Make sure that single-file apps can find assemblies that contains sinks

Before this commit, when single-file app was detected, the behaviour was to fallback on DLL scanning. But DLL scanning would not find anything for an app published as a single-file, by sheer definition of single-file app!

After this commit, an exception is thrown if the app is published as a single-file AND no `Serilog:Using` section is defined in the configuration. The error message explains that either a `Serilog:Using` section must be added or show how to explicitly configure assemblies through the `ConfigurationReaderOptions`.

src/Serilog.Settings.Configuration/ConfigurationLoggerConfigurationExtensions.cs

@@ -92,14 +92,10 @@ public static LoggerConfiguration ConfigurationSection(
         if (settingConfiguration == null) throw new ArgumentNullException(nameof(settingConfiguration));
         if (configSection == null) throw new ArgumentNullException(nameof(configSection));
 
-        var assemblyFinder = dependencyContext == null
-            ? AssemblyFinder.Auto()
-            : AssemblyFinder.ForDependencyContext(dependencyContext);
-
         return settingConfiguration.Settings(
             new ConfigurationReader(
                 configSection,
-                assemblyFinder,
+                AssemblyFinder.ForDependencyContext(dependencyContext),
                 configuration: null,
                 formatProvider: null));
     }
@@ -218,23 +214,15 @@ public static LoggerConfiguration Configuration(
     {
         var configurationReader = readerOptions switch
         {
-            { ConfigurationAssemblySource: {} } => GetConfigurationReader(configuration, readerOptions, readerOptions.ConfigurationAssemblySource.Value),
+            { ConfigurationAssemblySource: {} } => GetConfigurationReader(configuration, readerOptions, AssemblyFinder.ForSource(readerOptions.ConfigurationAssemblySource.Value)),
             { Assemblies: {} } => GetConfigurationReader(configuration, readerOptions, readerOptions.Assemblies),
-            _ => GetConfigurationReader(configuration, readerOptions ?? new ConfigurationReaderOptions(), readerOptions?.DependencyContext),
+            _ => GetConfigurationReader(configuration, readerOptions ?? new ConfigurationReaderOptions(), AssemblyFinder.ForDependencyContext(readerOptions?.DependencyContext)),
         };
         return settingConfiguration.Settings(configurationReader);
     }
 
-    static ConfigurationReader GetConfigurationReader(IConfiguration configuration, ConfigurationReaderOptions readerOptions, DependencyContext dependencyContext)
-    {
-        var assemblyFinder = dependencyContext == null ? AssemblyFinder.Auto() : AssemblyFinder.ForDependencyContext(dependencyContext);
-        var section = configuration.GetSection(readerOptions.SectionName);
-        return new ConfigurationReader(section, assemblyFinder, readerOptions.FormatProvider, configuration);
-    }
-
-    static ConfigurationReader GetConfigurationReader(IConfiguration configuration, ConfigurationReaderOptions readerOptions, ConfigurationAssemblySource source)
+    static ConfigurationReader GetConfigurationReader(IConfiguration configuration, ConfigurationReaderOptions readerOptions, AssemblyFinder assemblyFinder)
     {
-        var assemblyFinder = AssemblyFinder.ForSource(source);
         var section = configuration.GetSection(readerOptions.SectionName);
         return new ConfigurationReader(section, assemblyFinder, readerOptions.FormatProvider, configuration);
     }

src/Serilog.Settings.Configuration/Settings/Configuration/Assemblies/AssemblyFinder.cs

@@ -5,43 +5,26 @@ namespace Serilog.Settings.Configuration.Assemblies;
 
 abstract class AssemblyFinder
 {
+    public virtual bool CanFindAssemblies => true;
     public abstract IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind);
 
     protected static bool IsCaseInsensitiveMatch(string text, string textToFind)
     {
         return text != null && text.ToLowerInvariant().Contains(textToFind.ToLowerInvariant());
     }
 
