26.7.2 The query expression pattern
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 lambda expressions 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<A,R>(A arg);

class C<T>
{
    public C<T> Where(Func<T,bool> predicate);
    public C<S> Select<S>(Func<T,S> selector);
    public C<S> SelectMany<S>(Func<T,C<S>> selector);
    public O<T> OrderBy<K>(Func<T,K> keyExpr);
    public O<T> OrderByDescending<K>(Func<T,K> keyExpr);
    public C<G<K,T>> GroupBy<K>(Func<T,K> keyExpr);
    public C<G<K,E>> GroupBy<K,E>(Func<T,K> keyExpr, Func<T,E> elemExpr);
}
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>
{
    public K Key { get; }
    public C<T> Group { get; }
}

26.7.2 The query expression pattern
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 lambda expressions 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.

iT>>Но, конечно, открывается простор для багов, если ПОЗЖЕ в класс добавили метод, ранее введенный extension'ом, но с чуть-чуть другим поведением.

    static class StringExt
    {
        public static string[] Split(this string s)
        {
            return new string[] {};
        }
    }
    ...
    Console.WriteLine("1 2 3 4 5 6 7 8 9 10".Split().Length); // Выведет 10

_FR>Гхм. Если не сложно, а что за история?

int? i = null;
Console.WriteLine(i == null); // Выводится True, что нормально.
object o = i;
Console.WriteLine(o == null); // Выводится False, что, мягко говоря, не очень логично.

AVK>var dict2 = new Dictionary<string, int>
AVK>{
AVK>    new { Key = "one", Value = 1 },
AVK>    new { Key = "two", Value = 2 },
AVK>    new { Key = "three", Value = 3 }
AVK>};
AVK>

var scoreRecords = new [] { new {Name = "Alice", Score = 50},
                                                        new {Name = "Bob"  , Score = 40},
                                                        new {Name = "Cathy", Score = 45}
                                                };

var scoreRecordsDict = scoreRecords.ToDictionary(sr => sr.Name);

Console.WriteLine("Bob's score: {0}", scoreRecordsDict["Bob"]);