{"id":8971,"date":"2021-10-21T19:02:55","date_gmt":"2021-10-21T19:02:55","guid":{"rendered":"http:\/\/putridparrot.com\/blog\/?p=8971"},"modified":"2021-10-21T19:02:55","modified_gmt":"2021-10-21T19:02:55","slug":"more-pattern-matching-in-c","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/more-pattern-matching-in-c\/","title":{"rendered":"More Pattern Matching in C#"},"content":{"rendered":"

A while back I wrote a post C# 8.0 enhancements with pattern matching<\/a> which was very light on the subject. Let’s look a little more in depth at options for pattern matching.<\/p>\n

is\/as in pattern matching syntax<\/strong><\/p>\n

Often (if you’ve been writing C# for a while) you’ll used to writing code like this<\/p>\n

\r\nvar person = o as Person;\r\nif (person != null)\r\n{\r\n   Console.WriteLine(person.FirstName);\r\n}\r\n<\/pre>\n
i.e. we have an object o<\/em> which we don’t know the type of, so we use as<\/em> to convert this to a Person type or null if it’s not of that type. <\/p>\n
This can be replaced with a slightly more concise syntax (and what’s known as the declaration pattern<\/a>).<\/p>\n
\r\nif (o is Person person)\r\n{\r\n  Console.WriteLine(person.FirstName);\r\n}\r\n<\/pre>\n
Null checks<\/strong><\/p>\n
This is a pattern known as a constant pattern<\/a><\/p>\n
\r\nif (o is null)\r\n{\r\n  Console.WriteLine("Is null");\r\n}\r\n<\/pre>\n
Along with a logical pattern<\/a> using not<\/em> we can also write <\/p>\n
\r\nif (o is not null)\r\n{\r\n  Console.WriteLine("Is not null");\r\n}\r\n<\/pre>\n
We can also use this pattern with types, for example<\/p>\n
\r\nif (o is not (string or null))\r\n{\r\n  Console.WriteLine("Is NOT string or null");\r\n}\r\n\r\n<strong>Pattern matching when values match a criteria<\/strong>\r\n\r\nIf we extend the above Pattern Matching to not just check the type is a Person but also that the Age property is greater than 4, so we can now replace\r\n\r\n[code language="csharp"]\r\nif (o is Person p && p.Age > 4)\r\n{\r\n  Console.WriteLine($"Older than 4 {p.FirstName}");\r\n}\r\n<\/pre>\n
with the following<\/p>\n
\r\nif (o is Person { Age: > 4 } p)\r\n{\r\n   Console.WriteLine($"Older than 4 {p.FirstName}");\r\n}\r\n<\/pre>\n
In the above we’re using a property pattern<\/a>.<\/p>\n
Switch patterns matching<\/strong><\/p>\n
Exhaustive switches can be used to match types using switch statements, for example<\/p>\n
\r\nvar result = o switch\r\n{\r\n  string s => $"String {s}",\r\n  Person p => $"Person {p.FirstName}",\r\n  _ => throw new ArgumentException("Unhandled type")\r\n};\r\n<\/pre>\n
Note the use of the _ (discard) ensuring this is an exhaustive switch.<\/p>\n
Better still we can also use other features of pattern matching in the switch like this<\/p>\n
\r\nvar result = o switch\r\n{\r\n  string s => $"String {s}",\r\n  Person { Age: > 4} p => $"Person {p.FirstName} > 4",\r\n  Person p => $"Person {p.FirstName}",\r\n  null => "In Null",\r\n  _ => throw new ArgumentException("Unhandled type")\r\n};\r\n<\/pre>\n
In the above we’re switching based upon type and also matching values.<\/p>\n
We can also use relational patterns<\/a>, in this example we’ll assume o<\/em> is an int<\/p>\n
\r\nvar result = o switch\r\n{\r\n  1 or 2 => "One or Two",\r\n  > 2 and < 4 => "Mid",\r\n  >= 4 and < 6 => "High",\r\n  6 => "Six",\r\n  _ => "Other"\r\n};\r\n<\/pre>\n
Before we leave the switch<\/em> statement we can also match against types using the “standard” switch syntax, i.e.<\/p>\n
\r\nswitch (o)\r\n{\r\n  case string s:\r\n    Console.WriteLine(s);\r\n    break;\r\n  case Person p:\r\n    Console.WriteLine(p.FirstName);\r\n    break;\r\n}\r\n<\/pre>\n
Tuples<\/strong><\/p>\n
Pattern matching also allows us to match against tuples and use the discard to ignore parts we’re not interested in, for example<\/p>\n
\r\nvar result = o switch\r\n{\r\n  (1, _) => "First One",\r\n  (_, 0) => "Second Zero",\r\n  _ => "Other"\r\n};\r\n<\/pre>\n
Creating new variables from patterns<\/strong><\/p>\n
We can also use the var pattern<\/a> to assign values from the patterns<\/p>\n
\r\nvar result = o switch\r\n{\r\n  var (a, b) when a < b => new Tuple<string, int, int>("First less than second", a, b),\r\n  var (a, b) when a > b => new Tuple<string, int, int>("Second greater than first", a, b),\r\n  var (a, b) => new Tuple<string, int, int>("Equals", a, b)\r\n};\r\n<\/pre>\n
In this example we’re deconstructing a tuple and assigning to some new value (in this case an extended Tuple, just because I couldn’t think of a better example – check the documentation link above for a better example).<\/p>\n","protected":false},"excerpt":{"rendered":"
A while back I wrote a post C# 8.0 enhancements with pattern matching which was very light on the subject. Let’s look a little more in depth at options for pattern matching. is\/as in pattern matching syntax Often (if you’ve been writing C# for a while) you’ll used to writing code like this var person […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[3],"tags":[],"class_list":["post-8971","post","type-post","status-publish","format-standard","hentry","category-c"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/8971","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/comments?post=8971"}],"version-history":[{"count":5,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/8971\/revisions"}],"predecessor-version":[{"id":8978,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/8971\/revisions\/8978"}],"wp:attachment":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/media?parent=8971"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/categories?post=8971"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/tags?post=8971"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}