LINQ query syntax to method syntax?

The following example shows a simple query expression and the semantically equivalent query written as a method-based query. The output from the two examples is identical. You can see that the type of the query variable is the same in both forms: IEnumerable.

To understand the method-based query, let's examine it more closely. On the right side of the expression, notice that the where clause is now expressed as an instance method on the numbers object, which as you will recall has a type of IEnumerable. If you are familiar with the generic IEnumerable interface, you know that it does not have a Where method.

However, if you invoke the IntelliSense completion list in the Visual Studio IDE, you will see not only a Where method, but many other methods such as Select, SelectMany, Join, and Orderby. These are all the standard query operators. Although it looks as if IEnumerable has been redefined to include these additional methods, in fact this is not the case.

The standard query operators are implemented as a new kind of method called extension methods. Extensions methods "extend" an existing type; they can be called as if they were instance methods on the type. The standard query operators extend IEnumerable and that is why you can write numbers.

To get started using LINQ, all that you really have to know about extension methods is how to bring them into scope in your application by using the correct using directives. This is explained additionally in How to: Create a LINQ Project. From your application's point of view, an extension method and a regular instance method are the same.

For more information about extension methods, see Extension Methods (C# Programming Guide). For more information about standard query operators, see Standard Query Operators Overview. Some LINQ providers, such as LINQ to SQL and LINQ to XML, implement their own standard query operators and additional extension methods for other types besides IEnumerable.