Syntax for applying an attribute to an anonymous method?

Question

This is related to the question about return type attributes and anonymous classes, but then for anonymous methods (or lambdas), but as far I could find this exact question does not seem to be on stackoverflow yet.

In code for business entities that we generate using CodeSmith we now have [DebuggerNonUserCode] attributes, so they don't count in code coverage results. Unfortunately, the generated code uses anonymous methods that now still show up in code coverage with names like Class.<>c__DisplayClass3c because of the way these are actually handled by the compiler.

Quick code example, with names and types changed to protect the innocent, so to speak:

    public delegate T ReturnSomething<T>();

    public static T SafeCall<T>(T whenNotSupported, ReturnSomething<T> method)
    {
        T result;
        try
        {
            result = method();
        }
        catch (NotSupportedException)
        {
            result = whenNotSupported;
        }
        return result;
    }

    public static void CodeExample()
    {
        string foo = SafeCall<string>("OOPS!", delegate
        {
            //throw new NotSupportedException();
            return "Ok";
        });
    }

Is there a way to get [DebuggerNonUserCode] attributes on these methods so we could get rid of the name-mangled anonymous method names from our generated code from our code coverage results? Or do we need to rewrite that generated code to no longer use anonymous methods?

Putting the [DebuggerNonUserCode] on the method parameter of the SafeCall method definition (before the ReturnSomething<T> parameter type) does not compile and maybe would not do exactly what we would like if it would. The following also does not compile:

    public static void CodeExample()
    {
        string foo = SafeCall<string>("OOPS!", [DebuggerNonUserCode] delegate
        {
            //throw new NotSupportedException();
            return "Ok";
        });
    }

I've tried to have a quick look at the CSharp Language Specification, but have not had any luck finding a syntax that would allow applying attributes to anonymous methods (or lambdas). Did I miss it, or is this (currently?) impossible...?

Cromorne

Don't think this will get high priority (if it'll ever get done), but I have added the suggestion to MS Connect at connect.microsoft.com/VisualStudio/feedback/… – Cromorne

Answer 1

You cannot, unfortunately. It is listed on page 401 of the C#3.0 language specification:

Attributes can be specified at global scope (to specify attributes on the containing assembly or module) and for type-declarations (§9.6), class-member-declarations (§10.1.5), interface-member-declarations (§13.2), struct-member-declarations (§11.2), enum-member-declarations (§14.3), accessor-declarations (§10.7.2), event-accessor-declarations (§10.8.1), and formal-parameter-lists (§10.6.1).

Holman

Thanks, that's the reference I was looking for. :-( – Cromorne

Answer 2

Unfortunately C# does not allow attributes to be applied to anonymous methods.

Are you sure that you really want to apply a DebuggerNonUserCode attribute to this anonymous method? The method is user code and I would think that you would want to be able to step into it if need be.

Legible

Like I said, the SafeCall method is also called in CodeSmith generated code. There we would like to put a [DebuggerNonUserCode] on the anonymous method, yes. – Cromorne

Answer 3

Since your goal is to exclude the methods from Code Coverage, you can do so by creating a .runsettings file instead of adding an attribute to the method:

http://msdn.microsoft.com/en-us/library/jj159530.aspx

Ruisdael

Answer 4

Starting with C# 10, the custom attributes can be applied to anonymous methods, like so:

var choose = [DebuggerNonUserCode] object (bool b) => b ? 1 : "two";

More information: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-expressions#attributes

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