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?

Answer 1

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.