Is is as or is as is?

Today a question about the is and as operators: is the is operator implemented as a syntactic sugar for the as operator, or is the as operator implemented as a syntactic sugar for the is operator? More briefly, is is as or is as is?

Perhaps some sample code would be more clear. This code

bool b = x is Foo;

could be considered as a syntactic sugar for

bool b = (x as Foo) != null;

in which case is is a syntactic sugar for as. Similarly,

Foo f = x as Foo;

could be considered to be a syntactic sugar for

var temp = x;
Foo f = (temp is Foo) ? (Foo)temp : (Foo)null;

in which case as is a syntactic sugar for is. Clearly we cannot have both of these be sugars because then we have an infinite regress!

The specification is clear on this point: as (in the non-dynamic case) is defined as a syntactic sugar for is.

However, in practice the CLR provides us instruction isinst, which ironically acts like as. We have an instruction which implements the semantics of as pretty well, from which we can build an implementation of is. In short, de jure is is is, and as is as is is, but de facto is is as and as is isinst.

I now invite you to leave the obvious jokes about President Clinton in the comments.


Commentary from 2020:

  • This article was based on a serious question I had been asked, but of course I relished the opportunity to make silly sentences such as “is is as or is as is?”
  • The bit at the end about President Clinton confused some readers, understandably. Read the explainer here for details. The TLDR is: President Clinton famously attempted to explain a lie with “it depends on what the meaning of ‘is’ is”.
  • In the original comments a reader asked why anyone should care about this level of implementation detail. My response to that question gets straight to the philosophy of why I’ve written a blog for 17 years:

I reject the premise of the question; I’m not saying that you or anyone else should care about this detail. I do not necessarily write about things that are important for developers to understand; I write about things that are fabulous! And the person who gets to decide what is fabulous is me. If some of those fabulous things happen to be important, that’s super, but that’s not my aim. The name of the blog is not “Important Stuff You Should Know About Coding“.


I blog because I’ve learned a bunch of interesting stuff — interesting to me — and I like to share things that I find interesting.

    • Another good question in the comments was “does the compiler statically analyze is and as expressions?”  Yes! If you write something like if(x is string) and we already know that x is some class type Foo then the expression will always be false and you get a warning.However, there are subtle problems here. The rules of isinst and the rules of C# are slightly different. In particular, it is illegal in C# to convert int[] to uint[] via reference conversion, but legal in the CLR. You can therefore get into situations where the compiler will warn about “impossible” situations that are actually possible.
    • Several readers pointed out that there is a missing language feature: once we know that x is Foo is true, it should be legal to use the members of Foo from x. A form of this feature was added to C# 6; if(x is Foo f)

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