I have a functionality in my code, but it will be available from next version. Is there any tags to notify that it is not in use, but will be available in future. I just have one way ie, [Obsolete] with custom message. Expecting other than this.Thanks in advance.
If it is not available in the current version, don't make it part of the public API. Don't expose stuff that shouldn't be used by consumers of your library.
And even when it's not a library, don't add code that you don't yet need. That violates the YAGNI principle. Add the code only when it is actually needed.
You can create throw a CustomError message from that method saying "This method is not available for the current version. It is for future" something like that.
Or else you can make the method as Obselete and specify the reason in the message.
[Obsolete("I have written this method for future use. Do not use it in the current version.")]
Judging from your question, I assume you want to generate a compiler warning when someone attempts to call this method, stating that it has not yet been implemented.
ObsoleteAttribute is hard coded into the C# compiler as something that should generate a warning message. Therefore there is no way to mark a method as 'to be implemented in the future' without modifying the compiler itself.
From the C# Spec:
The attribute Obsolete is used to mark types and members of types that
should no longer be used.
If a program uses a type or member that is decorated with the Obsolete attribute, the compiler issues a warning or an error.
Specifically, the compiler issues a warning if no error parameter is
provided, or if the error parameter is provided and has the value
false. The compiler issues an error if the error parameter is
specified and has the value true.
I suggest to not expose methods that are not yet implemented, since it makes no sense to use them. If this is part of a team development effort, specifying API contracts before developing specific modules, you should consider throwing a NotImplementedException or provide a dummy implementation.
Reviving this question as there is now a real solution thanks to Roslyn: Code analyzers.
Just add this Nuget package to your project and you'll be able to create attributes that will be picked up by the compiler!
The GitHub repo readme has the instructions, and it's really simple.
Related
I would like to create an C# attribute that accepts a Type parameter.
[MyAttribute(typeof(FooBar))]
public class MyClass
{
Adding this attribute to a class should, among other things, tell Visual Studio implicitly that this class has a member variable of that type, with a name derived from the type.
For example, adding the attribute with the parameter typeof(FooBar) should implicitly create the member variable private FooBar _foobar;.
I would like to be able to access that member via autocomplection in the IDE, so doing this at runtime would be too late.
Is this possible at all? How could I achieve this?
Not without something that rewrites the IL after the compiler has done its thing.
Except for a short list of attributes built into .NET and specifically handled by the compiler, attributes doesn't impact code at all, they're just metadata stored in the final assembly.
One way would be to use something like Postsharp which does the very thing I describe above.
Basically, Postsharp and similar systems takes the final assembly produced by the .NET compiler apart, reads the metadata (attributes and other things) then rebuilds the assembly from the ground up, injecting more code depending on those attributes.
However, with stock .NET and C#, no, this is not possible.
As #MarcinJuraszek comments, this may be possible with Roslyn, the new compiler platform that Microsoft is going to launch. I have little experience with this so my answer is related to current stable releases, which includes .NET 4.5 and C# 5.0.
Microsoft.CSharp is required to use dynamic feature.
I understand there are binders, evaluators and helpers in the assembly.
But why it has to be language-specific?
Why Microsoft.CSharp and not Microsoft.Dynamic or System.Dynamic?
Please, explain.
Let's say we have d.x where d is dynamic.
C# compiler
1. applies C# language rules
2. gets "property or field access"
3. emits (figurally) Binder.GetPropertyOrField(d, "x")
Now, being asked to reference Microsoft.CSharp may make one think that language-agnostic binder can't handle this case, and C#-only something got its way through compilation and requires special library.
Compiler had a bad day?
To your first question, it is language-specific because it needs to be.
In C# you call a method with too many arguments and you get an error. In Javascript, the extra arguments are simply ignored. In C# you access a member that doesn't exist and get an error, while in Javascript you get undefined. Even if you discovered all these varying feature sets and put it all into System.Core, the next language fad of the month is sure to have some super neat feature that it wouldn't support. It's better to be flexible.
There is common code in .NET core, under the System.Dynamic and System.Runtime.CompilerServices namespaces. It just can't all be common.
And as for your second question, the need for the "special C# library" could of course be removed by transforming these language-specific behaviors inline, but why? That will needlessly bloat your IL code size. It is the same reasoning for you not writing your own Int32.Parse every time you need to read in a number.
