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InfoQ Homepage News C# 9: Range Operators in Switch Constructs and Pattern-Matching Expressions

C# 9: Range Operators in Switch Constructs and Pattern-Matching Expressions

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Since C# was first introduced, developers have complained about the lack of a range operator in switch constructs. This made switches far less useful in C# than they were in VB. As part of the pattern-matching enhancements for C# 9, this limitation has been addressed.

In this feature, the following patterns will be allowed following the case or is keyword.

  • < constant-expression
  • constant-expression
  • <= constant-expression
  • >= constant-expression

This is still more limited than VB in the sense that only constants are allowed. This becomes a problem when you are dealing with dates, times, or other comparable structs, as they do not have a constant representation in C#. (Date/time values can be expressed as a constant in VB, but it is discouraged because one runs into DateTime.Kind issues.) When asked why, Neal Gafter wrote,

It doesn't do the same thing that pattern-matching was designed to do. Specifically, it was designed to work with constants specifically so that the compiler can analyze, diagnose, and optimize the totality of the set of pattern-matching operations. If the compiler doesn't know the value it is matching against, it cannot do any of that. That's not to say it is necessarily a bad idea, but it is a pretty different animal. Do ordinary expressions not satisfy that need?

Mariusz Pawelski explains why this may be a problem,

I just think there will be many people who will see using is and or keywords as just a different way of writing boolean expressions. Just another small C# feature that makes code more succinct. Similar to tons of others we got in recent years ( expression-bodied members, out variables, tuples, null-conditional operators operator, "default" literal, etc). They won't know it's part of greater feature like "pattern-matching" which "was designed to work with constants". They will just be confused when instead of constant they will use variable name and get "CS0150: A constant value is expected" error.

In our previous article, we discussed the and, or, and not pattern-matching operators. These can be used in conjunction with ranges. For example,

bool IsLetter(char c) => c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';

Admittedly, this is a bit difficult to read. So, another feature included in C# 9 is the ability to use parenthesis around patterns just as you would for other types of expressions to make the precedence explicit.

bool IsLetter(char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z');

According to the C# 9 feature status, these features are marked as having been merged into the master branch.

Update: A previous version of this report included the to operator. We have been informed that the current plan for C# 9 does not include this feature.

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