Why is this switch on type case considered confusing?

Question

I was looking for a way to refactor and simplify one function where I have to do data sorting depending on input class type, and got stuck at switch(input.GetType()):

Quick search led me to Why doesn't C# switch statement allow using typeof/GetType()? with a link to http://blogs.msdn.com/peterhal/archive/2005/07/05/435760.aspx

I read the documentation, but I don't get the justification that the situation is confusing.

From the article:

Unfortunately, like many 'simple' language features, type switch is not as simple as it first appears. The troubles start when you look at a more significant, and no less important, example like this:

class C {}
interface I {}
class D : C, I {}

switch typeof(e) {
case C: ... break;
case I: ... break;
default: ... break;
}

What's not simple about it? The call typeof(e) cannot return - this is a I D and C. It has to return a Type not an array of interface and class types - Type[]. So the type of the class D is D. And D corresponds to a default: branch.

An I missing something?

see also stackoverflow.com/questions/156467/switch-pattern-matching-idea stackoverflow.com/questions/7252186/switch-case-on-type-c-sharp stackoverflow.com/questions/7542793/… stackoverflow.com/questions/4478464/c-sharp-switch-on-type stackoverflow.com/questions/298976/… — Mikhail
and stackoverflow.com/questions/94305/… stackoverflow.com/questions/5947343/… stackoverflow.com/questions/10115028/… stackoverflow.com/questions/2551773/… — Mikhail

artbristol artbristol · Accepted Answer · 2011-06-10T10:21:12

It seems you don't expect the switch to match on subclasses. But this would break the Liskov Substitution Principle. (where if you passed in a C object, the code would work, but not with a D, even though D is a subclass of C).

Why is this switch on type case considered confusing?

2 Answers