I read C# spec and googled for it, but found nothing.
I am 99% sure there is no such feature like unit namespace directive in C#, but the question is: why? Are there idiomatic or technical reasons?
It is convenient, especially when most of our file consist of single namespace.
Is there any feature requests or proposals out there? Maybe we can make one?
// with unit namespace
namespace Foo;
class Bar { ... } // Class Bar declared inside Foo Namespace
struct Baz { ... } // Baz is inside Foo too
// without
namespace Foo {
class Bar { ... }
class Baz { ... }
}
Or maybe there is a way to re-export global symbols?
I mean first you declare everything inside global namespace, and then
publish public symbols in selected namespace?
The deep nesting of C# code is one of the most annoying thing for me.
I really enjoyed C++ ability to forward declare even nested classes and then define them without even 1 extra level of nesting.
Thanks for your time.
Logically, namespaces are block, just like classes, except that they can only contain types, not members.
Having a special syntax for this kind of block would be pointless and confusing.
First, this is not the same as the many highly upvoted questions on this exact topic unless I'm missing one of them. All of them point that the issue is I have a namespace with the same name as the class. This is not the case (but it was).
I started out creating a new console application called BatchResizer and put a couple of classes there, but then decided to move this into a class library, called BatchResizer.Components; I then renamed the original console application to BatchResizer.ConsoleRunner, changed all classes in that project to namespace BatchResizer.ConsoleRunner.[...], set the assembly name and default namespace to the same.
There is a class titled BatchResizer but there are no namespaces titled [...].BatchResizer in the project anymore, but when I do var batchResizer = new BatchResizer() I get the error that the namespace is used like a class. There are items named like BatchResizer.ConsoleRunner.[...] or BatchResizer.Components.[...], but nothing ending in BatchResizer.
I've tried "cleaning" and rebulding the project, deleting the .suo file, deleting the /bin folder of all projects in the solution, and I've went through every class in all related projects for namespace collisions.
BatchResizer is still a namespace name, though. If it's also the same name as a class, you'll have to be more explicit:
var batchResizer = new Components.BatchResizer();
You could also add a using statement within your namespace:
namespace BatchResizer.ConsoleRunner
{
using Components;
internal class Program
{
private static void Main(string[] args)
{
var batchResizer = new BatchResizer();
}
}
}
If you want to get a bit geeky, then the C# 5.0 spec has this to say:
9.2 Namespace declarations
...The qualified-identifier of a namespace-declaration may be a single identifier or a sequence of identifiers separated by “.” tokens. The latter form permits a program to define a nested namespace without lexically nesting several namespace declarations. For example,
namespace N1.N2
{
class A {}
class B {}
}
is semantically equivalent to
namespace N1
{
namespace N2
{
class A {}
class B {}
}
}
So even if, as you say, no class is declared in the namespace BatchResizer, BatchResizer is declared as a namespace.
First, this is not the same as the many highly upvoted questions on this exact topic unless I'm missing one of them. All of them point that the issue is I have a namespace with the same name as the class. This is not the case (but it was).
BatchResizer may not be a 'final' namespace, but it' still a namespace
Namespace : Foo.BatchResizer.Components
Foo.BatchResizer.ConsoleRunner
Class : Foo.BatchResizer
I'm running into an issue where I can't make a reference to a class in a different namespace. I have 2 classes:
namespace Foo
{
public class Class1 { ... }
}
namespace My.App.Foo
{
public class Class2
{
public void SomeMethod()
{
var x = new Foo.Class1; // compile error!
}
}
}
The compile error is:
The type or namespace name 'Class1'
does not exist in the namespace
'My.App.Foo'
In this situation, I can't seem to get Visual Studio to recognize that "Foo.Class1" refers to the first class. If I mouse-over "Foo", it shows that its trying to resolve that to "My.App.Foo.Class1"
If I put the line:
using Foo;
at the top of the .cs file that contains Class2, then it also resolves that to "My.App.Foo".
