Custom Compiler Warnings

Question

When using the ObsoleteAtribute in .Net it gives you compiler warnings telling you that the object/method/property is obsolete and somthing else should be used. I'm currently working on a project that requires a lot of refactoring an ex-employees code. I want to write a custom attribute that I can use to mark methods or properties that will generate compiler warnings that give messages that I write. Something like this

[MyAttribute("This code sux and should be looked at")]
public void DoEverything()
{
}

<MyAttribute("This code sux and should be looked at")>
Public Sub DoEverything()
End Sub

I want this to generate a compiler warning that says, "This code sux and should be looked at". I know how to create a custom attribute, the question is how do I cause it to generate compiler warnings in visual studio.

Answer 1

Update

This is now possible with Roslyn (Visual Studio 2015). You can build a code analyzer to check for a custom attribute

---

I don't believe it's possible. ObsoleteAttribute is treated specially by the compiler and is defined in the C# standard. Why on earth is ObsoleteAttribute not acceptable? It seems to me like this is precisely the situation it was designed for, and achieves precisely what you require!

Also note that Visual Studio picks up the warnings generated by ObsoleteAttribute on the fly too, which is very useful.

Don't mean to be unhelpful, just wondering why you're not keen on using it...

Unfortunately ObsoleteAttribute is sealed (probably partly due to the special treatment) hence you can't subclass your own attribute from it.

From the C# standard:-

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.

Doesn't that sum up your needs?... you're not going to do better than that I don't think.

Answer 2

Don't know if this will work but it's worth a try.

You can't extend Obsolete, because its final, but maybe you can create your own attribute, and mark that class as obsolete like this:

[Obsolete("Should be refactored")]
public class MustRefactor: System.Attribute{}

Then when you mark your methods with the "MustRefactor" attribute, the compile warnings might show.

I said "maybe" and "might" because I haven't tried this. Please tell me if it doesn't work so I'll remove the answer.

Regards!

UPDATE: Tested it. It generates a compile time warning, but the error message looks funny, you should see it for yourself and choose. This is very close to what you wanted to achieve.

UPDATE2: With this code It generates this warnings (not very nice, but I don't think there's something better).

public class User
{
    private String userName;

    [TooManyArgs] // Will show warning: Try removing some arguments
    public User(String userName)
    {
        this.userName = userName;   
    }

    public String UserName
    {
        get { return userName; }
    }
    [MustRefactor] // will show warning: Refactor is needed Here
    public override string ToString()
    {
        return "User: " + userName;
    }
}
[Obsolete("Refactor is needed Here")]
public class MustRefactor : System.Attribute
{

}
[Obsolete("Try removing some arguments")]
public class TooManyArgs : System.Attribute
{

}

Answer 3

In some compilers you can use #warning to issue a warning:

#warning "Do not use ABC, which is deprecated. Use XYZ instead."

In Microsoft compilers, you can typically use the message pragma:

#pragma message ( "text" )

You mentioned .Net, but didn't specify whether you were programming with C/C++ or C#. If you're programming in C#, than you should know that C# supports the #warning format.

Answer 4

We're currently in the middle of a lot of refactoring where we couldn't fix everything right away. We just use the #warning preproc command where we need to go back and look at code. It shows up in the compiler output. I don't think you can put it on a method, but you could put it just inside the method, and it's still easy to find.

public void DoEverything() {
   #warning "This code sucks"
}

Answer 5

In VS 2008 (+sp1) #warnings don't show properly in Error List after Clean Soultion & Rebuild Solution, no all of them. Some Warnings are showed in the Error List only after I open particular class file. So I was forced to use custom attribute:

[Obsolete("Mapping ToDo")]
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Property)]
public class MappingToDo : System.Attribute
{
    public string Comment = "";

    public MappingToDo(string comment)
    {
        Comment = comment;
    }

    public MappingToDo()
    {}
}

So when I flag some code with it

[MappingToDo("Some comment")]
public class MembershipHour : Entity
{
    // .....
}

It produces warnings like this:

Namespace.MappingToDo is obsolete: 'Mapping ToDo'.

