Question

I know there are several posts already concerning the difference between casts and the as operator. They all mostly restate the same facts:

• The as operator will not throw, but return null if the cast fails
• Consequently, the as operator only works with reference types
• The as operator will not use user-defined conversion operators

Answers then tend to debate endlessly the how to use or not use the one or the other and the pros and cons of each, even their performance (which interests me not at all).

But there is something more at work here. Consider:

static void MyGenericMethod<T>(T foo)
{
    var myBar1 = foo as Bar;  // compiles
    var myBar2 = (Bar)foo;    // does not compile ('Cannot cast expression of
                              // type 'T' to type 'Bar')
}

Please never mind whether this obviously contrite example is good practice or not. My concern here is the very interesting disparity between the two in that the cast will not compile whereas the as does. I really would like to know if anyone could shed some light on this.

As is often noted, the as operator disregards user-defined conversions, but in the above example, it is clearly the more capable of the two. Note that as far as the compiler is concerned, there is no known connection between the (unknown at compile-time) type T and Bar. The cast is entirely ‘run-time’. Should we suspect that the cast is resolved, wholly or partly, at compile time and the as operator not?

By the way, adding a type constraint unsurprisingly fixes the cast, thus:

static void MyGenericMethod<T>(T foo) where T : Bar
{
    var myBar1 = foo as Bar;  // compiles
    var myBar2 = (Bar)foo;    // now also compiles
}

Why does the as operator compile and the cast not?

Answer 1

To address your first question: it is not just that the as operator disregards user-defined conversions, though that is relevant. What is more relevant is that the cast operator does two contradictory things. The cast operator means either:

1. I know that this expression of compile-time type Foo will actually be an object of runtime-type Bar. Compiler, I am telling you this fact now so that you can make use of it. Please generate code assuming that I am correct; if I am incorrect, then you may throw an exception at runtime.

2. I know that this expression of compile-time type Foo will actually be of runtime type Foo. There is a standard way of converting some or all instances of Foo to an instance of Bar. Compiler, please generate such a conversion, and if it turns out at runtime that the value being converted is not convertible, then throw an exception at runtime.

Those are opposites. Neat trick, to have an operator that does opposite things.

The as operator by contrast only has the first sense. An as only does boxing, unboxing and representation-preserving conversions. A cast can do all of those plus additional representation-changing conversions. For example, casting int to short changes the representation from a four-byte integer to a two-byte integer.

That’s why “raw” casts are not legal on unconstrained generics; because the compiler does not have enough information to figure out what kind of cast it is: boxing, unboxing, representation-preserving or representation-changing. The expectation of users is that a cast in generic code has all the semantics of a cast in more strongly typed code, and we have no way to generate that code efficiently.

Consider:

void M<T, U>(T t, out U u)
{
    u = (U)t;
}

Do you expect that to work? What code do we generate that can handle:

M<object, string>(...); // explicit reference conversion
M<string, object>(...); // implicit reference conversion
M<int, short>(...); // explicit numeric conversion
M<short, int>(...); // implicit numeric conversion
M<int, object>(...); // boxing conversion
M<object, int>(...); // unboxing conversion
M<decimal?, int?>(...); // lifted conversion calling runtime helper method
// and so on; I could give you literally hundreds of different cases.

Basically we would have to emit code for the test that started the compiler again, did a full analysis of the expressions, and then emitted new code. We implemented that feature in C# 4; it’s called “dynamic” and if that’s the behaviour you want, you can feel free to use it.

We have none of these problems with as, because as only does three things. It does boxing conversions, unboxing conversions, and type tests, and we can easily generate code that does those three things.