Get rid of RegistryItems<> - it is the same as IReadOnlySet (#255)

* Get rid of `RegistryItems<>` - it is the same as `IReadOnlySet`

* Make EntitySetChangeRegistry readonly struct

* EntitySetChangeRegistry = new()

* Make `EntityEventBroker` struct

* Fix readonly modifier

* Make NonPairedReferenceChangesRegistry  struct

* [0] instead of .First()

* Minor improvements

* Make CommandProcessorContextProvider struct

* Make KeyMapper struct

* Make ReferenceFieldsChangesRegistry struct

Author: SergeiPavlov

.editorconfig

@@ -36,7 +36,7 @@ tab_width = 2
 
 # New line preferences
 end_of_line = lf
-insert_final_newline = false
+insert_final_newline = true
 
 #### .NET Coding Conventions ####
 # https://docs.microsoft.com/en-us/visualstudio/ide/editorconfig-language-conventions?view=vs-2019#net-code-style-settings

Orm/Xtensive.Orm.Tests/Storage/CommandProcessorContextProviderTest.cs

@@ -19,8 +19,7 @@ protected override void PopulateData()
     public void GetContextTest()
     {
       var session = Session.Demand();
-      var provider = session.CommandProcessorContextProvider;
-      var context = provider.ProvideContext();
+      var context = session.CommandProcessorContextProvider.ProvideContext();
 
       Assert.That(context, Is.Not.Null);
       Assert.That(context.AllowPartialExecution, Is.False);
@@ -30,15 +29,14 @@ public void GetContextTest()
       Assert.That(context.ProcessingTasks, Is.Not.Null);
       Assert.That(context.ProcessingTasks.Count, Is.EqualTo(0));
 
-      Assert.That(provider.ProvideContext(), Is.Not.SameAs(provider.ProvideContext()));
+      Assert.That(session.CommandProcessorContextProvider.ProvideContext(), Is.Not.SameAs(session.CommandProcessorContextProvider.ProvideContext()));
     }
 
     [Test]
     public void GetContextForPartialExecutionTest()
     {
       var session = Session.Demand();
-      var provider = session.CommandProcessorContextProvider;
-      var context = provider.ProvideContext(true);
+      var context = session.CommandProcessorContextProvider.ProvideContext(true);
 
       Assert.That(context, Is.Not.Null);
       Assert.That(context.AllowPartialExecution, Is.True);
@@ -48,7 +46,7 @@ public void GetContextForPartialExecutionTest()
       Assert.That(context.ProcessingTasks, Is.Not.Null);
       Assert.That(context.ProcessingTasks.Count, Is.EqualTo(0));
 
-      Assert.That(provider.ProvideContext(true), Is.Not.SameAs(provider.ProvideContext(true)));
+      Assert.That(session.CommandProcessorContextProvider.ProvideContext(true), Is.Not.SameAs(session.CommandProcessorContextProvider.ProvideContext(true)));
     }
   }
-}
+}

Orm/Xtensive.Orm/Orm/CommandProcessorContextProvider.cs

@@ -10,19 +10,15 @@
 
 namespace Xtensive.Orm
 {
-  public sealed class CommandProcessorContextProvider : SessionBound
+  internal struct CommandProcessorContextProvider()
   {
-    private readonly ConcurrentDictionary<CommandProcessorContext, CommandProcessorContext> providedContexts
-      = new ConcurrentDictionary<CommandProcessorContext, CommandProcessorContext>();
+    private readonly ConcurrentDictionary<CommandProcessorContext, CommandProcessorContext> providedContexts = new();
 
     /// <inheritdoc/>
-    public int AliveContextCount { get { return providedContexts.Count; } }
+    public int AliveContextCount => providedContexts.Count;
 
     /// <inheritdoc/>
-    public CommandProcessorContext ProvideContext()
-    {
-      return ProvideContext(false);
-    }
+    public CommandProcessorContext ProvideContext() => ProvideContext(false);
 
     /// <inheritdoc/>
     public CommandProcessorContext ProvideContext(bool allowPartialExecution)
@@ -40,10 +36,5 @@ private void RemoveDisposedContext(object sender, EventArgs args)
       providedContexts.TryRemove(context, out _);
       context.Disposed -= RemoveDisposedContext;
     }
-    
-    internal CommandProcessorContextProvider(Session session)
-      : base(session)
-    {
-    }
   }
-}
+}

Orm/Xtensive.Orm/Orm/EntityEventBroker.cs

@@ -14,7 +14,7 @@ namespace Xtensive.Orm
   /// <summary>
   /// Manages events related to <see cref="Entity"/>.
   /// </summary>
-  public sealed class EntityEventBroker
+  public struct EntityEventBroker
   {
     private Dictionary<(Key, FieldInfo, object), Delegate> subscribers;
 
@@ -258,4 +258,4 @@ public void RemapKeys(KeyMapping keyMapping)
       }
     }
   }
-}
+}

Orm/Xtensive.Orm/Orm/Internals/ChangeRegistries/EntityChangeRegistry.cs

@@ -61,8 +61,7 @@ internal void Register(EntityState item)
     /// </summary>
     /// <param name="state">The state of items to get.</param>
     /// <returns>The sequence of items with specified state.</returns>
-    public RegistryItems<EntityState> GetItems(in PersistenceState state) =>
-      new(GetContainer(state));
+    public IReadOnlySet<EntityState> GetItems(in PersistenceState state) => GetContainer(state);
 
     /// <summary>
     /// Clears the registry.

Orm/Xtensive.Orm/Orm/Internals/ChangeRegistries/EntitySetChangeRegistry.cs

@@ -11,14 +11,14 @@ namespace Xtensive.Orm.Internals
   /// <summary>
   /// Contains <see cref="EntitySetState"/>s which modified during the bounded session.
   /// </summary>
-  public sealed class EntitySetChangeRegistry : SessionBound
+  public readonly struct EntitySetChangeRegistry()
   {
     private readonly HashSet<EntitySetState> modifiedEntitySets = new();
 
     /// <summary>
     /// Count of registered <see cref="EntitySetState"/>.
     /// </summary>
-    public int Count { get { return modifiedEntitySets.Count; } }
+    public int Count => modifiedEntitySets.Count;
 
     /// <summary>
     /// Register the specified <see cref="EntitySetState"/>.
@@ -30,21 +30,11 @@ public sealed class EntitySetChangeRegistry : SessionBound
     /// Gets all registered items.
     /// </summary>
     /// <returns></returns>
-    public RegistryItems<EntitySetState> GetItems() => new(modifiedEntitySets);
+    public IReadOnlySet<EntitySetState> GetItems() => modifiedEntitySets;
 
     /// <summary>
     /// Clears the registry.
     /// </summary>
-    public void Clear() => modifiedEntitySets.Clear();
-
-    /// <summary>
-    /// Initializes a new instance of this class.
-    /// </summary>
-    /// <param name="session"><see cref="Session"/>, to which current instance 
-    /// is bound.</param>
-    public EntitySetChangeRegistry(Session session)
-      : base(session)
-    {
-    }
+    public void Clear() => modifiedEntitySets?.Clear();
   }
 }