Is there some trick to referencing the right "Foo" namespace without just renaming the namespaces so they don't conflict? Both of these namespaces are in the same assembly.
You can use global:: to globally qualify a namespace: global::Foo.Class1 should work,.
You could also alias global::Foo to make things easier. At the top of your source file, below your using statements, add:
using AliasClass1=global::Foo.Class1;
Now you can use:
AliasClass1 c = new AliasClass1();
// and so on.
Of course, you can use a better name than AliasClass :-)
var x = new global::Foo.Class1();
In addition to LBushkin's answer, you might be interested in these articles by Eric Lippert :
Do not name a class the same as its namespace, Part One
Do not name a class the same as its namespace, Part Two
Do not name a class the same as its namespace, Part Three
Do not name a class the same as its namespace, Part Four
They are not directly related to your problem, but they give an interesting insight on naming strategies
Restatement of the question
I'm resurrecting this question because I just ran into this error again today, and I'm still utterly confused why the C# compiler bothers to check for collisions between namespaces and types in contexts where it makes no sense for a namespace to exist.
If I have...
public Foo MyFoo { get; set; }
...why would the compiler care that Foo is both a namespace and a type? Can you declare a property as a namespace instead of a type?
What is the logic behind the "namespace used like type" compiler error? What problem is this saving me from?
[And how do I tag Eric Lippert? :)]
Original Question
The problem
I have a project "Foo" with default namespace CompanyName.Foo. I have a database that's also called "Foo".
And when I run SqlMetal.exe on the database, it generates a class CompanyName.Foo.Models.Foo.
Then, when I attempt to create a property with this class as the type, like this...
using CompanyName.Foo.Models;
...
public Foo DataContext { get; set; }
...I get the error:
'CompanyName.Foo' is a 'namespace' but is used like a 'type'.
I am forced to do...
public CompanyName.Foo.Models.Foo Foo { get; set; } // :-(
Questions:
Why does this error occur? My property declaration doesn't contain CompanyName, so why is this a problem? Simply put: Foo != CompanyName.Foo. Also, just to be sure, I did a search of my entire solution for namespace Foo and came up with zero hits (if I had actually used a namespace Foo, I could understand getting an error).
[answered] Is there any way around fully qualifying Foo every time I want to use it?
[answered] Is there any way to get SqlMetal to name the class anything other than Foo (w/o changing the name of my database)? I can change the namespace using a switch, but I don't know of a way to change the actual class name.
Update
Still seeking an answer to (1).
O.K.W. nailed (2) & (3).
Usings
A request was made for all my using statements:
using System;
using System.ComponentModel;
using System.Data.Linq;
using System.Linq;
using MyCompany.Foo.Models;
how do I tag Eric Lippert?
If you have something you want brought to my attention you can use the "contact" link on my blog.
I'm still utterly confused why the C# compiler bothers to check for collisions between namespaces and types in contexts where it makes no sense for a namespace to exist.
Indeed, the rules here are tricky. Coincidentally, two weeks ago I wrote and posted a series of blog articles about some of these issues closely related to this very issue; they'll actually go live in early March. Watch the blog for details.
UPDATE: The articles mentioned above are here:
Link
Why does this error occur?
Let me rephrase the question into several questions.
What sections of the specification justify the production of this error?
I think that's already been covered satisfactorily in other answers. The type resolution algorithm is extremely well-specified. But just to sum up: being inside something of the right name "binds more tightly" than using something of the right name from the outside. When you say:
using XYZ;
namespace ABC.DEF
{
class GHI : DEF { }
}
that is the same as
using XYZ;
namespace ABC
{
namespace DEF
{
class GHI : DEF { }
}
}
So now we must determine the meaning of DEF. We go from inside to outside. Is there a type parameter of GHI called DEF? No. Look at the container. Is there a member of DEF called DEF? No. Look at the container. Is there a member of ABC called DEF? YES. We're done; we have determined the meaning of DEF, it is a namespace. We discover the meaning of DEF before we ask "does XYZ have a member DEF?"
