The documentation uses is in the example for “declaration patterns” but the book I am reading uses a switch statement. But when I try to run the code I run into errors.
using System;
public class Program
{
public static void Main()
{
var o = 42;
switch (o) {
case string s:
Console.WriteLine($"A piece of string is {s.Length} long");
break;
case int i:
Console.WriteLine($"That's numberwang! {i}");
break;
}
}
}
Error:
Compilation error (line 7, col 6): An expression of type 'int' cannot be handled by a pattern of type 'string'.
EDIT
Changing from
var o = 42;
to
object o = 42;
worked.
Full code: https://github.com/idg10/prog-cs-10-examples/blob/main/Ch02/BasicCoding/BasicCoding/Patterns.cs
An integer will never be a string. Originally you create an integer variable so it’s telling you the string case is pointless.
Your code does not follow the pattern matching syntax; I don’t see “is” anywhere. That’s what is actually doing the casting
Edit: I think I’m completely wrong about “is” being required
I see.
The book uses a very specific scenario where
ois an object that would accept any type. So using the object data type worked. Check the OP for the edit.I see, using “is” could be a downcast from any type. But from object it would always be an upcast so you don’t need an explicit casting operator
So c# in runtime already knows the type of o(unless you do some silly magic ofcourse) If you wanna change o for debug purposes you can try .GetType() and typeOf
You can check the type with
bool isString = o.GetType() == typeof(string);(Sorry for any errors I’m on phone so code fiddlers aren’t that great)
The book uses a very specific scenario where
ois an object that would accept any type. So using the object data type worked. Check the OP for the edit.
The “is” is a part of pattern matching which I don’t believe regular switches can do. Only switch expressions. The example in the link you gave is checking the type by casting using “is”.