Refactored.

src/Volo.Abp/Volo/Abp/Modularity/DependsOnAttribute.cs

@@ -1,4 +1,5 @@
 using System;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity
 {
@@ -12,6 +13,7 @@ namespace Volo.Abp.Modularity
         /// <summary>
         /// Types of depended modules.
         /// </summary>
+        [NotNull]
         public Type[] DependedModuleTypes { get; }
 
         /// <summary>
@@ -20,9 +22,10 @@ namespace Volo.Abp.Modularity
         /// <param name="dependedModuleTypes">Types of depended modules</param>
         public DependsOnAttribute(params Type[] dependedModuleTypes)
         {
-            DependedModuleTypes = dependedModuleTypes;
+            DependedModuleTypes = dependedModuleTypes ?? new Type[0];
         }
 
+        [NotNull]
         public virtual Type[] GetDependedModuleTypes()
         {
             return DependedModuleTypes;

src/Volo.Abp/Volo/Abp/Modularity/IDependedModuleTypesProvider.cs

@@ -1,9 +1,11 @@
 using System;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity
 {
     public interface IDependedModuleTypesProvider
     {
+        [NotNull]
         Type[] GetDependedModuleTypes();
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/IModuleLifecycleContributer.cs

@@ -1,11 +1,12 @@
-using Volo.DependencyInjection;
+using JetBrains.Annotations;
+using Volo.DependencyInjection;
 
 namespace Volo.Abp.Modularity
 {
     public interface IModuleLifecycleContributer : ISingletonDependency
     {
-        void Initialize(IAbpModule module);
+        void Initialize([NotNull] IAbpModule module);
 
-        void Shutdown(IAbpModule module);
+        void Shutdown([NotNull] IAbpModule module);
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/IModuleLoader.cs

@@ -1,4 +1,5 @@
 using System;
+using JetBrains.Annotations;
 using Microsoft.Extensions.DependencyInjection;
 using Volo.Abp.Modularity.PlugIns;
 
@@ -6,6 +7,7 @@ namespace Volo.Abp.Modularity
 {
     public interface IModuleLoader
     {
-        AbpModuleDescriptor[] LoadModules(IServiceCollection services, Type startupModuleType, PlugInSourceList plugInSources);
+        [NotNull]
+        AbpModuleDescriptor[] LoadModules([NotNull] IServiceCollection services, [NotNull] Type startupModuleType, [NotNull] PlugInSourceList plugInSources);
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/IModuleManager.cs

@@ -1,9 +1,11 @@
 using System.Collections.Generic;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity
 {
     public interface IModuleManager
     {
+        [NotNull]
         IReadOnlyList<AbpModuleDescriptor> Modules { get; }
 
         void InitializeModules();

src/Volo.Abp/Volo/Abp/Modularity/ModuleLoader.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 using Microsoft.Extensions.DependencyInjection;
 using Volo.Abp.Modularity.PlugIns;
 using Volo.ExtensionMethods.Collections.Generic;
@@ -9,8 +10,13 @@ namespace Volo.Abp.Modularity
 {
     public class ModuleLoader : IModuleLoader
     {
-        public AbpModuleDescriptor[] LoadModules(IServiceCollection services, Type startupModuleType, PlugInSourceList plugInSources)
+        [NotNull]
+        public AbpModuleDescriptor[] LoadModules([NotNull] IServiceCollection services, [NotNull] Type startupModuleType, [NotNull] PlugInSourceList plugInSources)
         {
+            Check.NotNull(services, nameof(services));
+            Check.NotNull(startupModuleType, nameof(startupModuleType));
+            Check.NotNull(plugInSources, nameof(plugInSources));
+
             var modules = new List<AbpModuleDescriptor>();
 
             FillModules(modules, services, startupModuleType, plugInSources);

src/Volo.Abp/Volo/Abp/Modularity/PlugIns/FolderPlugInSource.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using JetBrains.Annotations;
 using Volo.Abp.Reflection;
 using Volo.ExtensionMethods.Collections.Generic;
 
