C# expression evaluates to a namespace

Question

MSDN docs state "An expression is a fragment of code that can be evaluated to a single value, object, method, or namespace."

Could someone please explain what it means for an expression to evaluate to a namespace - how can that be?

edit: fixed typo

Answer 1

foo = System;

System evaluates to a namespace.

(Of course, this won't compile, because you can't assign a namespace to a variable, but you get the idea.)

Answer 2

This is how the grammar is defined. Look at:

System.String

is an expression containing a dot operator that operates on a couple different expressions. System alone is considered an expression. An expression can be as simple as a single identifier or literal (hint: it's defined recursively.)

Expressions (C# 3.5 spec section §7.1: Expression classifications)

An expression is classified as one of the following:
...
A namespace. An expression with this classification can only appear as the left hand side of a member-access (§7.5.4). In any other context, an expression classified as a namespace causes a compile-time error.

Not being able to use it as, say, an argument to a method doesn't disqualify it as being considered an expression.

Answer 3

You can use an expression like this:

using OutlookStuff = Microsoft.Office.Outlook.This.Is.Really.Long.Isnt.It;

and then be able to use the expression later in code. This can be useful when there is namespace contention like if you have a class like MailItem or something that is common across multiple namespaces.

So you can now do this:

var mailItem = new OutlookStuff.MailItem();

EDIT I think Mehrdad's answer is the more correct answer, but I thought this was worth noting.

Answer 4

On this page on MSDN it says:

However, although a namespace name is classified as an expression, it does not evaluate to a value and therefore can never be the final result of any expression. You cannot pass a namespace name to a method parameter, or use it in a new expression, or assign it to a variable. You can only use it as a sub-expression in a larger expression. The same is true for types (as distinct from System..::.Type objects), method group names (as distinct from specific methods), and event add and remove accessors.

So in reality you can't really do anything with a namespace being a expression it always work in the same way there is nothing dynamic about it that you can influence but for the parser a namespace has to be something :). The reason it is an expression is due to the grammar of the C# language that's used to parse the code during the compilation process. It consists out of statements, expressions, operators etc... so in the case of System.Guid yourGuid = System.Guid.NewGuid() the System part would be a expression containing a namespace, the . would be an opperator and the Guid would be a type to the C# parser.