C# Language Tutorial => Chaining methods

Example

Many LINQ functions both operate on an IEnumerable<TSource> and also return an IEnumerable<TResult>. The type parameters TSource and TResult may or may not refer to the same type, depending on the method in question and any functions passed to it.

A few examples of this are

public static IEnumerable<TResult> Select<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TResult> selector
)

public static IEnumerable<TSource> Where<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, int, bool> predicate
)

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

While some method chaining may require an entire set to be worked prior to moving on, LINQ takes advantage of deferred execution by using yield return ^MSDN which creates an Enumerable and an Enumerator behind the scenes. The process of chaining in LINQ is essentially building an enumerable (iterator) for the original set -- which is deferred -- until materialized by enumerating the enumerable.

This allows these functions to be fluently chained ^wiki, where one function can act directly on the result of another. This style of code can be used to perform many sequence based operations in a single statement.

For example, it's possible to combine Select, Where and OrderBy to transform, filter and sort a sequence in a single statement.

var someNumbers = { 4, 3, 2, 1 };

var processed = someNumbers
        .Select(n => n * 2)   // Multiply each number by 2
        .Where(n => n != 6)   // Keep all the results, except for 6
        .OrderBy(n => n);     // Sort in ascending order

Output:

2
4
8

Live Demo on .NET Fiddle

Any functions that both extend and return the generic IEnumerable<T> type can be used as chained clauses in a single statement. This style of fluent programming is powerful, and should be considered when creating your own extension methods.

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