One reason I can think of - Visual Basic.NET has had late binding in it from day one, primarily oriented around how it interoperates with COM IDispatch interfaces - so if they wanted a language agnostic binder, they'd have had to adopt the Visual Basic rules - which includes that member lookup only works with Public members.
Apparently, the C# designers didn't want to be so strict. You can call this class' DoStuff method from C# via a dynamic reference:
public class Class1
{
internal void DoStuff()
{
Console.WriteLine("Hello");
}
}
Whereas attempting to call the same via Visual Basic's Object results in a MissingMemberException at runtime.
So because the C# designers weren't the first to arrive at the late-binding party, they could either follow Visual Basic's lead or they could say "each language will have its own rules" - they went with the latter.
As shown here, attribute constructors are not called until you reflect to get the attribute values. However, as you may also know, you can only pass compile-time constant values to attribute constructors. Why is this? I think many people would much prefer to do something like this:
[MyAttribute(new MyClass(foo, bar, baz, jQuery)]
than passing a string (causing stringly typed code too!) with those values, turned into strings, and then relying on Regex to try and get the value instead of just using the actual value, and instead of using compile-time warnings/errors depending on exceptions that might be thrown somewhere that has nothing to do with the class except that a method that it called uses some attributes that were typed wrong.
What limitation caused this?
Attributes are part of metadata. You need to be able to reflect on metadata in an assembly without running code in that assembly.
Imagine for example that you are writing a compiler that needs to read attributes from an assembly in order to compile some source code. Do you really want the code in the referenced assembly to be loaded and executed? Do you want to put a requirement on compiler writers that they write compilers that can run arbitrary code in referenced assemblies during the compilation? Code that might crash, or go into infinite loops, or contact databases that the developer doesn't have permission to talk to? The number of awful scenarios is huge and we eliminate all of them by requiring that attributes be dead simple.
The issue is with the constructor arguments. They need to come from somewhere, they are not supplied by code that consumes the attribute. They must be supplied by the Reflection plumbing when it creates the attribute object by calling its constructor. For which it needs the constructor argument values.
This starts at compile time with the compiler parsing the attribute and recording the constructor arguments. It stores those argument values in the assembly metadata in a binary format. At issue then is that the runtime needs a highly standardized way to deserialize those values, one that preferably doesn't depend on any of the .NET classes that you'd normally use the de/serialize data. Because there's no guarantee that such classes are actually available at runtime, they won't be in a very trimmed version of .NET like the Micro Framework.
Even something as common as binary serialization with the BinaryFormatter class is troublesome, note how it requires the [Serializable] attribute on the class to allow it to do its job. Versioning would also be an enormous problem, clearly such a serializer class could never change for the risk of breaking attributes in old assemblies.
This is a rock and a hard place, solved by the CLS designers by heavily restricting the allowed types for an attribute constructor. They didn't leave much, just the simple values types, string, a simple one-dimensional array of them and Type. Never a problem deserializing them since their binary representation is simple and unambiguous. Quite a restriction but attributes can still be pretty expressive. The ultimate fallback is to use a string and decode that string in the constructor at runtime. Creating an object of MyClass isn't an issue, you can do so in the attribute constructor. You'll have to encode the arguments that this constructor needs however as properties of the attribute.
The probably most correct answer as to why you can only use constants for attributes is because the C#/BCL design team did not judge supporting anything else important enough to be added (i.e. not worth the effort).
When you build, the C# compiler will instantiate the attributes you have placed in your code and serialize them, so that they can be stored in the generated assembly. It was probably more important to ensure that attributes can be retrieved quickly and reliably than it was to support more complex scenarios.
Also, code that fails because some attribute property value is wrong is much easier to debug than some framework-internal deserialization error. Consider what would happen if the class definition for MyClass was defined in an external assembly - you compile and embed one version, then update the class definition for MyClass and run your application: boom!
On the other hand, it's seriously frustrating that DateTime instances are not constants.
What limitation caused this?
The reason it isn't possible to do what you describe is probably not caused by any limitation, but it's purely a language design decision. Basically, when designing the language they said "this should be possible but not this". If they really wanted this to be possible, the "limitations" would have been dealt with and this would be possible. I don't know the specific reasoning behind this decision though.
