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Using C# LINQ - Review

Congratulations on completing the Using C# LINQ course!

Here is a quick review of what you have learned:

Lambda expressions

Lambda expressions are defined using the lambda operator, =>, and allow you to define an anonymous method that can be stored and passed around in a variable. Here is an example of a lambda expression:

Func<int, int> multiplyByFive = num => num * 5;
// Returns 35
int result = multiplyByFive(7);

Many LINQ methods take a method reference (called a delegate) as a parameter, and lambda expressions are a convenient way of defining these delegates.


LINQ methods are extensions to the IEnumerable<T> interface, which is an abstraction for a sequence of values of data type T. Any LINQ method can be called on anything that implements IEnumerable<T>, including C# arrays and collections.

Any IEnumerable<T> can be converted into an array or a list (which may be more convenient to work with) by calling the ToArray() or ToList() LINQ methods.

MSDN documentation

Every LINQ method is documented on MSDN at the following address:

Query and method syntax

There are two different ways to express a LINQ statement in C#: query syntax and method syntax. Query syntax looks similar to SQL, and may be more human-readable, but not all LINQ methods are supported by query syntax. Method syntax looks more like standard C# code, and supports the full set of LINQ methods. Query syntax is converted into method syntax by the compiler.

LINQ Methods

We discussed 27 different LINQ methods, arranged into five categories:

  • Methods to extract a single element from a sequence: First(), Last(), Single(), FirstOrDefault(), LastOrDefault(), and SingleOrDefault()
  • Methods to extract multiple elements from a sequence: Skip(), Take(), SkipWhile(), TakeWhile(), Distinct(), Intersect(), and Where()
  • Methods to change the order of the elements in a sequence: Reverse(), OrderBy() and ThenBy()
  • Methods to calculate a single value based on a sequence: Count(), Sum(), Min(), Max(), Any(), All(), SequenceEqual(), and Aggregate()
  • Methods to calculate a new sequence based on a source sequence: Cast<U>(), Select(), and SelectMany()
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