C# 7 introduced pattern matching with the extension of the switch statement and the is operator offering the const pattern, the type pattern, and the var pattern. With C# 8 an extension of pattern matching is planned, including the property pattern, the recursive pattern, and a new switch – the switch expression.
Pattern Matching with the switch Statement
With C# 7, pattern matching was introduced in C#. The following sample makes use of pattern matching in the
switch statement, and type pattern matches. With the first case, also the
when clause is used to filter only shapes where the size of the shape has a minimum height. The second and third cases match shapes with a smaller height, but only objects of type Ellipse and Rectangle. For all the other shapes, the default case is chosen:
Shape class makes use of tuples and deconstruction. The read-only property
Position is a tuple type containing two int values for x and y. The property
Size is a tuple containing two int values for height and width. The
Shape defines a constructor where the position and size is initialized. With the implementation of the constructor, on the right side a tuple is created containing the position and the size, and this tuple is deconstructed to fill the Position and Size properties.
Deconstruct method is used to deconstruct two tuples for the size and the position out of the
Shape class. This deconstruction will be used later with pattern matching.
Main method of the application, an array of
Shape objects is created, and in turn the
foreach statement is invoked. The object is passed to the
M1 method shown earlier making use of pattern matching.
Install C# 8
At the time of this writing, C# 8 is not yet released. However, you can try it out. At the time of this writing, to try C# 8 patterns, Visual Studio 2017 15.5-15.7 is needed – and the preview of C# Patterns and Ranges. You can easily install it, and uninstall it as described in the linked article.
Be aware that the features shown here might look different in the released product, or also might be removed or delayed to a later version of C#.
Using the switch expression
C# 7 extended the scenarios where you can use expression bodied members. However, as soon as you use the
switch statement, the method cannot be implemented using the expression syntax. This changes with the new switch expression.
The switch expression is simplified compared to the switch statement. First, the order of the switch keyword and the variable used is reversed. Instead of writing switch (shape), you write shape switch. The
case keyword is not needed with the new syntax. Every case is decided by a pattern, e.g. the type pattern Ellipse e where the variable
e is filled in this case. You can also use the
when filter as used in the earlier C# 7 pattern matching sample. This syntax is the same as before. The
break keyword is also not needed. The implementation of the case follows the lambda operator. After a comma, the next pattern specifies the next case. The new discard pattern with the
_ specifies the default case. This new pattern can also be used with the switch statement used previously in case of
What if multiple statements are needed in a single case? You can use local functions in such a scenario.
Property Pattern, Recursive Pattern
The new switch epression can also be simplified using more new C# 8 pattern matching features. The case matching the
Ellipse, now deconstruction is used to fill the
With the match for the
Rectangle, the position is ignored from the deconstruction – using the discard pattern.
The second case is a match for the
Shape itself. Because the variable
shape is of type
Shape, a declaration for the
Shape type is not needed. Using curly brackets, the property pattern is used. Here a match only happens if the
Size property has a value of
(200, 200) (a tuple). The result from the
Position property is assigned to the
With the match for the
CombinedShape class, a recursive pattern is used. The
CombinedShape class defines deconstruction to shape1 and shape2. The first shape of this CombinedShape is assigned to the variable
shape1. With the second shape, a recursive pattern allows deconstruction of the inner shape – the Position of the second shape is assigned to the
pos variable, and the inner size is ignored.
C# 7 introduced pattern matching with the type pattern, the const pattern, and the var pattern. C# 8 extends pattern matching with the discard pattern, the property pattern, and the recursive pattern. Patterns that can simplify code written today.
The new switch expression offers a more modern way for switch/case/break, whereas the case and break keywords are no longer needed.
What do you think about this new C# 8 feature?
Read C# 8 & No Nore NullReferenceExceptions – What about legacy code? for another feature on C# 8.
Get the complete sample from More Samples!
Before C# 8 is released, read about all the cool C# 7 features in the book Professional C# 7 and .NET Core 2.0!
Enjoy programming and learning,