extended-property-patterns.md

Extended property patterns

[!INCLUDESpecletdisclaimer]

Summary

Allow property subpatterns to reference nested members, for instance:

if (e is MethodCallExpression { Method.Name: "MethodName" })

Instead of:

if (e is MethodCallExpression { Method: { Name: "MethodName" } })

Motivation

When you want to match a child property, nesting another recursive pattern adds too much noise which will hurt readability with no real advantage.

Detailed design

The property_pattern syntax is modified as follow:

property_pattern
  : type? property_pattern_clause simple_designation?
  ;

property_pattern_clause
  : '{' (subpattern (',' subpattern)* ','?)? '}'
  ;

subpattern
- : identifier ':' pattern
+ : subpattern_name ':' pattern
  ;

+subpattern_name
+ : identifier 
+ | subpattern_name '.' identifier
+ ;

The receiver for each name lookup is the type of the previous member T0, starting from the input type of the property_pattern. if T is a nullable type, T0 is its underlying type, otherwise T0 is equal to T.

For example, a pattern of the form { Prop1.Prop2: pattern } is exactly equivalent to { Prop1: { Prop2: pattern } }.

Note that this will include the null check when T is a nullable value type or a reference type. This null check means that the nested properties available will be the properties of T0, not of T.

Repeated member paths are allowed. The compilation of pattern matching can take advantage of common parts of patterns.