What's the C# equivalent of perl warn?

Question

In Perl I can say

use warnings;
warn "Non-fatal error condition, printed to stderr.";

What is the equivalent of this in C# ASP.NET? Specifically what I'm looking for is something so that other members of my team can know when they're still using a deprecated routine. It should show up at run time when the code path is actually hit, not at compile time (otherwise they'd get warnings about code sitting at a compatibility layer which ought to never run anyway.)

My best option so far is to use Trace, which feels like a bad hack.

Possible duplicate: http://stackoverflow.com/questions/968249/c-create-custom-warning-in-visual-studio-if-certain-method-is-used-in-source-co — Michael La Voie, Mar 01 '10 at 19:37
I don't really think it's a duplicate. The frame of reference for the individual OP's is different. — David Morton, Mar 01 '10 at 19:39
I think in C# your trace method will probably be your best approach. Hopefully you're targeting the event log with your warning messages. Really think the Obsolete attribute is your bet though. — No Refunds No Returns, Mar 01 '10 at 19:47
The target I was hoping for is the browser. Developers may or may not ever look into a log file filled with warnings they can ignore, but they *will* look at page source and can't help but see things that are dumped there. — sorpigal, Mar 02 '10 at 16:55
I miss warnings in c#. They are handy in perl as you can raise a warning, and capture it, but the code keeps executing. Kind of like exceptions that don't stop the code. — Matthew Lock, Feb 13 '15 at 00:48

score 6 · Answer 1 · answered Mar 01 '10 at 19:36

6

Use ObsoleteAttribute. Decorate it over any method you want to mark as deprecated. They'll get a warning in their errors window, but the app will still compile.

public class Thing
{
     [Obsolete]
     public void OldMethod() { Console.WriteLine("I'm Old"); }
}

answered Mar 01 '10 at 19:36

David Morton

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One note about ObsoleteAttribute that is sometimes missed - you can mark a member as deprecated with a flag to cause a compile-time failure, but the member can still be accessed via reflection. If you truly want the member to be uncallable, it's best to also change the contents of the member to throw an exception. – plinth Mar 01 '10 at 19:41

Jon Skeet · Answer 2 · 2010-03-01T20:17:02.800

EDIT: I hadn't seen the bit in the question saying you wanted it at execution time.

The problem about writing out data execution time is that you need to know where to write it. What logging does your application use in general? Use the same form of logging as that, basically.

I would still favour the compile-time option listed below, however - you can turn off specific warnings in the compatibility layer using #pragma warn disable/restore, but it'll make it a lot easier to spot the problems than hoping someone reads a log file...

Old answer

Use the [Obsolete] attribute on any type or member. You can decide whether this should end up being a warning or an error. For example:

using System;

class Test
{
    static void Main()
    {
        OldMethod();
        BadMethod();
    }    

    [Obsolete("Use something else instead")]
    static void OldMethod() {}

    [Obsolete("This would destroy your machine!", true)]
    static void BadMethod() {}
}

Compiling this gives:

Test.cs(7,9): warning CS0618: 'Test.OldMethod()' is obsolete: 'Use something else instead' Test.cs(8,9): error CS0619: 'Test.BadMethod()' is obsolete: 'This would destroy your machine!'

Ideally, the message should explain what the effects of continuing to use the method would be, and the suggested alternative.

You are correct; the problem with tracing and other logging solutions is "Where do the messages go?" In perl web development warn + CGI::Carp + warningsToBrowser sends the messages to the page as comments. ASP.NET tracing is similar, but dumps a huge amount of data for formatting and such, which makes the resultant page a mess. It looks like Trace is the closest answer. — sorpigal, Mar 02 '10 at 16:49

score 2 · Answer 3 · answered Mar 01 '10 at 19:40

2

FYI there is a #warning directive that can be used for run time warning generation; however, the [Obsolete] attribute sound much more like what you need.

answered Mar 01 '10 at 19:40

Michael La Voie

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also at compile time, not run time. – No Refunds No Returns Mar 01 '10 at 19:42

score 2 · Answer 4 · answered Mar 01 '10 at 19:42

There is no equivalent to warn's __WARN__ handling, but the printing to STDERR can be accomplished with a simple Console.Error.WriteLine() call.

That said, what you're really trying to do is mark something obsolete or deprecated, and others have shown you how to do that. Use that method rather than inserting warn calls into your function.

score 2 · Accepted Answer · answered Mar 01 '10 at 21:07

You can use Trace.TraceWarning for run-time obsolete method usage detection, BUT you should really rethink your design and use ObsoleteAttribute and compile-time detection.

Run-time detection of such errors should be the last resort.

If your compatibility layer is auto-generated code, you can check for ObsoleteAttribute when generating the shims and mark such shims also [Obsolete]
If your compatibility layer uses reflection, it can check for ObsoleteAttribute and emit the warnings.
If your compatiblity layer is hand-coded you can write a tool that automatically inspects the IL in your compatibility layer and marks compatibility-layer methods [Obsolete] based on which methods they call.

In each case it would be best if all obsolete methods were actually marked with [Obsolete] so new code will not call them.

If your compatibility layer includes shims that call obsolete methods and these are also marked [Obsolete] you can safely compile it using this:

#pragma warning disable 618

This will hide the obsolete warnings when compiling the compatibility layer. Since your compatibility layer methods are also marked [Obsolete] you will get warnings at the correct locations.

Trace appears to be the best solution for run time warnings. I like your [Obsolete] + #pragma solution for nicer compile-time messages as well. Even though it wasn't what I was asking for I find it to be highly useful. — sorpigal, Mar 02 '10 at 16:49

dkackman · Answer 6 · 2010-03-01T20:50:52.397

I think the Trace class is your best bet. Depending on how you intrusive you want to be you can use different trace levels or even a Fail statement (which brings up the nasty assert dialog).

private void SomeOldMethod()
{
  Trace.Fail("You shouldn't call this method"); // <-- this will bring up an assert dialog
  Trace.TraceWarning("You shouldn't call this method");
  Trace.TraceError("You shouldn't call this method");
}

score 0 · Answer 7 · answered Mar 01 '10 at 19:39

the way to flag a method as deprecated in c# is to use the obsolete attribute, this is overloaded to take a message to output (on intellisense) and a bool as to whether it should allow compile or not. I am not 100% convinced this matches what you want though, I am unfamiliar with perl

[Obsolete("A message",false)]

What's the C# equivalent of perl warn?

7 Answers7