Feature: Lazy Loading of Transitive Shared Types [See Comments]

src/Plugins/Loader/AssemblyLoadContextBuilder.cs

@@ -26,6 +26,7 @@ public class AssemblyLoadContextBuilder
         private AssemblyLoadContext _defaultLoadContext = AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly()) ?? AssemblyLoadContext.Default;
         private string? _mainAssemblyPath;
         private bool _preferDefaultLoadContext;
+        private bool _lazyLoadReferences;
 
 #if FEATURE_UNLOAD
         private bool _isCollectible;
@@ -63,6 +64,7 @@ public AssemblyLoadContext Build()
                 resourceProbingPaths,
                 _defaultLoadContext,
                 _preferDefaultLoadContext,
+                _lazyLoadReferences,
 #if FEATURE_UNLOAD
                 _isCollectible,
                 _loadInMemory,
@@ -148,21 +150,46 @@ public AssemblyLoadContextBuilder PreferLoadContextAssembly(AssemblyName assembl
         /// <returns>The builder.</returns>
         public AssemblyLoadContextBuilder PreferDefaultLoadContextAssembly(AssemblyName assemblyName)
         {
-            if (assemblyName.Name == null || _defaultAssemblies.Contains(assemblyName.Name))
+            // Lazy loaded references have dependencies resolved as they are loaded inside the actual Load Context.
+            if (_lazyLoadReferences)
             {
-                // base cases
+                if (assemblyName.Name != null && !_defaultAssemblies.Contains(assemblyName.Name))
+                {
+                    _defaultAssemblies.Add(assemblyName.Name);
+                    var assembly = _defaultLoadContext.LoadFromAssemblyName(assemblyName);
+                    foreach (var reference in assembly.GetReferencedAssemblies())
+                    {
+                        if (reference.Name != null)
+                        {
+                            _defaultAssemblies.Add(reference.Name);
+                        }
+                    }
+                }
+
                 return this;
             }
 
-            _defaultAssemblies.Add(assemblyName.Name);
-
-            // Recursively load and find all dependencies of default assemblies.
-            // This sacrifices some performance for determinism in how transitive
-            // dependencies will be shared between host and plugin.
-            var assembly = _defaultLoadContext.LoadFromAssemblyName(assemblyName);
-            foreach (var reference in assembly.GetReferencedAssemblies())
+            var names = new Queue<AssemblyName>();
+            names.Enqueue(assemblyName);
+            while (names.TryDequeue(out var name))
             {
-                PreferDefaultLoadContextAssembly(reference);
+                if (name.Name == null || _defaultAssemblies.Contains(name.Name))
+                {
+                    // base cases
+                    continue;
+                }
+
+                _defaultAssemblies.Add(name.Name);
+
+                // Load and find all dependencies of default assemblies.
+                // This sacrifices some performance for determinism in how transitive
+                // dependencies will be shared between host and plugin.
+                var assembly = _defaultLoadContext.LoadFromAssemblyName(name);
+
+                foreach (var reference in assembly.GetReferencedAssemblies())
+                {
+                    names.Enqueue(reference);
+                }
             }
 
             return this;
@@ -187,6 +214,24 @@ public AssemblyLoadContextBuilder PreferDefaultLoadContext(bool preferDefaultLoa
             return this;
         }
 
+        /// <summary>
+        /// Instructs the load context to lazy load dependencies of all shared assemblies.
+        /// Reduces plugin load time at the expense of non-determinism in how transitive dependencies are loaded
+        /// between the plugin and the host.
+        ///
+        /// Please be aware of the danger of using this option:
+        /// <seealso href="https://github.com/natemcmaster/DotNetCorePlugins/pull/164#issuecomment-751557873">
+        /// https://github.com/natemcmaster/DotNetCorePlugins/pull/164#issuecomment-751557873
+        /// </seealso>
+        /// </summary>
+        /// <param name="isLazyLoaded">True to lazy load, else false.</param>
+        /// <returns>The builder.</returns>
+        public AssemblyLoadContextBuilder IsLazyLoaded(bool isLazyLoaded)
+        {
+            _lazyLoadReferences = isLazyLoaded;
+            return this;
+        }
+
         /// <summary>
         /// Add a managed library to the load context.
         /// </summary>

