Where does Console.WriteLine go in ASP.NET?

335
votes

In a J2EE application (like one running in WebSphere), when I use System.out.println(), my text goes to standard out, which is mapped to a file by the WebSphere admin console.

In an ASP.NET application (like one running in IIS), where does the output of Console.WriteLine() go? The IIS process must have a stdin, stdout and stderr; but is stdout mapped to the Windows version of /dev/null or am I missing a key concept here?

I'm not asking if I should log there (I use log4net), but where does the output go? My best info came from this discussion where they say Console.SetOut() can change the TextWriter, but it still didn't answer the question on what the initial value of the Console is, or how to set it in config/outside of runtime code.

12
asp.netiisconsole.writeline
It would actually go to the STDOUT of the ASP.NET Worker process. Where that is pointed to, I'm not sure. – FlySwat
That's the question - where does STDOUT go? – Kevin Hakanson
apparently no one knows, but everyone uses it in their examples. wtf – Jason
if you were looking for debugging purposes i would refer the @Greg Bernhardt reply below. – Ram
@KevinHakanson FWIW all these years later, stdout for any process is chosen by its parent, the process which started it. In this case, the parent would be IIS. This might point you in the right direction. – jpaugh

12 Answers

207
votes

If you look at the Console class in .NET Reflector, you'll find that if a process doesn't have an associated console, Console.Out and Console.Error are backed by Stream.Null (wrapped inside a TextWriter), which is a dummy implementation of Stream that basically ignores all input, and gives no output.

So it is conceptually equivalent to /dev/null, but the implementation is more streamlined: there's no actual I/O taking place with the null device.

Also, apart from calling SetOut, there is no way to configure the default.

Update 2020-11-02: As this answer is still gathering votes in 2020, it should probably be noted that under ASP.NET Core, there usually is a console attached. You can configure the ASP.NET Core IIS Module to redirect all stdout and stderr output to a log file via the stdoutLogEnabled and stdoutLogFile settings:

<system.webServer>
  <aspNetCore processPath="dotnet"
              arguments=".\MyApp.dll"
              hostingModel="inprocess"
              stdoutLogEnabled="true"
              stdoutLogFile=".\logs\stdout" />
<system.webServer>
778
votes

If you use System.Diagnostics.Debug.WriteLine(...) instead of Console.WriteLine(), then you can see the results in the Output window of Visual Studio.

29
votes

I've found this question by trying to change the Log output of the DataContext to the output window. So to anyone else trying to do the same, what I've done was create this:

class DebugTextWriter : System.IO.TextWriter {
   public override void Write(char[] buffer, int index, int count) {
       System.Diagnostics.Debug.Write(new String(buffer, index, count));
   }

   public override void Write(string value) {
       System.Diagnostics.Debug.Write(value);
   }

   public override Encoding Encoding {
       get { return System.Text.Encoding.Default; }
   }
}

Annd after that: dc.Log = new DebugTextWriter() and I can see all the queries in the output window (dc is the DataContext).

Have a look at this for more info: http://damieng.com/blog/2008/07/30/linq-to-sql-log-to-debug-window-file-memory-or-multiple-writers

18
votes

If you are using IIS Express and launch it via a command prompt, it will leave the DOS window open, and you will see Console.Write statements there.

So for example get a command window open and type:

"C:\Program Files (x86)\IIS Express\iisexpress" /path:C:\Projects\Website1 /port:1655

This assumes you have a website directory at C:\Projects\Website1. It will start IIS Express and serve the pages in your website directory. It will leave the command windows open, and you will see output information there. Let's say you had a file there, default.aspx, with this code in it:

<%@ Page Language="C#" %>
<html>
<body>
    <form id="form1" runat="server">
    Hello!

    <% for(int i = 0; i < 6; i++) %>
       <% { Console.WriteLine(i.ToString()); }%>

    </form>
</body>
</html>

Arrange your browser and command windows so you can see them both on the screen. Now type into your browser: http://localhost:1655/. You will see Hello! on the webpage, but in the command window you will see something like

Request started: "GET" http://localhost:1655/
0
1
2
3
4
5
Request ended: http://localhost:1655/default.aspx with HTTP status 200.0

I made it simple by having the code in a code block in the markup, but any console statements in your code-behind or anywhere else in your code will show here as well.

9
votes

System.Diagnostics.Debug.WriteLine(...); gets it into the Immediate Window in Visual Studio 2008.

Go to menu Debug -> Windows -> Immediate:

Enter image description here

6
votes

There simply is no console listening by default. Running in debug mode there is a console attached, but in a production environment it is as you suspected, the message just doesn't go anywhere because nothing is listening.

5
votes

Unless you are in a strict console application, I wouldn't use it, because you can't really see it. I would use Trace.WriteLine() for debugging-type information that can be turned on and off in production.

3
votes

The TraceContext object in ASP.NET writes to the DefaultTraceListener which outputs to the host process’ standard output. Rather than using Console.Write(), if you use Trace.Write, output will go to the standard output of the process.

You could use the System.Diagnostics.Process object to get the ASP.NET process for your site and monitor standard output using the OutputDataRecieved event.

1
votes

if you happened to use NLog in your ASP.net project, you can add a Debugger target:

<targets>
    <target name="debugger" xsi:type="Debugger"
            layout="${date:format=HH\:mm\:ss}|${pad:padding=5:inner=${level:uppercase=true}}|${message} "/>

and writes logs to this target for the levels you want:

<rules>
    <logger name="*" minlevel="Trace" writeTo="debugger" />

now you have console output just like Jetty in "Output" window of VS, and make sure you are running in Debug Mode(F5).

0
votes

This is confusing for everyone when it comes IISExpress. There is nothing to read console messages. So for example, in the ASPCORE MVC apps it configures using appsettings.json which does nothing if you are using IISExpress.

For right now you can just add loggerFactory.AddDebug(LogLevel.Debug); in your Configure section and it will at least show you your logs in the Debug Output window.

Good news CORE 2.0 this will all be changing: https://github.com/aspnet/Announcements/issues/255

0
votes

Mac, In Debug mode there is a tab for the Output. enter image description here

-3
votes

In an ASP.NET application, I think it goes to the Output or Console window which is visible during debugging.