Essential Books

GroupBy

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GroupBy is an easy way to sort a IEnumerable<T> collection of items into distinct groups.

Simple Example

In this first example, we end up with two groups, odd and even items. 

List<int> iList = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
var grouped = iList.GroupBy(x => x % 2 == 0);

//Groups iList into odd [13579] and even[2468] items 
       
foreach(var group in grouped)
{
    foreach (int item in group)
    {
        Console.Write(item); // 135792468  (first odd then even)
    }
}

More Complex Example

Let’s take grouping a list of people by age as an example. First, we’ll create a Person object which has two properties, Name and Age. 

public class Person
{
    public int Age {get; set;}
    public string Name {get; set;}
}

Then we create our sample list of people with various names and ages. 

List<Person> people = new List<Person>();
people.Add(new Person{Age = 20, Name = "Mouse"});
people.Add(new Person{Age = 30, Name = "Neo"});
people.Add(new Person{Age = 40, Name = "Morpheus"});
people.Add(new Person{Age = 30, Name = "Trinity"});
people.Add(new Person{Age = 40, Name = "Dozer"});
people.Add(new Person{Age = 40, Name = "Smith"});

Then we create a LINQ query to group our list of people by age. 

var query = people.GroupBy(x => x.Age);

Doing so, we can see the Age for each group, and have a list of each person in the group. 

foreach(var result in query)
{
    Console.WriteLine(result.Key);
                
    foreach(var person in result)
        Console.WriteLine(person.Name);
}

This results in the following output: 

20
Mouse
30
Neo
Trinity
40
Morpheus
Dozer
Smith

You can play with the live demo on .NET Fiddle

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LINQ Queries:
* LINQ Queries
* Chaining methods
* First FirstOrDefault Last LastOrDefault Single and SingleOrDefault
* Except
* SelectMany
* Any
* JOINS
* 0Skip and Take
* Zip
* 09Defining a variable inside a Linq query let keyword
* Range and Repeat
* All
* Basics
* Aggregate
* Distinct
* SelectMany Flattening a sequence of sequences
* Query collection by type cast elements to type
* GroupBy
* Enumerating the Enumerable
* Where
* Contains
* Using SelectMany instead of nested loops
* Using Range with various Linq methods
* Sum
* SkipWhile
* ToDictionary
* SequenceEqual
* Query Ordering - OrderBy ThenBy OrderByDescending ThenByDescending
* Any and FirstOrDefault - best practice
* GroupBy one or multiple fields
* OrderBy
* ElementAt and ElementAtOrDefault
* ToLookup
* Joining multiple sequences
* Joining on multiple keys
* DefaultIfEmpty
* GroupBy Sum and Count
* Count and LongCount
* Linq Quantifiers
* Build your own Linq operators for IEnumerableT
* GroupJoin with outer range variable
* Incrementally building a query
* Concat
* Reverse
* Union
* Select - Transforming elements
* OrderByDescending
* TakeWhile
* Select with FuncTSource int TResult selector - Use to get ranking of elements
Table Of Contents
0 Getting Started With C#
1 Literals
2 Operators
3 Conditional Statements
4 Equality Operator
5 Equals and GetHashCode
6 Null-Coalescing Operator
7 Null-Conditional Operators
8 nameof Operator
9 Verbatim String
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13 String Manipulation
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17 Regex
18 DateTime
19 Arrays
20 On Algorithm for circular rotation of an array
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23 Overview of C# collections
24 GUID
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31 Built-in Types
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63 LINQ to Objects
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69 C# 7.0 Features
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71 C# 5.0 Features
72 C# 4.0 Features
73 C# 3.0 Features
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77 Read and Understand Stack trackes
78 Diagnostics
79 Overflow
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81 Using json.net
82 Lambda Expressions
83 Lambda expressions
84 Generic Lambda Query Builder
85 Properties
86 Initializing Properties
87 INotifyPropertyChanged interface
88 Events
89 Expression Trees
90 Overload Resolution
91 Preprocessor directives
92 Structs
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95 File and Stream IO
96 Networking
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109 Threading
110 Async/Await, BackgroundWorker, Task and Thread examples
111 Async Await
112 Synchronization Context in Async/Await
113 BackgroundWorker
114 Task Parallel Library
115 Making a variable thread safe
116 Lock Statement
117 Yield Keyword
118 Task Parallel Library TPL Dataflow Constructs
119 Functional Programming
120 Func delegates
121 Function with multiple return values
122 Binary Serialization
123 ICloneable
124 IComparable
125 Accessing Databases
126 Using SQLite
127 Caching
128 Code Contracts
129 Code Contracts and Assertions
130 Structural Design Patterns
131 Creational Design Patterns
132 Implementing Decorating Design Pattern
133 Implementing Flyweight Design Pattern
134 System.Management.Automation
135 Pointers
136 Pointers and Unsafe Code
137 Simulating C Unions with C# Structs
138 Reactive Extensions Rx
139 AsseblyInfo.cs examples
140 Creating Console Application
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142 ObservableCollection
143 Hash Functions
144 Generating Random Numbers
145 Cryptography
146 Unsafe Code in .NET
147 C# Script
148 Runtime Compile
149 Interoperability
150 .NET Compiler Platform Roslyn
151 Creating Own MessageBox in Windows Form Application
152 Including Font Resources
153 Garbage Collector
154 Windows Communication Foundation
155 Contributors