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/Settings/Configuration/Assemblies/AssemblyFinder.cs

@@ -5,6 +5,7 @@ namespace Serilog.Settings.Configuration.Assemblies;
 
 abstract class AssemblyFinder
 {
+    public abstract bool CanFindAssemblies { get; }
     public abstract IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind);
 
     protected static bool IsCaseInsensitiveMatch(string text, string textToFind)
@@ -14,20 +15,7 @@ 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 +1,25 @@
+using System.Reflection;
+
+namespace Serilog.Settings.Configuration.Assemblies;
+
+class AutoAssemblyFinder : AssemblyFinder
+{
+    readonly AssemblyFinder[] _assemblyFinders;
+
+    public AutoAssemblyFinder(params AssemblyFinder[] assemblyFinders)
+    {
+        _assemblyFinders = assemblyFinders;
+    }
+
+    public override bool CanFindAssemblies => _assemblyFinders.Any(e => e.CanFindAssemblies);
+
+    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

@@ -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/Assemblies/DllScanningAssemblyFinder.cs

@@ -4,36 +4,11 @@ namespace Serilog.Settings.Configuration.Assemblies;
 
 sealed class DllScanningAssemblyFinder : AssemblyFinder
 {
+    public override bool CanFindAssemblies => GetProbeDirs().SelectMany(e => Directory.GetFiles(e, "*.dll")).Any();
+
     public override IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind)
     {
-        var probeDirs = new List<string>();
-
-        if (!string.IsNullOrEmpty(AppDomain.CurrentDomain.BaseDirectory))
-        {
-            probeDirs.Add(AppDomain.CurrentDomain.BaseDirectory);
-
-#if NETFRAMEWORK
-            var privateBinPath = AppDomain.CurrentDomain.SetupInformation.PrivateBinPath;
-            if (!string.IsNullOrEmpty(privateBinPath))
-            {
-                foreach (var path in privateBinPath.Split(';'))
-                {
-                    if (Path.IsPathRooted(path))
-                    {
-                        probeDirs.Add(path);
-                    }
-                    else
-                    {
-                        probeDirs.Add(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, path));
-                    }
-                }
-            }
-#endif
-        }
-        else
-        {
-            probeDirs.Add(Path.GetDirectoryName(typeof(AssemblyFinder).Assembly.Location));
-        }
+        var probeDirs = GetProbeDirs();
 
         var query = from probeDir in probeDirs
                     where Directory.Exists(probeDir)
@@ -58,4 +33,34 @@ static AssemblyName TryGetAssemblyNameFrom(string path)
             }
         }
     }
+
+    static IEnumerable<string> GetProbeDirs()
+    {
+        if (!string.IsNullOrEmpty(AppDomain.CurrentDomain.BaseDirectory))
+        {
+            yield return AppDomain.CurrentDomain.BaseDirectory;
+
+#if NETFRAMEWORK
+            var privateBinPath = AppDomain.CurrentDomain.SetupInformation.PrivateBinPath;
+            if (!string.IsNullOrEmpty(privateBinPath))
+            {
+                foreach (var path in privateBinPath.Split(';'))
+                {
+                    if (Path.IsPathRooted(path))
+                    {
+                        yield return path;
+                    }
+                    else
+                    {
+                        yield return Path.Combine(AppDomain.CurrentDomain.BaseDirectory, path);
+                    }
+                }
+            }
+#endif
+        }
+        else
+        {
+            yield return Path.GetDirectoryName(typeof(AssemblyFinder).Assembly.Location);
+        }
+    }
 }

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

@@ -368,13 +368,23 @@ 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.
+                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);
+        }
 
         foreach (var assemblyName in assemblyFinder.FindAssembliesContainingName("serilog"))
         {