Skip to main content

ObsoleteAttribute and the Expression Compiler

I was going to post this as a question over at SO, but as I got to the end I realised a blog would probably be better (apart from the fact that only five people will read it!).

The project I'm working uses runtime objects that are produced with configuration-driven delegates built using Expression trees.

I'm doing a big functionality merge at the moment and have reached a class that, because of another that has grown, is now obsolete. So I want to make it 'properly' obsolete (generate compiler errors), go through the static codebase and change all the references.

The problem is that the class library is used, through Expressions, by a few other apps so I can't yet get rid of the class completely - I'll perhaps look at farming that job off to somebody else.

So I thought I'd just check whether the expression compiler honours the ObsoleteAttribute when marked as an error, and it doesn't:

public class UnitTest1
    [Obsolete("obsolete", true)]
    public class ObsoleteClass


    public void TestObsoletedClassViaExpression()
        Type obsoleteType = typeof(UnitTest1).GetNestedType("ObsoleteClass");

        Func<object> f =

        var o = f();

        Assert.AreEqual(obsoleteType, o.GetType());

In my case this is actually an advantage as it means the existing projects will carry on working with the updated codebase since the only reference to this type will be through dynamically generated code.

Of course once I finally remove the type those apps will no longer work - but I won't do that until they've all been updated. That said, it would be a lot easier to go through these apps an augment them to the replacement type if the Expression compiler *did* honour the ObsoleteAttribute - just follow the runtime errors (! - sorry I mean run the tests of course ;) ).

I understand, and have exploited, the fact that the Expression compiler is allowed to do things that aren't normal - skip visibility checks etc. I also understand that attributes such as ObsoleteAttribute require a compiler to honour them as, at the IL level, they are meaningless.

However, given Extression trees' ubiquity and growing importance, shouldn't its compiler honour (or have the option to honour) the ObsoleteAttribute? On the DLR, for example, this means that a language designer must code in reflection at every step to look for Obsolete(true) declarations if he/she wants to have this functionality in their language.

Like I say, it’s a tricky one because in this particular example it’s going to work in my favour – I’m just not sure it should.


Popular posts from this blog

Asp.Net 2 and 4 default application pool generates CS0016 IIS7.5

Before I start – if you’ve found a bunch of other articles about this around the net, tried the fixes that are mentioned and still not getting any joy – then read on – you might find this solves your problem. Earlier today I discovered that when I run any ASP.Net 2 or 4 application through IIS7.5 using the default application pools (which use ApplicationPoolIdentity) on Windows 2008 R2 x64 I get an error message similar to this: Server Error in '/MvcApplication31' Application. Compilation ErrorDescription: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.
Compiler Error Message: CS0016: Could not write to output file 'c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\mvcapplication31\222b4fe6\4e80a86\App_global.asax.clb4bsnc.dll' -- 'The directory name is invalid. '
Source Error:[No relevant source lines]

Serializing to attributes in WCF with DataContractSerializer

It’s a common problem – you want to return an object from a WCF service as XML, but you either want, or need, to deliver some or all of the property values as XML Attributes instead of XML Elements; but you can’t because the DataContractSerializer doesn’t support attributes (you’re most likely to have seen this StackOverflow QA if you’ve done a web search).  Most likely you’ve then migrated all your WCF service code to using the XmlSerializer (with all the XmlElement/XmlAttribute/XmlType attributes et al) – and you’ve cursed loudly.Well, I’m here to rescue you, because it is possible – and the answer to the problem is actually inferred from the MSDN article entitled ‘Types supported by the Data Contract Serializer’.The example I’m going to give is purely for illustration purposes only.  I don’t have a lot of time, so work with me!Create a new Asp.Net WCF service application, you can use Cassini as your web server (probably easier – otherwise you might have to enable Asp.Net compatibil…

Adding ‘Deny’ functionality to AuthorizeAttribute in Asp.Net Web API

For the web service project I’m working on at the moment I need to be able to treat authorization differently based on the hostname of the URL that requests are made through.To state more clearly – these web services will have a ‘sandbox’ mode in addition to the real mode, and the mode a request will operate under is determined as part of the controller-selection phase early in the Web API request lifecycle.  So, say that my web services will be hosted on; the sandbox will simply be note – a discussion of how this is implemented is entirely outside the scope of this article; but I’ll just say that I’ve developed an in-house multi-tenancy layer for both MVC 4 and Web API that allows us to define ‘brands’ and, under those, you can then redefine content, controllers, and even the DI container that is used.These services are going to require caller-level authentication for most operations via SCRAM Authentication (RFC 5802), and as such m…