src/Plugins/Loader/ManagedLoadContext.cs

@@ -24,11 +24,12 @@ internal class ManagedLoadContext : AssemblyLoadContext
         private readonly IReadOnlyDictionary<string, ManagedLibrary> _managedAssemblies;
         private readonly IReadOnlyDictionary<string, NativeLibrary> _nativeLibraries;
         private readonly IReadOnlyCollection<string> _privateAssemblies;
-        private readonly IReadOnlyCollection<string> _defaultAssemblies;
+        private readonly ICollection<string> _defaultAssemblies;
         private readonly IReadOnlyCollection<string> _additionalProbingPaths;
         private readonly bool _preferDefaultLoadContext;
         private readonly string[] _resourceRoots;
         private readonly bool _loadInMemory;
+        private readonly bool _lazyLoadReferences;
         private readonly AssemblyLoadContext _defaultLoadContext;
 #if FEATURE_NATIVE_RESOLVER
         private readonly AssemblyDependencyResolver _dependencyResolver;
@@ -45,6 +46,7 @@ internal class ManagedLoadContext : AssemblyLoadContext
             IReadOnlyCollection<string> resourceProbingPaths,
             AssemblyLoadContext defaultLoadContext,
             bool preferDefaultLoadContext,
+            bool lazyLoadReferences,
             bool isCollectible,
             bool loadInMemory,
             bool shadowCopyNativeLibraries)
@@ -64,12 +66,13 @@ internal class ManagedLoadContext : AssemblyLoadContext
             _basePath = Path.GetDirectoryName(mainAssemblyPath) ?? throw new ArgumentException(nameof(mainAssemblyPath));
             _managedAssemblies = managedAssemblies ?? throw new ArgumentNullException(nameof(managedAssemblies));
             _privateAssemblies = privateAssemblies ?? throw new ArgumentNullException(nameof(privateAssemblies));
-            _defaultAssemblies = defaultAssemblies ?? throw new ArgumentNullException(nameof(defaultAssemblies));
+            _defaultAssemblies = defaultAssemblies != null ? defaultAssemblies.ToList() : throw new ArgumentNullException(nameof(defaultAssemblies));
             _nativeLibraries = nativeLibraries ?? throw new ArgumentNullException(nameof(nativeLibraries));
             _additionalProbingPaths = additionalProbingPaths ?? throw new ArgumentNullException(nameof(additionalProbingPaths));
             _defaultLoadContext = defaultLoadContext;
             _preferDefaultLoadContext = preferDefaultLoadContext;
             _loadInMemory = loadInMemory;
+            _lazyLoadReferences = lazyLoadReferences;
 
             _resourceRoots = new[] { _basePath }
                 .Concat(resourceProbingPaths)
@@ -105,6 +108,19 @@ internal class ManagedLoadContext : AssemblyLoadContext
                     var defaultAssembly = _defaultLoadContext.LoadFromAssemblyName(assemblyName);
                     if (defaultAssembly != null)
                     {
+                        // Add referenced assemblies to the list of default assemblies.
+                        // This is basically lazy loading
+                        if (_lazyLoadReferences)
+                        {
+                            foreach (var reference in defaultAssembly.GetReferencedAssemblies())
+                            {
+                                if (reference.Name != null && !_defaultAssemblies.Contains(reference.Name))
+                                {
+                                    _defaultAssemblies.Add(reference.Name);
+                                }
+                            }
+                        }
+
                         // Older versions used to return null here such that returned assembly would be resolved from the default ALC.
                         // However, with the addition of custom default ALCs, the Default ALC may not be the user's chosen ALC when
                         // this context was built. As such, we simply return the Assembly from the user's chosen default load context.

src/Plugins/PluginConfig.cs

@@ -62,10 +62,22 @@ public PluginConfig(string mainAssemblyPath)
         /// </summary>
         public bool PreferSharedTypes { get; set; }
 
-        /// <summary>
+        /// <summary>
+        /// If enabled, will lazy load dependencies of all shared assemblies.
+        /// Reduces plugin load time at the expense of non-determinism in how transitive dependencies are loaded
+        /// between the plugin and the host.
+        ///
+        /// Please be aware of the danger of using this option:
+        /// <seealso href="https://github.com/natemcmaster/DotNetCorePlugins/pull/164#issuecomment-751557873">
+        /// https://github.com/natemcmaster/DotNetCorePlugins/pull/164#issuecomment-751557873
+        /// </seealso>
+        /// </summary>
+        public bool IsLazyLoaded { get; set; } = false;
+
+        /// <summary>
         /// If set, replaces the default <see cref="AssemblyLoadContext"/> used by the <see cref="PluginLoader"/>.
         /// Use this feature if the <see cref="AssemblyLoadContext"/> of the <see cref="Assembly"/> is not the Runtime's default load context.
-        /// i.e. (AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly) != <see cref="AssemblyLoadContext.Default"/>
+        /// i.e. (AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly) != <see cref="AssemblyLoadContext.Default"/>
         /// </summary>
         public AssemblyLoadContext DefaultContext { get; set; } = AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly()) ?? AssemblyLoadContext.Default;
 