-    public static AssemblyFinder Auto()
-    {
-        try
-        {
-            // Need to check `Assembly.GetEntryAssembly()` first because
-            // `DependencyContext.Default` throws an exception when `Assembly.GetEntryAssembly()` returns null
-            if (Assembly.GetEntryAssembly() != null && DependencyContext.Default != null)
-            {
-                return new DependencyContextAssemblyFinder(DependencyContext.Default);
-            }
-        }
-        catch (NotSupportedException) when (typeof(object).Assembly.Location is "") // bundled mode detection
-        {
-        }
-
-        return new DllScanningAssemblyFinder();
-    }
-
     public static AssemblyFinder ForSource(ConfigurationAssemblySource configurationAssemblySource)
     {
         return configurationAssemblySource switch
         {
-            ConfigurationAssemblySource.UseLoadedAssemblies => Auto(),
+            ConfigurationAssemblySource.UseLoadedAssemblies => new DependencyContextAssemblyFinder(DependencyContext.Default),
             ConfigurationAssemblySource.AlwaysScanDllFiles => new DllScanningAssemblyFinder(),
             _ => throw new ArgumentOutOfRangeException(nameof(configurationAssemblySource), configurationAssemblySource, null),
         };
     }
 
     public static AssemblyFinder ForDependencyContext(DependencyContext dependencyContext)
     {
-        return new DependencyContextAssemblyFinder(dependencyContext);
+        return new DependencyContextAssemblyFinder(dependencyContext ?? DependencyContext.Default);
     }
 }

src/Serilog.Settings.Configuration/Settings/Configuration/Assemblies/DependencyContextAssemblyFinder.cs

@@ -9,11 +9,16 @@ sealed class DependencyContextAssemblyFinder : AssemblyFinder
 
     public DependencyContextAssemblyFinder(DependencyContext dependencyContext)
     {
-        _dependencyContext = dependencyContext ?? throw new ArgumentNullException(nameof(dependencyContext));
+        _dependencyContext = dependencyContext;
     }
 
+    public override bool CanFindAssemblies => _dependencyContext != null;
+
     public override IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind)
     {
+        if (_dependencyContext == null)
+            return Array.Empty<AssemblyName>();
+
         var query = from library in _dependencyContext.RuntimeLibraries
                     where IsReferencingSerilog(library)
                     from assemblyName in library.GetDefaultAssemblyNames(_dependencyContext)

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

@@ -363,13 +363,21 @@ static IReadOnlyCollection<Assembly> LoadConfigurationAssemblies(IConfigurationS
             {
                 if (string.IsNullOrWhiteSpace(simpleName))
                     throw new InvalidOperationException(
-                        "A zero-length or whitespace assembly name was supplied to a Serilog.Using configuration statement.");
+                        $"A zero-length or whitespace assembly name was supplied to a {usingSection.Path} configuration statement.");
 
                 var assembly = Assembly.Load(new AssemblyName(simpleName));
                 if (!assemblies.ContainsKey(assembly.FullName))
                     assemblies.Add(assembly.FullName, assembly);
             }
         }
+        else if (!assemblyFinder.CanFindAssemblies)
+        {
+            var message = $"The application is published as single-file and no {usingSection.Path} configuration section is defined.{Environment.NewLine}" +
+                          $"Either add a {usingSection.Path} section or explicitly specify assemblies that contains sinks and other types through the reader options. For example:{Environment.NewLine}" +
+                          $"var options = new ConfigurationReaderOptions(typeof(ConsoleLoggerConfigurationExtensions).Assembly, typeof(SerilogExpression).Assembly);{Environment.NewLine}" +
+                          "new LoggerConfiguration().ReadFrom.Configuration(configuration, options);";
+            throw new InvalidOperationException(message);
+        }
 
         foreach (var assemblyName in assemblyFinder.FindAssembliesContainingName("serilog"))
         {