Is it possible to make the following code valid in C# without changing my namespace or type name. I'm wondering if there is a trick or keyword around this problem?
namespace NS
{
public class Foo { }
}
namespace NS.Foo
{
public class Bar { }
}
The error I'm getting is "The namespace 'NS' already contains a definition for 'Foo'".
No - Imagine having a subclass in Foo called Bar:
namespace NS
{
public class Foo { public class Bar {} }
}
namespace NS.Foo
{
public class Bar { }
}
How can you ever tell the difference if one does: new NS.Foo.Bar()?
Essentially, no there isn't. Even if you could somehow resolve the immediate ambiguity, this problem goes all the way down: what happens if you declare a property of Foo named Bar?
Rename one of your Foos.
No - there's no way to disambiguate from NS.Foo the namespace and NS.Foo the class.
Related
Does anyone come across with some older C# code as below, what would the equivalent in order to over come the later compiler error "'car' is a 'namespace' but is used like a 'type'".
Many thanks in advance.
namespace something.car
{
public class Display : UserControl
{
private car _car; // comppiler error here
public car carConfig // comppiler error here
{
get
{
return this._car;
}
set
{
this._car = value;
}
}
}
}
Change your first line from namespace something.car to
namespace Something.SomethingElse
and make sure you have a class car defined somewhere which you use in lines private car _car; and public car carConfig.
As a side comment, it is quite common to spell namespaces and class names in Pascal notation, e. g. namespace Something and public class Car.
To be able to use it without changing the namespace name ("but for sure change the namespace name is the right way").
but you can use the file path for the class when you use it, so if the car class is namespace "SomenameSpace.Viechels"
then you could use like below to not have the above conflict:
SomenameSpace.Viechels.car _car;
SomenameSpace.Viechels.car _carConfig;
Let's say we have a base class Rectangle and a derived class Square:
namespace Shapes {
using System.Foo;
public class Rectangle {
public Rectangle(int l, int w){}
}
}
namespace Shapes {
public class Square : Rectangle
public Square(int l, int w){}
}
Does the Square class have to explicitly say that it is using System.Foo? I'm getting erratic results. In one project the using directives seem to be inherited and in a web application they aren't.
using statements, in this context, don't compile to code -- they are helpers to make your code read cleaner for others. As a result, they are not "inherited".
So, to answer your question, your Square class needs to reference System.Foo - either with a using statement, or by using a fully qualified class name.
A using statement will only propagate to the next set of closing braces (}) from the level it was declared on within the same the file.
//From File1.cs
using System.Baz;
namespace Example
{
using System.Foo;
//The using statement for Foo and Baz will be in effect here.
partial class Bar
{
//The using statement for Foo and Baz will be in effect here.
}
}
namespace Example
{
//The using statement for Baz will be in effect here but Foo will not.
partial class Bar
{
//The using statement for Baz will be in effect here but Foo will not.
}
}
//From File2.cs
namespace Example
{
//The using statement for Foo and Baz will NOT be in effect here.
partial class Bar
{
//The using statement for Foo and Baz will NOT be in effect here.
}
}
using directives are only shared if the classes are in the same file and they are not nested in the classes themselves like in your example.
For instance:
using System.Foo;
namespace N
{
class A {}
class B {}
}
If this is all in one file, A and B can both use Foo.
I think everyone is missing the point when it comes to using directives. They really have nothing to do with the class at all. using directives in a code file (.cs, .vb, etc...) are not part of the classes defined within the file. They are used by the compiler to resolve namespaces when compiling.
using System.Foo;
namespace Shapes {...
Importing should always be top most and not within a namespace. This will allow the entire structure of the class to rely on that import when needed.
I Have created two .cs files with namespaces ,classes and methods . I want to call the classes of one .cs file into another .cs file. Can u help me how to declare namespace and use the namespace so that i can call the classes of the preceding .cs file.
Please forgive if my explanation is not correct.
Suppose i have the following code.