/.../ passing a string (causing stringly typed code too!) with those values, turned into strings, and then relying on Regex to try and get the value instead of just using the actual value /.../
I have been in similar situations. I sometimes wanted to use attributes with lambda expressions to implement something in a functional way. But after all, c# is not a functional language, and if I wrote the code in a non-functional way I haven't had the need for such attributes.
In short, I think like this: If I want to develop this in a functional way, I should use a functional language like f#. Now I use c# and I do it in a non-functional way, and then I don't need such attributes.
Perhaps you should simply reconsider your design and not use the attributes like you currently do.
UPDATE 1:
I claimed c# is not a functional language, but that is a subjective view and there is no rigourous definition of "Functional Language". I agree with the Adam Wright, "/.../ As such, I wouldn't class C# as functional in general discussion - it is, at best, multi-paradigm, with some functional flavour." at Why is C# a functional programmming language?
UPDATE 2:
I found this post by Jon Skeet: https://stackoverflow.com/a/294259/1105687 It regards not allowing generic attribute types, but the reasoning could be similar in this case:
Answer from Eric Lippert (paraphrased): no particular reason, except
to avoid complexity in both the language and compiler for a use case
which doesn't add much value.
Can I restrict classes from a specific namespace from referencing classes in another specific namespace? Both namespaces exist in the same .NET assembly.
Example:
namespace LegacyCode
{
class LegacyClass { ... }
}
namespace NewCode
{
class NewClass {...}
}
I do not want classes from 'NewCode' to be able to reference classes in 'LegacyCode'.
Options:
Have different assemblies (makes deployment harder, build takes longer)
Using a tool like NDetect (costs money!)
Does anyone have any other ideas?
Consider marking the classes with the Obsolete attribute. This will cause any code that isn't itself marked as 'Obsolete' to generate a warning during compilation.
Enable 'Treat warnings as errors' setting on the 'Build' tab of the project file to cause this warning to fail compilation with an error instead.
Edit:
Agree that seperate assemblies is a good strategy to facilitate fading out this code. This won't stop people referring to it though. The obsolete attribute makes it clear that this code is, um, obsolete.
Edit #2:
Thanks to Dan Tao for pointing out the overloaded constructor of the Obsolete attribute. This means you can enforce whether usage of a something should be treated as an error or not, without having to enable treat warnings as errors. There is also usefully the option to specify a message instructing the user of a workaround. This message is displayed during compilation in the error/warning.
Document the design, talk to people, review code. Don't try to throw technology at people problems. (The review part can become more effective with tools like NDetect, though.)
If you really need the isolation of design changes, go for separate assemblies: that's the intended design mechanism. But be sure you have a reasonable versioning scheme both for the interface and the implementation.
I think separate assemblies are the only possible solution.
MS uses the System.ObsoleteAttribute attribute to mark obsolete/legacy code. This attribute provides an ctor that creates a compiler error. Though, I'd use this if there are not too many legacy classes.
As others have said, use the obsolete attribute (Even if you have to rename it).
But go one step further. DELETE ANY Legacy method that is NO longer used as soon as possible. This will prevent someone from using it later. You should start to see the Compiler warnings due to the obsolete attributes to drop over time.
You might even make it a daily one hour long test to eliminate as many compiler warnings as you can... Maybe you pitch in to buy the daily winner a beer (or soft drink..;) after work.
I have some classes, which have several methods which I don't really want to be there, but are there simply because the XML Serializer needs them. Is there anyway to generate compile-time errors/warnings if they get called from user-code?
I am aware that I can implement IXmlSerializable, and I am also aware that I can separate out the classes into purely data storage classes, however, I am not asking a question about how I should design such a system, I am simply asking if there is a way to generate compile-time errors/warnings if they are called by anything that is not the XML serializer...
You can add
[Obsolete]
to the method. The IsError property of ObsoleteAttribute controls whether an error or warning is generated, and you can provide an explanatory message too.
You could decorate the members in question with the ObsoleteAttribute. Its intention is a bit different, but it will generate compiler warnings (or errors) when called from user code.
You can hide the methods from users intellisense using the [EditorBrowsable] attribute, and from property designer using [Browsable], attribute.
I don't recommend using the [ObsoleteAttribute], because it conveys a different meaning to what method state actually is. Instead use a comment indicating that the method should not be used from user code.
Also keep in mind that there are lot's of users that compile their code with threat warnings as errors, which will make impossible for them to compile valid code, in this case.