What the meaning of is is

Today a follow-up to my 2010 article about the meaning of the is operator. Presented as a dialog, as is my wont!

I’ve noticed that the is operator is inconsistent in C#. Check this out:

string s = null; // Clearly null is a legal value of type string
bool b = s is string; // But b is false!

What’s up with that?

Let’s suppose you and I are neighbours.

Um… ok, I’m not sure where this is going, but sure.

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Static analysis of “is”

Returning now to the subject we started discussing last time on FAIC: sometimes the compiler can know via static analysis[1. That is, analysis done knowing only the compile-time types of expressions, rather than knowing their possibly more specific run-time types] that an is operator expression is guaranteed to produce a particular result.
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An “is” operator puzzle, part one

It is possible for a program with some local variable x

bool b = x is FooBar;

to assign true to b at runtime, even though there is no conversion, implicit or explicit, from x to FooBar allowed by the compiler! That is to say that

FooBar foobar = (FooBar)x;

would not be allowed by the compiler in that same program.

Can you create a program to demonstrate this fact?

This is not a particularly hard puzzle but it does illustrate some of the subtleties of the is operator that we’ll discuss in the next episode.