C# 7 switch case with null checks

Question

C#7 introduces a new feature called patterns, which you can use with Is-Expression or Switch cases like this:

string str = null; 
switch(str){
    case string x:
        Console.WriteLine("string " + x);
        break;
    default:
        Console.WriteLine("default");
        break;
}

and you would expect that it will goes inside case #1, as it is the same type, but it didn't.

Answer 1

Despite what you might think, string x = null actually isn't a string at all. It is 'nothing', assigned to a variable of type string.

The check in your switch is basically the same as null is string which is false for a long time already. This is a common issue when evaluating types with generics, but it has its plus sides too.

Under the hood, is uses as, with a null check. So that is why it can't return true. You could say the logic for the is operator is as follows:

is = (null as string) != null