What design principles influence this design?
One design principle is "names mean the same thing no matter how you use them". The language does not 100% obey this principle; there are situations in which the same name can be used to refer to two different things in the same code. But in general, we strive for a situation where when you see "Foo" two times in the same context, it means the same thing. (See my article on The Color Color Problem for some details on this, as well as my articles on identifying violations of the "simple name" rules.)
One design principle is "no backtracking". We do not ever say in C# "I see that you used a name to refer to something that is not legal to refer to in this context. Let me abandon the result of name binding and start over, looking for something that might work."
A larger principle that underlies the "no backtracking" principle is that C# is not a "guess what the user meant" language. You wrote a program where the best possible binding of an identifier identified a namespace when a type was expected. There are two possibilities. Possibility one: you've made an error that you want to be told about so that you can take action to correct it. Possibility two: you meant for a less-good binding to be the one we choose, and so we should guess from amongst all the possible less-good bindings to figure out which one you probably meant.
That's a good design principle in languages like JScript -- JScript is all about muddling on through when the developer does something crazy. C# is not that kind of language; the feedback we get loud and clear from our developers is tell me when something is broken so I can fix it.
The thing about "no backtracking" is that it makes the language much easier to understand. Suppose you have something like this mess:
namespace XYZ.DEF
{
public class GHI {}
}
namespace QRS.DEF.GHI
{
public class JKL { }
}
...
using QRS;
namespace TUV
{
using XYZ;
namespace ABC
{
namespace DEF
{
class GHI { }
class MNO : DEF.GHI.JKL { }
}
}
}
Work out the base type of MNO. With no backtracking we say "DEF is ABC.DEF". Therefore GHI is ABC.DEF.GHI. Therefore JKL is ABC.DEF.GHI.JKL, which does not exist, error. You must fix the error by giving a type name that lets the compiler identify which DEF you meant.
If we had backtracking, what would we have to do? We get that error, and then we backtrack. Does XYZ contain a DEF? Yes. Does it contain a GHI? Yes. Does it contain a JKL? No. Backtrack again. Does QRS contain an DEF.GHI.JKL? Yes.
That works, but can we logically conclude from the fact that it works that it is the one the user meant?
Who the heck knows in this crazy siutation? We got all kinds of good bindings in there that then went bad very late in the game. The idea that we stumbled upon the desired answer after going down many blind alleys seems highly suspect.
The correct thing to do here is not to backtrack multiple times and try out all kinds of worse bindings for every stage of the lookup. The correct thing to do is to say "buddy, the best possible match for this lookup gives nonsensical results; give me something less ambiguous to work with here please."
An unfortunate fact about writing a language where the compiler by design complains loudly if the best match is something that doesn't work, is that developers frequently say "well, sure, in general I want the compiler to point out all my mistakes -- or, rather, all my coworker's mistakes. But for this specific case, I know what I am doing, so please, compiler, do what I mean, not what I say."
Trouble is, you can't have it both ways. You can't have both a compiler that both enforces rigid rules that make it highly likely that suspicious code will be aggressively identified as erroneous and allow crazy code via compiler heuristics that figure out "what I really meant" when you write something that the compiler quite rightly sees as ambiguous or wrong.
For an object lesson in how lots of pro devs vehemently dislike the effects of a language design that aggressively identifies errors rather than guessing that the developer meant for the worse result to be chosen, see the 116 comments to this article on a minor and rather unimportant aspect of overload resolution:
(Note that I am no longer responding to comments on this issue; I've explained my position over ten times. If all those explanations are not convincing, that's because I'm not a very good convincer.)
And finally, if you really want to test your understanding of how the name resolution rules work in C#, try out this little puzzle. Almost everyone gets it wrong, or gets it right for the wrong reasons. The answer is here.
