Throw for missing configured assemblies only when required

Fixes #389

Author: 0xced

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

@@ -17,6 +17,9 @@ class ConfigurationReader : IConfigurationReader
 {
     const string LevelSwitchNameRegex = @"^\${0,1}[A-Za-z]+[A-Za-z0-9]*$";
 
+    // Section names that can be handled by Serilog itself (hence builtin) without requiring any additional assemblies.
+    static readonly string[] BuiltinSectionNames = { "LevelSwitches", "MinimumLevel", "Properties" };
+
     readonly IConfiguration _section;
     readonly IReadOnlyCollection<Assembly> _configurationAssemblies;
     readonly ResolutionContext _resolutionContext;
@@ -383,7 +386,10 @@ static IReadOnlyCollection<Assembly> LoadConfigurationAssemblies(IConfiguration
             assemblies.Add(assumed);
         }
 
-        if (assemblies.Count == 1)
+        // We don't want to throw if the configuration contains only sections that can be handled by Serilog itself, without requiring any additional assembly.
+        // See https://github.com/serilog/serilog-settings-configuration/issues/389
+        var requiresAdditionalAssemblies = section.GetChildren().Select(e => e.Key).Except(BuiltinSectionNames).Any();
+        if (assemblies.Count == 1 && requiresAdditionalAssemblies)
         {
             var message = $"""
                 No {usingSection.Path} configuration section is defined and no Serilog assemblies were found.

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

@@ -88,6 +88,15 @@ public async Task RunTestApp_ConsoleAndThread(PublishMode publishMode, string st
         stdErr.Should().BeEmpty();
     }
 
+    [Theory]
+    [ClassData(typeof(PublishModeTheoryData))]
+    public async Task RunTestApp_ConfigureMinimumLevelOnly(PublishMode publishMode)
+    {
+        var (_, stdOut, stdErr) = await RunTestAppAsync(publishMode, "--minimum-level-only");
+        stdOut.Should().Be("(Main thread) [Information] Expected success");
+        stdErr.Should().BeEmpty();
+    }
+
     async Task<(bool IsSingleFile, string StdOut, string StdErr)> RunTestAppAsync(PublishMode publishMode, params string[] args)
     {
         // Determine whether the app is a _true_ single file, i.e. not a .NET Core 3.x version which

test/TestApp/Program.cs

@@ -21,13 +21,20 @@
 SelfLog.Enable(Console.Error);
 
 Thread.CurrentThread.Name = "Main thread";
+const string outputTemplate = "({ThreadName}) [{Level}] {Message}{NewLine}";
 
-var configurationValues = new Dictionary<string, string?>
+var configurationValues = new Dictionary<string, string?>();
+var minimumLevelOnly = args.Contains("--minimum-level-only");
+if (minimumLevelOnly)
 {
-    ["Serilog:Enrich:0"] = "WithThreadName",
-    ["Serilog:WriteTo:0:Name"] = "Console",
-    ["Serilog:WriteTo:0:Args:outputTemplate"] = "({ThreadName}) [{Level}] {Message}{NewLine}",
-};
+    configurationValues["Serilog:MinimumLevel"] = "Verbose";
+}
+else
+{
+    configurationValues["Serilog:Enrich:0"] = "WithThreadName";
+    configurationValues["Serilog:WriteTo:0:Name"] = "Console";
+    configurationValues["Serilog:WriteTo:0:Args:outputTemplate"] = outputTemplate;
+}
 
 if (args.Contains("--using-thread")) configurationValues["Serilog:Using:Thread"] = "Serilog.Enrichers.Thread";
 if (args.Contains("--using-console")) configurationValues["Serilog:Using:Console"] = "Serilog.Sinks.Console";
@@ -40,7 +47,14 @@
 {
     var configuration = new ConfigurationBuilder().AddInMemoryCollection(configurationValues).Build();
     var options = assemblies.Count > 0 ? new ConfigurationReaderOptions(assemblies.ToArray()) : null;
-    var logger = new LoggerConfiguration().ReadFrom.Configuration(configuration, options).CreateLogger();
+    var loggerConfiguration = new LoggerConfiguration().ReadFrom.Configuration(configuration, options);
+    if (minimumLevelOnly)
+    {
+        loggerConfiguration
+            .Enrich.WithThreadName()
+            .WriteTo.Console(outputTemplate: outputTemplate);
+    }
+    var logger = loggerConfiguration.CreateLogger();
     logger.Information("Expected success");
     return 0;
 }