Example of using custom prefixes with Serilog when using AppSettings

App.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="custom1:serilog:minimum-level" value="Warning" />
    <add key="custom2:serilog:minimum-level" value="Error" />
  </appSettings>
</configuration>

AppSettingsSettings.cs

using System;
using System.Configuration;
using System.Linq;
using Serilog;
using Serilog.Configuration;

namespace Serilog
{
    internal class AppSettingsSettings : ILoggerSettings
    {
        readonly string SettingPrefix;

        public AppSettingsSettings(string settingPrefix)
        {
            SettingPrefix = string.Format("{0}:", settingPrefix);
        }

        public void Configure(LoggerConfiguration loggerConfiguration)
        {
            if (loggerConfiguration == null)
            {
                throw new ArgumentNullException("loggerConfiguration");
            }

            var settings = ConfigurationManager.AppSettings;

            var pairs = settings.AllKeys
                .Where(k => k.StartsWith(SettingPrefix))
                .ToDictionary(k => k.Substring(SettingPrefix.Length), k => Environment.ExpandEnvironmentVariables(settings[k]));

            // Add the FullNetFx assembly by default so that all built-in Serilog sinks are available without "using"
            pairs.Add("using:_ImpliedSerilogFullNetFx", typeof(AppSettingsSettings).Assembly.FullName);

            loggerConfiguration.ReadFrom.KeyValuePairs(pairs);
        }
    }
}

SerilogExtensions.cs

using System;
using Serilog;
using Serilog.Configuration;

namespace Serilog
{
    public static class SerilogExtensions
    {
        /// <summary>
        /// Reads the &lt;appSettings&gt; element of App.config or Web.config, searching for for keys
        /// that look like: <code>serilog:*</code>, which are used to configure
        /// the logger. To add a sink, use a key like <code>serilog:write-to:File.path</code> for
        /// each parameter to the sink's configuration method. To add an additional assembly
        /// containing sinks, use <code>serilog:using</code>. To set the level use
        /// <code>serilog:minimum-level</code>.
        /// </summary>
        /// <param name="settingConfiguration">Logger setting configuration</param>
        /// <param name="settingPrefix">Prefix to use when reading keys in appSettings</param>
        /// <returns>An object allowing configuration to continue.</returns>
        public static LoggerConfiguration AppSettingsWithPrefix(
            this LoggerSettingsConfiguration settingConfiguration, string settingPrefix = "serilog")
        {
            if (settingConfiguration == null)
            {
                throw new ArgumentNullException("settingConfiguration");
            }

            return settingConfiguration.Settings(new AppSettingsSettings(settingPrefix));
        }
    }
}

SerilogExtensionsTests.cs

using System.Configuration;
using NUnit.Framework;
using Serilog;
using Serilog.Events;

namespace Serilog.UnitTests
{
    [TestFixture]
    public class SerilogExtensionsTests
    {
        [Test]
        public void CanUseCustomPrefixToConfigureSettings()
        {
            const string prefix1 = "custom1:serilog";
            const string prefix2 = "custom2:serilog";

            // Make sure we have the expected keys in the App.config
            Assert.AreEqual("Warning", ConfigurationManager.AppSettings[prefix1 + ":minimum-level"]);
            Assert.AreEqual("Error", ConfigurationManager.AppSettings[prefix2 + ":minimum-level"]);

            var log1 = new LoggerConfiguration()
                .ReadFrom.AppSettingsWithPrefix(prefix1)
                .CreateLogger();

            var log2 = new LoggerConfiguration()
                .ReadFrom.AppSettingsWithPrefix(prefix2)
                .CreateLogger();

            Assert.IsFalse(log1.IsEnabled(LogEventLevel.Information));
            Assert.IsTrue(log1.IsEnabled(LogEventLevel.Warning));

            Assert.IsFalse(log2.IsEnabled(LogEventLevel.Warning));
            Assert.IsTrue(log2.IsEnabled(LogEventLevel.Error));
        }
    }
}