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`.

Author: 0xced

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

@@ -14,20 +14,7 @@ protected static bool IsCaseInsensitiveMatch(string? text, string textToFind)
 
     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();
+        return new AutoAssemblyFinder(new DependencyContextAssemblyFinder(DependencyContext.Default), new DllScanningAssemblyFinder());
     }
 
     public static AssemblyFinder ForSource(ConfigurationAssemblySource configurationAssemblySource)

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

@@ -0,0 +1,23 @@
+using System.Reflection;
+
+namespace Serilog.Settings.Configuration.Assemblies;
+
+class AutoAssemblyFinder : AssemblyFinder
+{
+    readonly AssemblyFinder[] _assemblyFinders;
+
+    public AutoAssemblyFinder(params AssemblyFinder[] assemblyFinders)
+    {
+        _assemblyFinders = assemblyFinders;
+    }
+
+    public override IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind)
+    {
+        var assemblyNames = new List<AssemblyName>();
+        foreach (var assemblyFinder in _assemblyFinders)
+        {
+            assemblyNames.AddRange(assemblyFinder.FindAssembliesContainingName(nameToFind));
+        }
+        return assemblyNames;
+    }
+}

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

@@ -5,22 +5,25 @@ namespace Serilog.Settings.Configuration.Assemblies;
 
 sealed class DependencyContextAssemblyFinder : AssemblyFinder
 {
-    readonly DependencyContext _dependencyContext;
+    readonly DependencyContext? _dependencyContext;
 
-    public DependencyContextAssemblyFinder(DependencyContext dependencyContext)
+    public DependencyContextAssemblyFinder(DependencyContext? dependencyContext)
     {
-        _dependencyContext = dependencyContext ?? throw new ArgumentNullException(nameof(dependencyContext));
+        _dependencyContext = dependencyContext;
     }
 
     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)
                     where IsCaseInsensitiveMatch(assemblyName.Name, nameToFind)
                     select assemblyName;
 
-        return query.ToList().AsReadOnly();
+        return query.ToList();
 
         static bool IsReferencingSerilog(Library library)
         {

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

@@ -44,7 +44,7 @@ where IsCaseInsensitiveMatch(assemblyFileName, nameToFind)
                     where assemblyName != null
                     select assemblyName;
 
-        return query.ToList().AsReadOnly();
+        return query.ToList();
 
         static AssemblyName? TryGetAssemblyNameFrom(string path)
         {

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

@@ -368,7 +368,7 @@ static IReadOnlyCollection<Assembly> LoadConfigurationAssemblies(IConfiguration
             {
                 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))
@@ -383,7 +383,19 @@ static IReadOnlyCollection<Assembly> LoadConfigurationAssemblies(IConfiguration
                 assemblies.Add(assumed.FullName, assumed);
         }
 
-        return assemblies.Values.ToList().AsReadOnly();
+        if (assemblies.Count == 1)
+        {
+            var message = $"""
+                No {usingSection.Path} configuration section is defined and no Serilog assemblies were found.
+                This is most likely because the application is published as single-file.
+                Either add a {usingSection.Path} section or explicitly specify assemblies that contains sinks and other types through the reader options. For example:
+                var options = new ConfigurationReaderOptions(typeof(ConsoleLoggerConfigurationExtensions).Assembly, typeof(SerilogExpression).Assembly);
+                new LoggerConfiguration().ReadFrom.Configuration(configuration, options);
+                """;
+            throw new InvalidOperationException(message);
+        }
+
+        return assemblies.Values;
     }
 
     void CallConfigurationMethods(ILookup<string, Dictionary<string, IConfigurationArgumentValue>> methods, IReadOnlyCollection<MethodInfo> configurationMethods, object receiver)