Add support for overrides from environment variables

src/Serilog.Settings.Configuration/Settings/Configuration/ConfigurationReader.cs

@@ -160,10 +160,19 @@ void ApplyMinimumLevel(LoggerConfiguration loggerConfiguration)
             loggerConfiguration.MinimumLevel.ControlledBy(globalMinimumLevelSwitch);
         }
 
+        // Level overrides may be set via environment variables. In that case it may be problematic to use dots in
+        // variable name, see https://github.com/dotnet/AspNetCore.Docs/issues/17378, so all dots should be replaces
+        // with double underscore. The consequence is that configuration becomes "nested" and GetSection("Override").GetChildren()
+        // call returns nothing since this method returns only immediate descendant configuration sub-sections.
+        // The workaround is to use AsEnumerable(true) method to retrieve all descendants and then manually replace : with .
+        var flattenDescendants = minimumLevelDirective.GetSection("Override").AsEnumerable(true).ToList();
+
         foreach (var overrideDirective in minimumLevelDirective.GetSection("Override").GetChildren())
         {
             var overridePrefix = overrideDirective.Key;
             var overridenLevelOrSwitch = overrideDirective.Value;
+            // filter out "normal" keys
+            flattenDescendants.RemoveAll(p => p.Key == overridePrefix);
             if (Enum.TryParse(overridenLevelOrSwitch, out LogEventLevel _))
             {
                 ApplyMinimumLevelConfiguration(overrideDirective, (configuration, levelSwitch) =>
@@ -180,6 +189,29 @@ void ApplyMinimumLevel(LoggerConfiguration loggerConfiguration)
             }
         }
 
+        // Process remaining configuration values if any. Note that this code path does not call
+        // SubscribeToLoggingLevelChanges as ApplyMinimumLevelConfiguration does above. This can
+        // be improved some time later.
+        foreach (var pair in flattenDescendants)
+        {
+            var overridePrefix = pair.Key.Replace(':', '.');
+            var overridenLevelOrSwitch = pair.Value;
+            if (Enum.TryParse(overridenLevelOrSwitch, out LogEventLevel _))
+            {
+                var minimumLevel = ParseLogEventLevel(overridenLevelOrSwitch!);
+
+                var levelSwitch = new LoggingLevelSwitch(minimumLevel);
+                loggerConfiguration.MinimumLevel.Override(overridePrefix, levelSwitch);
+                _resolutionContext.ReaderOptions.OnLevelSwitchCreated?.Invoke(overridePrefix, levelSwitch);
+            }
+            else if (!string.IsNullOrEmpty(overridenLevelOrSwitch))
+            {
+                var overrideSwitch = _resolutionContext.LookUpLevelSwitchByName(overridenLevelOrSwitch!);
+                // not calling ApplyMinimumLevel local function because here we have a reference to a LogLevelSwitch already
+                loggerConfiguration.MinimumLevel.Override(overridePrefix, overrideSwitch);
+            }
+        }
+
         void ApplyMinimumLevelConfiguration(IConfigurationSection directive, Action<LoggerMinimumLevelConfiguration, LoggingLevelSwitch> applyConfigAction)
         {
             var minimumLevel = ParseLogEventLevel(directive.Value!);

test/Serilog.Settings.Configuration.Tests/ConfigurationSettingsTests.cs

@@ -405,6 +405,35 @@ public void TestMinimumLevelOverridesForChildContext()
         Assert.NotNull(evt);
     }
 
+    [Fact]
+    public void TestMinimumLevelOverridesForChildContext_With_Environment_Variables()
+    {
+        Environment.SetEnvironmentVariable("My_Serilog__MinimumLevel__Default", "Warning");
+        Environment.SetEnvironmentVariable("My_Serilog__MinimumLevel__Override__System", "Warning");
+        Environment.SetEnvironmentVariable("My_Serilog__MinimumLevel__Override__System__Threading", "Debug");
+
+        var builder = new ConfigurationBuilder().AddEnvironmentVariables("My_");
+        var configuration = new LoggerConfiguration().ReadFrom.Configuration(builder.Build());
+
+        LogEvent? evt = null;
+
+        var log = configuration
+            .WriteTo.Sink(new DelegatingSink(e => evt = e))
+            .CreateLogger();
+
+        log.Write(Some.DebugEvent());
+        Assert.Null(evt);
+
+        var custom = log.ForContext(Constants.SourceContextPropertyName, typeof(System.Threading.Tasks.Task).FullName + "<42>");
+        custom.Write(Some.DebugEvent());
+        Assert.NotNull(evt);
+
+        evt = null;
+        var systemThreadingLogger = log.ForContext<System.Threading.Tasks.Task>();
+        systemThreadingLogger.Write(Some.DebugEvent());
+        Assert.NotNull(evt);
+    }
+
     [Fact]
     public void SinksWithAbstractParamsAreConfiguredWithTypeName()
     {

test/Serilog.Settings.Configuration.Tests/Serilog.Settings.Configuration.Tests.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT'">net48</TargetFrameworks>
@@ -19,6 +19,7 @@
   <ItemGroup>
     <PackageReference Include="CliWrap" Version="3.6.0" />
     <PackageReference Include="FluentAssertions" Version="6.10.0" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="7.0.0" />
     <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="7.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.5.0" />
     <PackageReference Include="NuGet.Frameworks" Version="6.5.0" />