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What is LINQ and how do I use it?

C# 2 min read Full lesson: Build a Calculator App in Visual C#
Short answer

LINQ adds query methods to any IEnumerable<T>. Where, Select, OrderBy, First, Any and Sum cover most day-to-day use.

csharp
using System.Linq;

var videos = new List<Video> { /* … */ };

var popular = videos
    .Where(v => v.Views > 200)
    .OrderByDescending(v => v.Views)
    .Select(v => v.Title)
    .ToList();

#The core methods

csharp
videos.Where(v => v.Views > 200);       // filter
videos.Select(v => v.Title);            // transform
videos.OrderBy(v => v.Title);           // sort
videos.First(v => v.Views > 500);       // throws if none
videos.FirstOrDefault(v => …);          // null if none
videos.Any(v => v.Views > 500);         // bool
videos.All(v => v.Views > 0);           // bool
videos.Count(v => v.Views > 200);       // int
videos.Sum(v => v.Views);
videos.GroupBy(v => v.Track);
videos.Distinct();
videos.Take(5);  videos.Skip(10);

#Deferred execution

A LINQ query does not run when you write it. It runs when you enumerate it:

csharp
var query = videos.Where(v => v.Views > 200);   // nothing has happened yet
videos.Add(new Video { Views = 999 });
var result = query.ToList();                    // NOW it runs — includes the new one

This surprises people the first time. Call .ToList() or .ToArray() to force it immediately.

#Query syntax exists too

csharp
var popular = from v in videos
              where v.Views > 200
              orderby v.Views descending
              select v.Title;

Identical result. Method syntax is more common and handles everything; query syntax reads better for complex joins and group by.

#First vs Single

csharp
.First()            // first match, throws if none
.FirstOrDefault()   // first match, default if none
.Single()           // throws unless there is EXACTLY one
.SingleOrDefault()  // throws if more than one

Use Single when more than one match indicates a bug — a lookup by primary key, for instance. The exception is the point.

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