ClassFile1
using system
namespace namespace1
{
class c1
{
Methods()
}
}
ClassFile2
using system
//here i need to declare the namespace1 .Can u help me how to declare namespace1 in this ClassFile2//
namespace namespace2
{
class c2
{
Methods()
}
}
You can reference the fully-qualified name of the class:
namespace SecondNamespace
{
public class SecondClass
{
private FirstNamespace.FirstClass someObject;
}
}
Or you can add a using directive to the file (note, this is at the file level, not the class level) to include a specific namespace when resolving type names:
using FirstNamespace;
namespace SecondNamespace
{
public class SecondClass
{
private FirstClass someObject;
}
}
Taken from here:
namespace SampleNamespace
{
class SampleClass
{
public void SampleMethod()
{
System.Console.WriteLine(
"SampleMethod inside SampleNamespace");
}
}
// Create a nested namespace, and define another class.
namespace NestedNamespace
{
class SampleClass
{
public void SampleMethod()
{
System.Console.WriteLine(
"SampleMethod inside NestedNamespace");
}
}
}
class Program
{
static void Main(string[] args)
{
// Displays "SampleMethod inside SampleNamespace."
SampleClass outer = new SampleClass();
outer.SampleMethod();
// Displays "SampleMethod inside SampleNamespace."
SampleNamespace.SampleClass outer2 = new SampleNamespace.SampleClass();
outer2.SampleMethod();
// Displays "SampleMethod inside NestedNamespace."
NestedNamespace.SampleClass inner = new NestedNamespace.SampleClass();
inner.SampleMethod();
}
}
}
Note also that sometimes in addition to the "using" entry (I'm not quite clear on how you app is structured, if it's all one project this is probably moot) you may also need to add the reference. Also not sure what environment you're using. From VSExpress while in the project/file that's the recipient click on Project - Add Reference, select solution and then select your namespace.
I have defined an assembly level attribute class FooAttribute like this:
namespace Bar
{
[System.AttributeUsage (System.AttributeTargets.Assembly, AllowMultiple=true)]
public sealed class FooAttribute : System.Attribute
{
public FooAttribute(string id, System.Type type)
{
// ...
}
}
}
and I use it to associate an id to classes, for instance:
[assembly: Bar.Foo ("MyClass", typeof (Bar.MyClass))]
namespace Bar
{
public class MyClass
{
private class Mystery { }
}
}
This all works fine. But what if I need to somehow reference the private class Mystery, defined in MyClass? Is this at all possible? Trying to reference it from the top-level [assembly: ...] directive does not work, as the type is not publicly visible:
[assembly: Bar.Foo ("Mystery", typeof (Bar.MyClass.Mystery))] // won't work
And trying to put the [assembly: ...] directive into MyClass in so that it could see Mystery is not legal, as [assembly: ...] must be defined at the top level:
namespace Bar
{
class MyClass
{
[assembly: FooAttribute (...)] // won't work either
...
}
}
There is a way to access internal types from outside of an assembly by declaring the user a friend of the assembly, but how about referencing private types inside an assembly? I guess it is not possible, and I just would have to declare Mystery to be internal instead, but I want to be sure I did not miss some subtlety.
Making it internal (which you already state you don't want to do) is the least effort approach. For the majority of code, allowing MyClass to expose (via a static property) the type instance (i.e. public static Type MysteryType { get { return typeof(Mystery); } } would work, but that won't work from an attribute (only constant values of a few basic types can be used).
The only alternative to internal, then, is to code it as a string literal, (i.e. [Foo("Bar.MyClass+Mystery")]) and use typeof(MyClass).Assembly.GetType(fullName) - but then you lose the compiler validation that typeof normally provides. (note also the + that the runtime uses to represent nested types, not . which is the C# representation)
Personally, I'd just make it internal.
Your assertions in your last paragraphs are correct. Your options would be to:
Make the nested class internal to enable typeof
or
Have an added constructor to FooAttribute which takes the fully qualified type name of the private nested class, and then uses reflection to get a System.Type representing it.
For instance:
public sealed class FooAttribute
{
public FooAttribute(string id, string typeName)
{
var type = Type.GetType(typeName);
// whatever the other ctor does with the System.Type...
