ToLookup in C#

A frozen one-to-many index over a sequence.

var files = new[] { "a.cs", "b.py", "c.cs", "d.md", "e.py" };

// a lookup is a frozen one-to-many dictionary
var byExt = files.ToLookup(f => Path.GetExtension(f));

Console.WriteLine(string.Join(",", byExt[".cs"]));
Console.WriteLine(string.Join(",", byExt[".py"]));

// a missing key yields an empty group, never an exception
Console.WriteLine(byExt[".txt"].Count());

How it works

1. ToLookup(f => extension) groups files by their suffix, once.
2. Indexing by key returns the group, ready to iterate.
3. A missing key yields an empty group, never an exception.

Keywords and builtins used here

• new
• string
• var

The run, in numbers

Lines

10

Characters to type

369

Tokens

85

Three-star pace

90 tpm

At the three-star pace of 90 tokens a minute, this run takes about 57 seconds.

Type this snippet

Step 3 of 3 in Joins & lookups, step 9 of 17 in LINQ in depth.

← Previous Next →