I can't change the text of the warning, 'Some comment' is not showed it Error List. But it will jump to proper place in file. So if you need to vary such warning messages, create various attributes.

Answer 6

What you are trying to do is a misuse of attributes. Instead use the Visual Studio Task List. You can enter a comment in your code like this:

//TODO:  This code sux and should be looked at
public class SuckyClass(){
  //TODO:  Do something really sucky here!
}

Then open View / Task List from the menu. The task list has two categories, user tasks and Comments. Switch to Comments and you will see all of your //Todo:'s there. Double clicking on a TODO will jump to the comment in your code.

Al

Answer 7

I don't think you can. As far as I know support for ObsoleteAttribute is essentially hardcoded into the C# compiler; you can't do anything similar directly.

What you might be able to do is use an MSBuild task (or a post-build event) that executes a custom tool against the just-compiled assembly. The custom tool would reflect over all types/methods in the assembly and consume your custom attribute, at which point it could print to System.Console's default or error TextWriters.

Answer 8

Looking at the source for ObsoleteAttribute, it doesn't look like it's doing anything special to generate a compiler warning, so I would tend to go with @technophile and say that it is hard-coded into the compiler. Is there a reason you don't want to just use ObsoleteAttribute to generate your warning messages?

Answer 9

There are several comments that suggest to insert warnings or pragma. Obsolete works in a very different way! Marking obsolete a function of a library L, the obsolete message raises when a program calls the function even if the caller program is not in the library L. Warning raises the message ONLY when L is compiled.

Answer 10

Here is the Roslyn Implementation, so you can create your own attributes that give warnings or errors on the fly.

I've create an attribute Type Called IdeMessage which will be the attribute which generates warnings:

[AttributeUsage(AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
public class IDEMessageAttribute : Attribute
{
    public string Message;

    public IDEMessageAttribute(string message);
}

In order to do this you need to install the Roslyn SDK first and start a new VSIX project with analyzer. I've omitted some of the less relevant piece like the messages, you can figure out how to do that. In your analyzer you do this

public override void Initialize(AnalysisContext context)
{
    context.RegisterSyntaxNodeAction(AnalyzerInvocation, SyntaxKind.InvocationExpression);
}

private static void AnalyzerInvocation(SyntaxNodeAnalysisContext context)
{
    var invocation = (InvocationExpressionSyntax)context.Node;

    var methodDeclaration = (context.SemanticModel.GetSymbolInfo(invocation, context.CancellationToken).Symbol as IMethodSymbol);

    //There are several reason why this may be null e.g invoking a delegate
    if (null == methodDeclaration)
    {
        return;
    }

    var methodAttributes = methodDeclaration.GetAttributes();
    var attributeData = methodAttributes.FirstOrDefault(attr => IsIDEMessageAttribute(context.SemanticModel, attr, typeof(IDEMessageAttribute)));
    if(null == attributeData)
    {
        return;
    }

    var message = GetMessage(attributeData); 
    var diagnostic = Diagnostic.Create(Rule, invocation.GetLocation(), methodDeclaration.Name, message);
    context.ReportDiagnostic(diagnostic);
}

static bool IsIDEMessageAttribute(SemanticModel semanticModel, AttributeData attribute, Type desiredAttributeType)
{
    var desiredTypeNamedSymbol = semanticModel.Compilation.GetTypeByMetadataName(desiredAttributeType.FullName);

    var result = attribute.AttributeClass.Equals(desiredTypeNamedSymbol);
    return result;
}

static string GetMessage(AttributeData attribute)
{
    if (attribute.ConstructorArguments.Length < 1)
    {
        return "This method is obsolete";
    }

    return (attribute.ConstructorArguments[0].Value as string);
}

There are no CodeFixProvider for this you can remove it from the solution.