Linq lets us express collection transformations really nicely much of the time, but its names for operations are sometimes not the most obvious, so sometimes it helps me to write out the most common operations and what they mean as a cheat sheet. This is a list in order that I find them most common/useful:
The Basics
Select
Name(s) in other languages: map, transform, apply
What it does:
[a,b,c] -> [A,B,C]
Given an IEnumerable and a function for an item in that set, apply that function to each item to produce a new IEnumerable . Lazy.
Examples
//Converts each string to uppercase
(new[] {"left", "right", "up", "down"}).Select(s => s.ToUpper()); // Produces: ["LEFT", "RIGHT", "UP", "DOWN"]
//Squares a list of numbers
(new[] {1,2,3,4}).Select(number => number * number); // Produces: [1, 4, 9, 16]
Where
Name(s) in other languages: filter, remove-if, select
What it does:
[a,b,c] -> [a,c]
Given an IEnumerable and a function that takes an element and returns true or false, keep items where the function returns true.Lazy.
Examples
//Filter out odd numbers
(new[] {1,2,3,4}).Where(number => number % 2 == 0); // Produces: [2, 4]
First
Name(s) in other languages: head, front
What it does:
[a,b,c] -> a
Given an IEnumerable, return the first thing. Throws if the list is empty.
OR
Given an IEnumerable and a function that returns true or false, returns the first thing that that function returns true for. Throws if the list is empty or there are no matching elements.
Examples
//Get the first item
(new[] {1,2,3,4}).First(); // Produces: 1
//Fails if the list is empty
(new int[] {}).First(); // Throws an InvalidOperationException
//Gets the first even number
(new[] {1,2,3,4}).First(number => number % 2 == 0); // Produces: 2
FirstOrDefault
Like First, but returns the default value instead of throwing an exception.
Single and SingleOrDefault
Like First and FirstOrDefault, but throws if there is more than one item in the list, or more than one item that matches the predicate. Means “I expect this list only has one thing in it” or “I expect this list has exactly one of these things in it” respectively.
Advanced
SelectMany
Name(s) in other languages: flatten, join
What it does:
[ [a,b,c], [d,e,f], [g,h,i] ] -> [a,b,c,d,e,f,g,h,i]
Takes a “list of lists” and returns a single list containing all elements of every list.
Examples
var listOfLists = new[]
{
new[] {1, 2, 3},
new[] {1, 2, 3},
};
listOfLists.SelectMany(x => x); // Produces: [1,2,3,1,2,3]
Aggregate
Name(s) in other languages: fold, reduce, accumulate
What it does:
[a,b,c] -> A
Given a starting seed, visit each element in the list and apply each item to a value.
Aggregate seems really complicated at first, but once you use it a few times, it’s actually pretty easy and surprisingly useful.
Examples
var list = new[] {1, 2, 3};
//Sum the elements of list
list.Aggregate(0, (current, number) => current + number); //Produces: 6 (because 0+1+2+3 is 6)