}
}
usage:
[assembly: Foo("Bar", typeof(Bar))]
[assembly: Foo("Baz", "Foo.Bar+Baz, MyAssembly")]
namespace Foo
{
public class Bar
{
private class Baz
{
}
}
}
I've got something like this:
namespace n1
{
namespace n2
{
class foo{}
}
}
In other file I write:
using n1;
Why I can't type now something like:
n2.foo smthing;
And how to make something like this possibile?
This is a deliberate rule of C#. If you do this:
namespace Frobozz
{
namespace Magic
{
class Lamp {}
}
class Foo
{
Magic.Lamp myLamp; // Legal; Magic means Frobozz.Magic when inside Frobozz
}
}
That is legal. But this is not:
namespace Frobozz
{
namespace Magic
{
class Lamp {}
}
}
namespace Flathead
{
using Frobozz;
class Bar
{
Magic.Lamp myLamp; // Illegal; merely using Frobozz does not bring Magic into scope
}
}
The rule of C# that describes this is in section 7.6.2 of the C# 4 spec. This is a very confusing section; the bit you want is the paragraph near the end that says
Otherwise, if the namespaces imported by the using-namespace-directives of the namespace declaration contain exactly one type having name I...
The key point is that it says "exactly one type", not "exactly one type or namespace". We deliberately disallow you "slicing" a namespace name like this when you are outside of that namespace because it is potentially confusing. As others have said, if you want to do that sort of thing, fully qualify it once in a using-alias directive and then use the alias.
Use namespace aliases:
using n2 = n1.n2;
...
n2.foo something;
What is before the class name should be a complete name space (with/or other class name(s) for nested types). A truncated namespace will not work.
By design, namespaces are there to help you define scope.
Unless you fully qualify it, you will get the error you're seeing.
Assuming File1 has something like this:
namespace n1
{
namespace n2
{
class Foo { }
}
}
You can do this two ways:
Fully qualified using
File2 contents:
namespace n3
{
using n1.n2;
class TestClass
{
private Foo something;
}
}
Use a namespace alias
namespace n3
{
using n2 = n1.n2;
class TestClass
{
private n2.Foo something;
}
}
Section 9.4.2 paragraph 4 in the C# language specification explains this behavior explicitly:
A using-namespace-directive imports
the types contained in the given
namespace, but specifically does not
import nested namespaces.
It even goes on to give an example that is very similar to your own.
namespace N1.N2
{
class A {}
}
namespace N3
{
using N1;
class B: N2.A {} // Error, N2 unknown
}
Of course had you done this:
namespace n1
{
public class Example
{
public static void Main()
{
n2.Foo a; // This is legal.
}
}
}
This would compile because n2 is accessible since it is referenced from within an ancestor namespace block.
You cannot write this, as n2 is inside of n1. If you want to access the n2 namespace, you can try typing using n2 = n1.n2 at the beginning of your other file.
One way to do this is to declare your nested namespace as class.
You can have:
namespace NS1
{
public class NNS1
{
public class C { }
}
}
namespace NS2
{
public class NNS2
{
public class C { }
}
}
Then use your namespaces by:
using NS1;
using NS2;
class C
{
public C()
{
var c1 = new NNS1.C();
var c2 = new NNS2.C();
}
}
However, this cannot create union of namespaces. If you have:
namespace NS1
{
public class NNS
{
public class C1 { }
}
}
namespace NS2
{
public class NNS
{
public class C2 { }
}
}
And
using NS1;
using NS2;
class C
{
public C()
{
var c1 = new NNS.C1();
var c2 = new NNS.C2();
}
}
The compiler will complain about NNS for ambiguity, since it can refer to two class symbol in this context. I have tried to declare namespaces as interfaces, and use a class to implement them for creating union of namespaces. However, implementing an interface does not automatically import nested classes symbols in it.
I do not know why C# does not support reference of nested namespace, and I have no idea about the drawbacks for declaring namespaces as classes. Please comment if anyone knows.