The clash is between namespace CompanyName.Foo and CompanyName.Foo.Models.Foo, and not Foo. I'm not exactly sure how/why the compiler can't distinguish both though.
You can try using namespace alias to shorten full qualifying Foo
e.g. using coyModels = CompanyName.Foo.Models
From the reference, seems like you can use /context:<type> and /namespace:<name> to specify the data context class (instead of using table name) and namespace.
C# compiler doesn't compile when there is an ambiguity between a class and a namespace with the same name. Unfortunately you just have to namespace the class explicitly or rename the database. In your case the compiler didn't even get to the conflict, it died after resolving Foo as a namespace.
Whenever you have something like this:
using CompanyName.Foo.Models;
namespace CompanyName.Foo {
class Test {
public Foo Model { get; set; } // error CS0118: 'CompanyName.Foo' is a 'namespace' but is used like a 'type'
public Foo1 Model { get; set; } //OK
}
}
namespace CompanyName.Foo.Models {
class Foo1 {
}
class Foo {
}
}
What actually happens is every preceeding level of the namespace is implicitly imported at each level. This makes sense since the nested namespace syntax using dot is the same as nesting namespaces:
namespace CompanyName {
using CompanyName; //<--using1 - Implicit using, since we should be able to access anything within CompanyName implicitly.
namespace Foo {
using CompanyName.Foo; //<-- using2 Same as above
class Test {
public Foo Model { get; set; } //At this stage due to using1 Foo is actually CompanyName.Foo, hence the compiler error
}
}
}
So inside class Test there are two implicit usings:
using CompanyName;
using CompanyName.Foo;
Hence Foo is resolved to the namespace hence the error.
EDIT Good point. I've dug this up from MSDN:
The meaning of a namespace-or-type-name is determined as follows:
If the namespace-or-type-name consists of a single identifier:
If the namespace-or-type-name appears within
the body of a class or struct
declaration, then starting with that
class or struct declaration and
continuing with each enclosing class
or struct declaration (if any), if a
member with the given name exists, is
accessible, and denotes a type, then
the namespace-or-type-name refers to
that member. Note that non-type
members (constants, fields, methods,
properties, indexers, operators,
instance constructors, destructors,
and static constructors) are ignored
when determining the meaning of a
namespace-or-type-name.
Otherwise, starting with the namespace in which the
namespace-or-type-name occurs,
continuing with each enclosing
namespace (if any), and ending with
the global namespace, the following
steps are evaluated until an entity is
located:
If the namespace contains a namespace member with the given
name, then the namespace-or-type-name
refers to that member and, depending
on the member, is classified as a
namespace or a type.
Otherwise, if the namespace has a corresponding
namespace declaration enclosing the
location where the
namespace-or-type-name occurs, then:
If the namespace declaration contains a
using-alias-directive that associates
the given name with an imported
namespace or type, then the
namespace-or-type-name refers to that
namespace or type.
Otherwise, if the namespaces imported by the
using-namespace-directives of the
namespace declaration contain exactly
one type with the given name, then the
namespace-or-type-name refers to that
type.
...
(Bolding is mine) This means that when resolving Foo, matching it against CompanyName.Foo (first bold bit) happens before matching it against the using directive(second bold build).
why can't you just do
using CompanyName.Foo;
...
public Models.Foo DataContext { get; set; }
I had this issue pop up when I was referencing a class in a separate class library, where its type had the same name as the root of the namespace. Initially, when referencing this type in a separate console app project, there was no problem, everything compiled fine. However the reference from a Windows Service project was generating the is a 'namespace' but is used like a 'type'. message. Turns out the Windows Service Project had its Target Framework set to ".NET Framework 4 Client Profile". Changing this to ".NET Framework 4" eliminated the error. Hopefully this helps someone in a similar situation.
