I have a compiled class library with a root namespace Protege.MyLibrary.
It has a few root types, for example, CommonlyNamedType.
When I add the library to my consuming application, I'd like, for clarity in some situations, to be able to specify variables as:
using Protege;
...
MyLibrary.CommonlyNamedType oMyType = new MyLibrary.CommonlyNamedType;
rather than
using Protege.MyLibrary;
...
CommonlyNamedType oMyType = new CommonlyNamedType;
The former doesn't compile, indicating for the namespace Protege "Using directive is unnecessary", and that is can be removed.
This seems bizarre as I could go the other way and add additional namespaces, such as Protege.MyLibrary.AnotherNamespace.
I seem to be able to do this 100% okay in VB.NET - using either or both Imports Protege and/or Protege.MyLibrary and even optionally qualifying types with redundancy. But not in C#.NET.
I have had a good look around SO and other places and haven't seen an explanation for this behavior. Any ideas?
You can statically import a class like
using static System.IO.File;
It doesn't support for class, you need to use normal using for import the namespace.
I'm new and I don't know other way to explain so I posted my screenshot of project! Please help me to fix these errors... SCREENSHOT
You seem to be having a lot of problems with references and namespaces use.
First of all, you do not have a Card class defined. You only have a CardModel. Replace Card for CardModel and you will be good to go. Also, it seems you do not have a namespace declared on your class. Declare a namespace so you can use other classes in the same namespace (tipically the project name).
Second, if you are trying to use clases in another folder, you probably have to add the reference with the using keyword.
You're missing probably several using directives. Every class you write should be inside a 'namespace' You declare it after your using directives but before you start writing your classes, like this:
namespace WebShop.CardModel {
public class CardModel {
public string InsertCard(Card card){
And when you are working in the cardModel, unless Card is defined in the same namespace, you need:
using WebShop.Card;
Or whatever namespace you put Card in, that's what is throwing probably 99% of your errors, it is definitely the cause of all but one of the ones in the errors we can see in your screenshot.
I have the following two files:
IGlobalApiProvider.cs
using System.Collections.Generic;
using Vert.Slack;
namespace Vert.Interfaces
{
public interface IGlobalApiProvider
{
List<Im> ImList();
}
}
And the corresponding implementation: SlackApi.cs
using System.Collections.Generic;
using Vert.Interfaces;
namespace Vert.Slack
{
public class SlackApi : IGlobalApiProvider
{
public List<Im> ImList()
{
...
}
}
}
Now, Intellisense is telling me that when I use IM in IGlobalApiProvider it's resolving to Im, which is defined in a file named RtmStart.cs which has no namespace declaration. When I use IM in SlackApi.cs, it's resolving to Vert.Slack.Im which is defined in the Vert.Slack namespace in a file named Im.cs. The weird behavior alerted me to the redundant definition, so I removed it and things are working fine.
However, I'm confused about why Visual Studio behaved differently in these two ways. I can tell something was scanning for the class names in a different pattern in the two situations. I can also tell that being used in the same namespace vs being used in a class that uses the namespace seems to be the trigger. What I don't know is what mechanism controls the logic behind this behavior.
Can anyone shed light on this?
Everything you see is contained in Vert.dll, which consists of one project, Vert.csproj
Link to the four files mentioned in this post as they existed at the time of writing.
This has to do with the difference between the global and Vert.Slack namespaces.
The compiler looks for the most explicit namespace with the proper class defined.
In this example, when the compiler looks for the definition of Im in IGlobalInterfaceProvider.cs, there is no namespace defined (or used) in this file that contains the class, but Im is also defined in this file - which is declared in the global namespace.
When the compiler looks for the definition of Im in SlackApi.cs, Im is found in the explicit Vert.Slack namespace, and utilizes that class.
The answer here is a similar topic and may provide more insight.
This may be related to the fact that your namespaces are in the wrong place ;-)
http://www.stylecop.com/docs/SA1200.html
This answer here gives a good explanation: Should 'using' statements be inside or outside the namespace?
Is there any difference between:
namespace Outer.Inner
{
}
And
namespace Outer
{
namespace Inner
{
}
}
in C#?
Assuming you don't put any other declarations or using directives in the Outer namespace, there's no difference at all.
Given that you would very very rarely declare members in multiple namespaces within a single file, I'd suggest using the first form - aside from anything else it saves a level of indentation. Note that "brace at the start of a new line" is a more conventional bracing style for C# though:
namespace Outer.Inner
{
...
}
No difference whatsoever, those are the same thing, however the first one is more common.
No, but the first variant is the most used in c# code.
The second variant is what you'd have to write in C++ and I'm not sure I ever saw it in real c# code yet.
They both are the same as noted by other answers and per documentation here.
Also the fact the one namespace is defined inside another doesn't really mean that it has any special relationship with the parent or that it brings any particular features in relation with parent or that it depends on parent namespace. There is no relationship other than it's just defined inside parent (in context of using statement to bring them in).
As such, it is important to note that if you include the parent namespace, it doesn't meant the child names are now accessible. See this answer which explains this.
So despite the fact they are nested, for all practically purposes, each namespace is its own independent thing, no matter if its the top most parent or a child. You will always need to use fully qualified name of the nested namespace if you want to include it.
As such the dot(.) almost becomes part of the namespace much like you can have a gmail id as first.last#gmail.com
So why nested?
It really is just to logically place them as if they are in a hierarchy for benefit of organizing them but you can think of the dot in the name as part of the namespace (for all practical uses).
for example you can define this:
namespace Outer.Inner
{
class MyClass{}
}
But if you wanted, you can rename the above to the following for all practical purposes:
namespace Solar
{
class MyClass{}
}
In both cases if you want to use MyClass, you will need to include fully qualified name of the namespace where the class is defined which is:
using Outer.Inner;
or
using Solar;
This can best be understood and summarized in looking at C# library namespaces.
using System;
using System.Threading;
using System.Threading.Tasks;
So although, the above three lines contain nested namespaces, each line really is fully qualified name to a unique namespace (as if the dot is part of the namespace) and that should be useful to know in terms of the differences and usage.
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