How can I change the file location programmatically?

Question

log4net can handle this for you. Any appender property of type string can be formatted, in this case, using the log4net.Util.PatternString option handler. PatternString even supports the SpecialFolder enum which enables the following elegant config:

<appender name="LogFileAppender" type="log4net.Appender.RollingFileAppender" >
    <file type="log4net.Util.PatternString" 
        value="%envFolderPath{CommonApplicationData}\\test.txt" />
    ...
</appender>

Here's a unit test that proofs the pudding:

[Test]
public void Load()
{
    XmlConfigurator.Configure();
    var fileAppender = LogManager.GetRepository()
        .GetAppenders().First(appender => appender is RollingFileAppender);

    var expectedFile = 
        Path.Combine(
            Environment.GetFolderPath(
                Environment.SpecialFolder.CommonApplicationData),
                "test.txt");

    Assert.That(fileAppender, 
        Is.Not.Null & Has.Property("File").EqualTo(expectedFile));
}

The following test verifies that log4net actually writes to disk (which basically makes this an "integration" test, not a unit test, but we'll leave it at that for now):

[Test]
public void Log4net_WritesToDisk()
{
    var expectedFile = 
        Path.Combine(
            Environment.GetFolderPath(
                Environment.SpecialFolder.CommonApplicationData),
                "test.txt");

    if (File.Exists(expectedFile))
        File.Delete(expectedFile);

    XmlConfigurator.Configure();

    var log = LogManager.GetLogger(typeof (ConfigTest));
    log.Info("Message from test");

    LogManager.Shutdown();

    Assert.That(File.ReadAllText(expectedFile), 
        Text.Contains("Message from test"));
}

NB: I strongly suggest using the compact property syntax demonstrated in the above sample. Removing all those "<property name=" makes your config that much more readable.

I found a mutation of this code in the interwebs:

XmlConfigurator.Configure();
log4net.Repository.Hierarchy.Hierarchy h =
(log4net.Repository.Hierarchy.Hierarchy) LogManager.GetRepository();
foreach (IAppender a in h.Root.Appenders)
{
    if (a is FileAppender)
    {
        FileAppender fa = (FileAppender)a;
        // Programmatically set this to the desired location here
        string logFileLocation = @"C:\MySpecialFolder\MyFile.log";

        // Uncomment the lines below if you want to retain the base file name
        // and change the folder name...
        //FileInfo fileInfo = new FileInfo(fa.File);
        //logFileLocation = string.Format(@"C:\MySpecialFolder\{0}", fileInfo.Name);

        fa.File = logFileLocation;
        fa.ActivateOptions();
        break;
    }
}

This works for me. Our application needs to put the log file in a folder that contains the version number of the app based on the AssemblyInfo.cs file.

You should be able to set the logFileLocation programmatically (e.g. you can use Server.MapPath() if this is a web application) to suit your needs.

Looks like Peter's answer doesn't work for Log4net v1.2.10.0. An alternative method is described here.

Basically the method is to implement a custom pattern converter for the log4net config file.

First add this class to your project:

public class SpecialFolderPatternConverter : log4net.Util.PatternConverter
{
    override protected void Convert(System.IO.TextWriter writer, object state)
    {
        Environment.SpecialFolder specialFolder = (Environment.SpecialFolder)Enum.Parse(typeof(Environment.SpecialFolder), base.Option, true);
        writer.Write(Environment.GetFolderPath(specialFolder));
    }
}

Then set up the File parameter of your FileAppender as follows:

<file type="log4net.Util.PatternString">
    <converter>
      <name value="folder" />
      <type value="MyAppName.SpecialFolderPatternConverter,MyAppName" />
    </converter>
    <conversionPattern value="%folder{CommonApplicationData}\\SomeOtherFolder\\log.txt" />
  </file>

Basically the %folder tells it to look at the converter called folder which points it to the SpecialFolderPatternConverter class. It then calls Convert on that class, passing in the CommonApplicationData (or whatever) enum value.

How about a simple:

XmlConfigurator.LogFullFilename = @"c:\ProgramData\MyApp\Myapp.log";

Why is it so complex to do a really simple thing?