After discussing the query methods provided by .NET, this part demonstrates how to define custom query methods:

Besides the built-in query methods (standard query operators) provided by System.Linq.Enumerable, Microsoft also provides additional query methods through the System.Interactive NuGet package (aka [In

Understanding of internals of query methods is very helpful for using them accurately and effectively, and is also helpful for defining custom query methods, which is discussed later in this chapter.

As fore mentioned, when a generator method (method contains yield statement and returns IEnumerable<T>) is compiled to a pure function, which constructs a generator and return it to caller. So at runt

After understanding how to use LINQ to Objects, starting from this part, the implementation of query methods is discussed. Most LINQ to Object query methods are implemented with iteration pattern and

This part discusses the usages of built-in LINQ to Objects query methods and query expressions. As fore mentioned, these query methods (also called [standard query operators](http://msdn.microsoft.com

LINQ to Objects queries sequences of .NET objects in local memory of current .NET application or service. Its data source and the queries are represented by IEnumerable<T>.

Pattern matching is a common feature in functional languages. C# 7.0 introduces basic pattern matching, including constant value as pattern and type as pattern, and C# 7.1 supports generics in pattern

Asynchronous function can improve the responsiveness and scalability of the application and service. C# 5.0 introduces async and await keywords to greatly simplify the async programming model.

Functional programming encourages modeling operations with pure functions.