linq-cheatsheet.md

LINQ Cheat Sheet (.NET)

Quick reference for common LINQ (Language Integrated Query) operations in C#.

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📄 Basic Syntax

var result = from item in collection
             where item.Property == value
             select item;

OR using method syntax:

var result = collection.Where(item => item.Property == value);

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🔍 Filtering

Operation	Example
Where	list.Where(x => x.Age > 30)
OfType	objects.OfType<string>()

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📊 Projection

Operation	Example
Select	list.Select(x => x.Name)
SelectMany	list.SelectMany(x => x.Children)

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🔢 Ordering

Operation	Example
OrderBy	list.OrderBy(x => x.Name)
OrderByDescending	list.OrderByDescending(x => x.Score)
ThenBy	list.OrderBy(x => x.Name).ThenBy(x => x.Age)

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🧮 Aggregation

Operation	Example
Count	list.Count()
Sum	list.Sum(x => x.Amount)
Average	list.Average(x => x.Age)
Min / Max	list.Min(x => x.Price)

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🔗 Joining

Operation	Example
Join	list1.Join(list2, a => a.Id, b => b.Id, (a, b) => ...)
GroupJoin	categories.GroupJoin(products, c => c.Id, p => p.CategoryId, (c, p) => ...)

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📦 Grouping

var groups = list.GroupBy(x => x.Category);

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🧪 Quantifiers

Operation	Example
Any	list.Any(x => x.IsActive)
All	list.All(x => x.IsValid)
Contains	list.Contains(value)

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🔄 Element Operations

Operation	Example
First / FirstOrDefault	list.First(x => x.IsReady)
Single / SingleOrDefault	list.Single(x => x.Id == 1)
Last / LastOrDefault	list.LastOrDefault()
ElementAt / ElementAtOrDefault	list.ElementAt(2)

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🧹 Set Operations

Operation	Example
Distinct	list.Distinct()
Union	list1.Union(list2)
Intersect	list1.Intersect(list2)
Except	list1.Except(list2)

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🧠 Useful Tips

• Use ToList() or ToArray() to execute queries immediately.
• LINQ queries are lazy by default — they only execute when enumerated.
• Use .AsQueryable() to enable LINQ-to-Entities in EF.