@@ -12,13 +13,16 @@ namespace Volo.Abp.Modularity.PlugIns
 
         public SearchOption SearchOption { get; set; }
 
-        public FolderPlugInSource(string folder, SearchOption searchOption = SearchOption.TopDirectoryOnly)
+        public FolderPlugInSource([NotNull] string folder, SearchOption searchOption = SearchOption.TopDirectoryOnly)
         {
+            Check.NotNull(folder, nameof(folder));
+
             Folder = folder;
             SearchOption = searchOption;
         }
 
-        public List<Type> GetModules()
+        [NotNull]
+        public Type[] GetModules()
         {
             var modules = new List<Type>();
 
@@ -41,7 +45,7 @@ namespace Volo.Abp.Modularity.PlugIns
                 }
             }
 
-            return modules;
+            return modules.ToArray();
         }
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/PlugIns/IPlugInSource.cs

@@ -1,10 +1,11 @@
 using System;
-using System.Collections.Generic;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity.PlugIns
 {
     public interface IPlugInSource
     {
-        List<Type> GetModules();
+        [NotNull]
+        Type[] GetModules();
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/PlugIns/PlugInSourceExtensions.cs

@@ -1,18 +1,21 @@
 using System;
-using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity.PlugIns
 {
     public static class PlugInSourceExtensions
     {
-        public static List<Type> GetModulesWithAllDependencies(this IPlugInSource plugInSource)
+        [NotNull]
+        public static Type[] GetModulesWithAllDependencies([NotNull] this IPlugInSource plugInSource)
         {
+            Check.NotNull(plugInSource, nameof(plugInSource));
+
             return plugInSource
                 .GetModules()
                 .SelectMany(AbpModuleHelper.FindAllModuleTypes)
                 .Distinct()
-                .ToList();
+                .ToArray();
         }
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/PlugIns/PlugInSourceList.cs

@@ -1,17 +1,19 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity.PlugIns
 {
     public class PlugInSourceList : List<IPlugInSource>
     {
-        internal List<Type> GetAllModules()
+        [NotNull]
+        internal Type[] GetAllModules()
         {
             return this
                 .SelectMany(pluginSource => pluginSource.GetModulesWithAllDependencies())
                 .Distinct()
-                .ToList();
+                .ToArray();
         }
     }
 }

src/Volo.Abp/Volo/Abp/Modularity/PlugIns/PlugInSourceListExtensions.cs

@@ -1,17 +1,22 @@
 using System;
 using System.IO;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity.PlugIns
 {
     public static class PlugInSourceListExtensions
     {
-        public static void AddFolder(this PlugInSourceList list, string folder, SearchOption searchOption = SearchOption.TopDirectoryOnly)
+        public static void AddFolder([NotNull] this PlugInSourceList list, [NotNull] string folder, SearchOption searchOption = SearchOption.TopDirectoryOnly)
         {
+            Check.NotNull(list, nameof(list));
+
             list.Add(new FolderPlugInSource(folder, searchOption));
         }
 
-        public static void AddTypes(this PlugInSourceList list, params Type[] moduleTypes)
+        public static void AddTypes([NotNull] this PlugInSourceList list, params Type[] moduleTypes)
         {
+            Check.NotNull(list, nameof(list));
+
             list.Add(new PlugInTypeListSource(moduleTypes));
         }
     }

src/Volo.Abp/Volo/Abp/Modularity/PlugIns/PlugInTypeListSource.cs

@@ -1,6 +1,5 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
+using JetBrains.Annotations;
 
 namespace Volo.Abp.Modularity.PlugIns
 {
@@ -10,12 +9,13 @@ namespace Volo.Abp.Modularity.PlugIns
 
         public PlugInTypeListSource(params Type[] moduleTypes)
         {
-            _moduleTypes = moduleTypes;
+            _moduleTypes = moduleTypes ?? new Type[0];
         }
 
-        public List<Type> GetModules()
+        [NotNull]
+        public Type[] GetModules()
         {
-            return _moduleTypes.ToList();
+            return _moduleTypes;
         }
     }
 }