Flatten sequences with the C# LINQ SelectMany operator

Suppose that we have an object with a collection of other objects, like a customer with order items. Then we can also have a sequence of customers where each customer will have her own list of orders. It happens that we want to analyse all orders regardless of the customer, like how many of product A have been sold. There are several options to collect all orders from all customers and place them into one unified collection for further analysis.

The C# SelectMany operator has been specifically designed to extract collections of objects and flatten those collections into one. This post will provide a couple of examples to demonstrate its usage.

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An overview of grouping collections with LINQ in .NET

Introduction

The LINQ GroupBy operator is one of the most powerful ones among all LINQ operators. It helps us group elements in a collection in various ways and lets us control the element and result selection process. However, with its many overloads and Func parameters the GroupBy operator can be a bit difficult to understand at first. At least it is more complex than say the Min() and Max() LINQ operators.

This post will go through the GroupBy operator and many of its overloads with a number of examples.

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An example of grouping and joining collections in .NET: calculate total scores by student and subject

Imagine that we have two collections. First we have a student collection with IDs and names. Then we also have a collection that holds the scores the students got in various subjects on several occasions. This latter collection also holds a reference to a student by the student ID. The goal is to join the two collections and calculate the total score of each student by subject.

There are various ways to solve this problem. The goal of this post is to show an example of using the LINQ GroupBy and GroupJoin operators to build an object with the information we need.

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Determine the presence of an element in a sequence with LINQ C#

Say we have the following string list:

string[] bands = { "ACDC", "Queen", "Aerosmith", "Iron Maiden", "Megadeth", "Metallica", "Cream", "Oasis", "Abba", "Blur", "Chic", "Eurythmics", "Genesis", "INXS", "Midnight Oil", "Kent", "Madness", "Manic Street Preachers"
, "Noir Desir", "The Offspring", "Pink Floyd", "Rammstein", "Red Hot Chili Peppers", "Tears for Fears"
, "Deep Purple", "KISS"};

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Converting a sequence to a dictionary using the ToDictionary LINQ operator

Say you have a sequence of objects that you’d like to convert into a Dictionary for efficient access by key. Ideally the objects have some kind of “natural” key for the dictionary such as an ID:

public class Singer
{
	public int Id { get; set; }
	public string FirstName { get; set; }
	public string LastName { get; set; }
}

IEnumerable<Singer> singers = new List<Singer>() 
		{
			new Singer(){Id = 1, FirstName = "Freddie", LastName = "Mercury"} 
			, new Singer(){Id = 2, FirstName = "Elvis", LastName = "Presley"}
			, new Singer(){Id = 3, FirstName = "Chuck", LastName = "Berry"}
			, new Singer(){Id = 4, FirstName = "Ray", LastName = "Charles"}
			, new Singer(){Id = 5, FirstName = "David", LastName = "Bowie"}
		};

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Convert a sequence of objects to a sequence of specific type in LINQ .NET

Say that you have some old style collection such as this:

ArrayList stringList = new ArrayList() { "this", "is", "a", "string", "list" };

You can easily turn this into a proper string list using the Cast operator:

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Ordering a .NET data sequence with various LINQ operators

A sequence of any object can be easily ordered with the following LINQ operators in .NET:

• OrderBy
• ThenBy
• OrderByDescending
• ThenByDescending

Consider the following sequence:

string[] bands = { "ACDC", "Queen", "Aerosmith", "Iron Maiden", "Megadeth", "Metallica", "Cream", "Oasis", "Abba", "Blur", "Chic", "Eurythmics", "Genesis", "INXS", "Midnight Oil", "Kent", "Madness", "Manic Street Preachers", "Noir Desir", "The Offspring", "Pink Floyd", "Rammstein", "Red Hot Chili Peppers", "Tears for Fears"					 , "Deep Purple", "KISS"};

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Selecting a subset of elements in LINQ C# with the TakeWhile operator

The TakeWhile extension method in LINQ is similar to Take. With Take you can specify the number of elements to select from a sequence. In the case of TakeWhile we can specify a condition – a boolean function – instead. The operator will take elements from the sequence while the condition is true and then stop.

Data collection:

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