I am new to c# and I came into contact with this error upon decompiling a c# application, saving as a project, the attempting to immediately recompile... why the application was able to compile in the first place is beyond me.. however... the problem and solution is quite simple: by default, upon adding a new class, c# uses the same name for a namespace as it does for the class within the namespace!!!!! This is bad because without some hidden identifier explicitly telling which (namespace or type) you are referring to, the compiler can't tell the difference!!!!! doh! way to go c#!!!! ... THE SOLUTION: Instead of renaming a thousand things and double checking all corrections, run the project, when you have the list of errors in front of you, click each in turn to go to each problem. Once at the "foo" in question type a dot (.) after said "foo" such that it displays: foo. .. this should bring up the menu of classes contained within. In this list, double-click "foo" (or just retype the name) changing the original "foo" to "foo.foo" ... Do this for each error and problem solved!!! Voila!!!! I did this to an entire application with complex names, and it worked great! Happy coding! - Tim H.
Because you've used dot notation to separate Company and Foo, you are implicitly creating a Company namespace, with a nested Foo namespace, not Company.Foo as you believe.
That's why this doesn't work:
namespace Company.Foo
{
}
namespace Company.Foo.Models
{
public class TestClass {
public Foo DataContext { get; set; }
}
}
The closest thing to Foo is the nested Foo namespace in the Company namespace. You can however do this:
using Company.Foo;
using Company.Foo.Models;
namespace Company.Foo
{
class Program {
public static void Main() {}
}
}
namespace Company.Foo.Models
{
public class Foo { }
}
public class DataContextClass
{
public Foo DataContext { get; set; } /* Foo here is automatically resolved to Company.Foo.Models.Foo */
}
Edit
Igor said the same thing, but was more technical.
This also happens if you generate unit tests when you have a namespace and a class with the same name. Which you should never do as explained by Eric Lippert here:
http://blogs.msdn.com/b/ericlippert/archive/2010/03/09/do-not-name-a-class-the-same-as-its-namespace-part-one.aspx
I have a problem here. I have a class library namely ClLib, and has the following 8 cs files.
Express (parent class)
- 1E1
- 1E2
- 1E3
Normal (parent class)
- 1N1
- 1N2
- 1N3
Also, a method is also included in each 8 class file. For example,
public class 1E1:Express
{
public int subtractNumExp1E1(int firstNum, int secNum)
{
return firstNum - secNum;
}
}
The 1E1:Express is used to display the relationship between the parent class(Express) and subclass (1E1).
I have also created a Windows App in Visual Studio, and I need to create a directive like "using ClLib.Express.1E1". How do I go about it? I am stuck in this situation whereby when I want to change the namespace of 1E1 from "namespace ClLib" to "namespace ClLib.Express.1E1". An error occurs, as the namespace ClLib has already contain one definition for Express.
A great thanks and appreciation in advance for all kind souls who are willing to help me:)
You can't have a type and a namespace with the same name ("CILib.Express").
If you want a CILib.Express namespace, the simplest option is to move/rename the CILib.Express type.
You can, however, also have nested types. You could, for example, have:
namespace CILib {
public class Express {
public class SomeType {}
}
}
If separate files are a concern:
file 1:
namespace CILib {
public partial class Express {
// "Express" code
}
}
file 2:
namespace CILib {
public partial class Express {
public class SomeType {
// "SomeType" code
}
}
}
However, using directives only relate to namespaces; I don't think you could have using CILib.Express, as that is a type not a namespace.
In that case you just have to delete the namespace ClLib and make sure that there is exactly one copy of ClLib.Express.1E1
P/S: Are you sure that the proper namespace is ClLib.Express.1E1? 1E1 is a cs file, and it's usually a class. So you don't have to write using ClLib.Express, that would be sufficient.
I think there is confusion around the namespace and base class.
Namespace: ClLib.Express
Class: public class 1E1:Express
Rename your base class to ExpressBase to differentiate it.
If you need to reference the namespace:
using ClLib.Express
Hope I have understood your project structure!