector, Func<T,C<U>,V> resultSelector);
public O<T> OrderBy<K>(Func<T,K> keySelector);
public O<T> OrderByDescending<K>(Func<T,K> keySelector);
public C<G<K,T>> GroupBy<K>(Func<T,K> keySelector);
public C<G<K,E>> GroupBy<K,E>(Func<T,K> keySelector,
Func<T,E> elementSelector);
}
class O<T> : C<T>
{
public O<T> ThenBy<K>(Func<T,K> keySelector);
public O<T> ThenByDescending<K>(Func<T,K> keySelector);
}
class G<K,T> : C<T>
{
public K Key { get; }
}The methods above use the generic delegate types Func<T1, R> and
Func<T1, T2, R>, but they could equally well have used other delegate or
expression tree types with the same relationships in parameter and result
types.
Notice the recommended relationship between C<T> and O<T> which ensures
that the ThenBy and ThenByDescending methods are available only on the
result of an OrderBy or OrderByDescending. Also notice the recommended
shape of the result of GroupBy—a sequence of sequences, where each
inner sequence has an additional Key property.