@@ -104,9 +116,9 @@ public bool LoadInMemory
         /// </remarks>
         public bool EnableHotReload { get; set; }
 
-        /// <summary>
+        /// <summary>
         /// Specifies the delay to reload a plugin, after file changes have been detected.
-        /// Default value is 200 milliseconds.
+        /// Default value is 200 milliseconds.
         /// </summary>
         public TimeSpan ReloadDelay { get; set; } = TimeSpan.FromMilliseconds(200);
 #endif

src/Plugins/PluginLoader.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Reflection;
 using System.Runtime.Loader;
@@ -112,9 +113,15 @@ public static PluginLoader CreateFromAssemblyFile(string assemblyFile, Type[] sh
                     {
                         if (sharedTypes != null)
                         {
+                            var uniqueAssemblies = new HashSet<Assembly>();
                             foreach (var type in sharedTypes)
                             {
-                                config.SharedAssemblies.Add(type.Assembly.GetName());
+                                uniqueAssemblies.Add(type.Assembly);
+                            }
+
+                            foreach (var assembly in uniqueAssemblies)
+                            {
+                                config.SharedAssemblies.Add(assembly.GetName());
                             }
                         }
                         configure(config);
@@ -374,6 +381,7 @@ private static AssemblyLoadContextBuilder CreateLoadContextBuilder(PluginConfig
             }
 #endif
 
+            builder.IsLazyLoaded(config.IsLazyLoaded);
             foreach (var assemblyName in config.SharedAssemblies)
             {
                 builder.PreferDefaultLoadContextAssembly(assemblyName);

src/Plugins/PublicAPI.Unshipped.txt

@@ -0,0 +1,3 @@
+McMaster.NETCore.Plugins.PluginConfig.IsLazyLoaded.get -> bool
+McMaster.NETCore.Plugins.PluginConfig.IsLazyLoaded.set -> void
+McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder.IsLazyLoaded(bool isLazyLoaded) -> McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder!

src/Plugins/releasenotes.props

@@ -2,7 +2,8 @@
   <PropertyGroup>
     <PackageReleaseNotes Condition="'$(VersionPrefix)' == '1.4.0'">
 Changes:
-* @Sewer56 - search in additional probing paths (PR #172)
+* @Sewer56 - feature: add option to support lazy loading of transitive dependencies to increase performance (PR #164)
+* @Sewer56 - bugfix: search in additional probing paths (PR #172)
     </PackageReleaseNotes>
     <PackageReleaseNotes Condition="'$(VersionPrefix)' == '1.3.1'">
 Changes:

test/Plugins.Tests/SharedTypesTests.cs

@@ -42,13 +42,12 @@ public void PluginsCanForceSharedTypes()
         /// accounted for. Without this, the order in which code loads
         /// could cause different assembly versions to be loaded.
         /// </summary>
-        [Fact]
-        public void TransitiveAssembliesOfSharedTypesAreResolved()
+        [Theory]
+        [InlineData(true)]
+        [InlineData(false)]
+        public void TransitiveAssembliesOfSharedTypesAreResolved(bool isLazyLoaded)
         {
-            using var loader = PluginLoader.CreateFromAssemblyFile(
-                    TestResources.GetTestProjectAssembly("TransitivePlugin"),
-                    sharedTypes: new[] { typeof(SharedType) });
-
+            using var loader = PluginLoader.CreateFromAssemblyFile(TestResources.GetTestProjectAssembly("TransitivePlugin"), sharedTypes: new[] { typeof(SharedType) }, config => config.IsLazyLoaded = isLazyLoaded);
             var assembly = loader.LoadDefaultAssembly();
             var configType = assembly.GetType("TransitivePlugin.PluginConfig", throwOnError: true)!;
             var config = Activator.CreateInstance(configType);