Converting a sequence of objects into a Lookup with LINQ C#

A Lookup in .NET is one of the lesser known data structures. It is similar to a Dictionary but the keys are not unique. You can insert multiple elements for the same key.

Say you have the following object and collection:

public class Singer
{
	public int Id { get; set; }
	public string FirstName { get; set; }
	public string LastName { get; set; }
	public int BirthYear { get; set; }
}
IEnumerable<Singer> singers = new List<Singer>() 
	{
		new Singer(){Id = 1, FirstName = "Freddie", LastName = "Mercury", BirthYear=1964}
		, new Singer(){Id = 2, FirstName = "Elvis", LastName = "Presley", BirthYear = 1954}
		, new Singer(){Id = 3, FirstName = "Chuck", LastName = "Berry", BirthYear = 1954}
		, new Singer(){Id = 4, FirstName = "Ray", LastName = "Charles", BirthYear = 1950}
		, new Singer(){Id = 5, FirstName = "David", LastName = "Bowie", BirthYear = 1964}
	};

You can group the singers into an ILookup as follows:

ILookup<int, Singer> singersByBirthYear = singers.ToLookup(s => s.BirthYear);
IEnumerable<Singer> filtered = singersByBirthYear[1964];
foreach (Singer s in filtered)
{
	Console.WriteLine(s.LastName);
}

…which outputs “Mercury” and “Bowie”.

You can also set the elements inserted into the ILookup using an overloaded variant where you specify the element selector:

ILookup<int, string> singerNamesByBirthYear = singers.ToLookup(s => s.BirthYear, si => string.Concat(si.LastName, ", ", si.FirstName));
IEnumerable<string> filtered2 = singerNamesByBirthYear[1964];
foreach (string s in filtered2)
{
	Console.WriteLine(s);
}

…which prints “Mercury, Freddie” and “Bowie, David”.

You can view all LINQ-related posts on this blog here.

Advertisements

About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

ultimatemindsettoday

A great WordPress.com site

Elliot Balynn's Blog

A directory of wonderful thoughts

Robin Sedlaczek's Blog

Developer on Microsoft Technologies

Softwarearchitektur in der Praxis

Wissenswertes zu Webentwicklung, Domain-Driven Design und Microservices

Technology Talks

on Microsoft technologies, Web, Android and others

Software Engineering

Web development

Disparate Opinions

Various tidbits

chsakell's Blog

WEB APPLICATION DEVELOPMENT BEST PRACTICES WITH MICROSOFT STACK & ANGULAR

Cyber Matters

Bite-size insight on Cyber Security for the not too technical.

Guru N Guns's

OneSolution To dOTnET.

Johnny Zraiby

Measuring programming progress by lines of code is like measuring aircraft building progress by weight.

%d bloggers like this: