switch case syntax specifies a data type

Question

Today at work I we did a code review as we do now and again, but today I saw a syntax I haven't seen before. I have searched the web for it to no avail, and what is even more interesting is that, despite using the same version of Visual Studio, that is, 2017 Enterprise, I get a syntax error at home when I try to recreate what I saw at work today. It looked something like this:

switch (someObject) {
    case TypeOne valueOne: action1(); break;
    case TypeTwo valueTwo: action2(); break;
    // ... and so on
}

In other words, it looked like they checked both type and value in one go. But like I said, I can't find it on the internet and I get a syntax error at home. I know there's a lot of new stuff in C# 7, especially in terms of syntactic sugar. Can you explain this?

Check "Switch statements with patterns" in https://blogs.msdn.microsoft.com/dotnet/2017/03/09/new-features-in-c-7-0/ — KMoussa, Apr 28 '17 at 14:19
It is a new feature of C# 7 called pattern matching [MSDN](https://blogs.msdn.microsoft.com/dotnet/2016/08/24/whats-new-in-csharp-7-0/) search for `Pattern Matching` — Matthiee, Apr 28 '17 at 14:19
Make sure you set your compiler to use C# 7 http://stackoverflow.com/questions/43681882/expression-bodied-get-set-accessors — juharr, Apr 28 '17 at 14:27

score 13 · Accepted Answer · answered Apr 28 '17 at 14:19

This is a new C# 7 feature: pattern matching switch statement which matches on types.

What this code does, giving the first branch:

It checks if someObject is, derives from or implements type TypeOne.
If so, it casts someObject to type TypeOne which is assigned to valueOne.
Then it enters the case block where you can directly use valueOne.

switch case syntax specifies a data type

1 Answers1