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Steve Mak
Steve Mak

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Cheatsheet for LINQ

#cheatsheet #linq

Intro.

A query variable is always an enumerable type that will produce a sequence of elements when it is iterated over in a foreach statement or a direct call to its IEnumerator.MoveNext method.

Syntax 

from .. in .. [ join .. in .. on .. equals .. ] [ where | orderby .. ascending | descending | let ] [ group .. by .. into .. ] select new {}
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Group clause 

var queryCountryGroups = from country in countries group country by country.Name[0];
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Select clause 

var queryNameAndPop = from country in countries select new { Name = country.Name, Pop = country.Population };
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Orderby clause 

IEnumerable<Country> querySortedCountries = from country in countries orderby country.Area, country.Population descending select country;
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Join clause 

var categoryQuery = from cat in categories join prod in products on cat equals prod.Category select new { Category = cat, Name = prod.Name };
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Let clause 

string[] names = { "Svetlana Omelchenko", "Claire O'Donnell", "Sven Mortensen", "Cesar Garcia" }; IEnumerable<string> queryFirstNames = from name in names let firstName = name.Split(' ')[0] select firstName; foreach (string s in queryFirstNames) Console.Write(s + " "); //Output: Svetlana Claire Sven Cesar
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Subqueries in a query expression 

var queryGroupMax = from student in students group student by student.GradeLevel into studentGroup select new { Level = studentGroup.Key, HighestScore = (from student2 in studentGroup select student2.Scores.Average()) .Max() }; // percentileQuery is an IEnumerable<IGrouping<int, Country>> var percentileQuery = from country in countries let percentile = (int) country.Population / 10_000_000 group country by percentile into countryGroup where countryGroup.Key >= 20 orderby countryGroup.Key select countryGroup; // grouping is an IGrouping<int, Country> foreach (var grouping in percentileQuery) { Console.WriteLine(grouping.Key); foreach (var country in grouping) Console.WriteLine(country.Name + ":" + country.Population); }
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Standard query operators

Ref: https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.selectmany?view=netcore-3.1

Where Select SelectMany Skip SkipWhile Take TakeWhile Join GroupJoin Concate OrderBy OrderByDescending ThenBy ThenByDescending Reverse GroupBy Distinct Union Intersect Except AsEnumerable AsQueryable ToArray ToList ToDictionary ToLookup OfType Cast SequenceEqual First FirstOrDefault Last LastOrDefault Single SingleOfDefault ElementAt ElementAtOrDefault DefaultEmpty All Any Contains Count LongCout Sum Min Max Average Aggregate equal/Equals from/From in/In into/Into key let Group Range Repeat
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