What is this thing you call a "type"? Part Two

re: What is this thing you call a "type"? Part Two

Vlad — Sun, 20 Nov 2011 21:46:14 GMT

@Eric: I've just asked a question on SO about the type of null literal, which seems to be quite close to the topic of this discussion -- maybe you could comment there? stackoverflow.com/.../what-is-the-type-of-null-literal

re: What is this thing you call a "type"? Part Two

Michael — Thu, 22 Sep 2011 20:04:14 GMT

I always used the following to define "type" in a programming context:

A type is a schema and associated set of rules for interpreting the meaning behind the bits of a segment of memory.

re: What is this thing you call a "type"? Part Two

CarlD — Thu, 15 Sep 2011 19:33:16 GMT

@Marvin - That link is for WinRT which is a new runtime. It's not .NET 4.5.

re: What is this thing you call a "type"? Part Two

Marvin — Wed, 14 Sep 2011 23:57:53 GMT

I was looking at the notes for .Net 4.5 posted here: msdn.microsoft.com/.../br230302%28v=VS.85%29.aspx

I think I see massive breaking changes to System.Type. Is this line of posts somehow leading up to a discussion of those changes?

It is not. -- Eric

re: What is this thing you call a "type"? Part Two

Stuart — Tue, 13 Sep 2011 21:45:51 GMT

... tested those things. Apparently you can't - the "var" and the method call, respectively, are illegal. It's weird that default(Enumerable) is permitted and you can compare it against null, for example, but you can't store that value in a variable.

@DRBlaise - I don't think that anything that uses reflection is a valid counterpoint. The C# language spec defines what can be expressed in C#, not what the runtime and base class library support. So I dispute that your 1 and 2 are violations of the language spec (although it might still be a good idea if the runtime disallowed them). I agree that your 3, 4 and 5 seem like compiler bugs.

It is weird that you can get the default value of a static class! I'll mention that to Mads; it seems like an oversight that it was not made illegal. Thanks! -- Eric

re: What is this thing you call a "type"? Part Two

Stuart — Tue, 13 Sep 2011 21:39:29 GMT

So can you do "var x = default(Enumerable)" and get a declared variable of that type?

Or perhaps create a method void Foo<T>(T t) {...} and then call it with default(Enumerable) in order to bind Enumerable to a generic type parameter?

re: What is this thing you call a "type"? Part Two

DRBlaise — Tue, 13 Sep 2011 13:51:56 GMT

configurator: I did some more investigation that led me to the conclusion that a "static class" is just a regular type that is both abstract and sealed. It's base class is object, but it has no instance constructors. The 4.0 C# Spec section 10.1.1.3.1 does not match compiler behavior on several points: ".., it is an error for a static class to be used as a base class, a constituent type (§10.3.8) of a member, a generic type argument, or a type parameter constraint. Likewise, a static class cannot be used in an array type, a pointer type, a new expression, a cast expression, an is expression, an as expression, a sizeof expression, or a default value expression."

1. "generic type argument" - you can use reflection to create a valid generic type with a "static class" as the generic type.

2. "an array type" - you can use Array.CreateInstance or reflection to create a "static class" array and use SetValue and GetValue to reference the values in the Array which will always be null.

3. "is expression" - (x is Enumerable) is valid, but warns it is always false.

4. "as expression" - (x as Enumerable) is valid, but will always be null.

5. "default value expression" - default(Enumerable) is valid and will be null.

re: What is this thing you call a "type"? Part Two

configurator — Tue, 13 Sep 2011 12:29:44 GMT

DRBlaise: A static class is not a type. It's got a Type (which is why you can use typeof), but it's not a type (meaning you can't use it to describe a storage location).

re: What is this thing you call a "type"? Part Two

DRBlaise — Mon, 12 Sep 2011 13:36:25 GMT

I believe I found an exception to the 3rd type rule: If T is a type and n is a positive integer then "n-dimensional array of T" is also a type. A "static class" type can't be used as an array type. I was wondering if your definition of type is excluding "static class"?

The C# Specification has the ambiguous statement: "A static class cannot be instantiated, cannot be used as a type and can contain only static members." And yet a static class can be used in "typeof" and even "default". So is a "static class" a type or not?

re: What is this thing you call a "type"? Part Two

configurator — Sun, 11 Sep 2011 21:37:34 GMT

Do you have a link or reference to something which discusses such topologies? I find that really interesting and it's possibly quite useful as well!

Would an interface such as public interface IC<X> : IN<IN<IC<IC<X>>>> {} not be allowed in that case?