C# 5.0 Programmer's Reference

LINQ Extension Methods ❘ 187
}
};
CustBalance = customer.Balance
This code defines the methods that are passed as parameters to the query’s Where, OrderBy, and
Select calls, but where are Where, OrderBy, and Select defined They are called as if they are
methods provided by customers, but customers is simply an array of Customer objects and it
doesn’t define those methods.
It turns out that Where, OrderBy, and Select are extension methods added to the IEnumerable
interface by the LINQ library. Arrays implement that interface so they gain these extension methods.
Similarly, LINQ adds other extension methods to the IEnumerable interface such as Any, All,
Average, Count, Distinct, First, GroupBy, OfType, Repeat, Sum, Union, and many more.
Method-Based Queries with Lambda Functions
Lambda expressions make building method-based queries somewhat easier. When you use lambda
expressions, you don’t need to define separate methods to pass as parameters to LINQ methods such
as Where, OrderBy, and Select. Instead, you can pass a lambda expression directly into the method.
The following code shows a revised version of the previous method-based query. Here the method
bodies have been included as lambda expressions.
var query =
customers.Where(customer => customer.Balance < 0).
OrderBy(customer => customer.Balance).
Select(customer => new CustInfo()
{
CustName = customer.FirstName + " " + customer.LastName,
CustBalance = customer.Balance
}
);
This is more concise because it doesn’t require you to build separate methods, but it can be a
lot harder to read and understand. Passing a simple lambda expression to the Where or OrderBy
method may not be too confusing, but if you need to perform complex tests, you may be better
off making separate methods.
Whether you use methods or lambda expressions, the standard LINQ query syntax is usually
easier to understand, so you may prefer to use that version whenever possible. Unfortunately,
many references describe the LINQ extension methods as if you are going to use them in methodbased
queries rather than in LINQ queries. For example, the description of the OrderBy method
at msdn.microsoft.com/library/bb534966.aspx includes the following definition:
public static IOrderedEnumerable OrderBy(
this IEnumerable source,
Func keySelector
)
www.it-ebooks.info