types: fix crash when extending a type graph

This is a workaround-quality fix. A more robust solution would involve constructor changes in every model type. Base types would need to enter through the "wrap existing" constructor instead of the "extends" constructor.

Source/ActivityPub.Types/TypeMap.cs

@@ -187,15 +187,15 @@ public TObject AsModel<TObject>()
     /// <summary>
     ///     Extends the TypeGraph to include a new entity.
     ///     The entity must have a parameterless constructor.
-    ///     Throws an exception if the entity would be a duplicate or have unmet dependencies.
+    ///     If the entity already exists in the graph, then the existing instance is re-used.
+    ///     Throws an exception if the entity would have unmet dependencies.
     /// </summary>
     /// <remarks>
     ///     This function cannot accomodate entities with required members.
     ///     Instead, <see cref="Extend{TEntity}(TEntity)"/> must be used.
     /// </remarks>
-    /// <exception cref="InvalidOperationException">If the entity type already exists in the graph</exception>
     /// <exception cref="InvalidOperationException">If the entity requires another entity that is missing from the graph</exception>
-    /// <see cref="Extend{TEntity}(TEntity)"/>
+    /// <seealso cref="Extend{TEntity}(TEntity)"/>
     /// <seealso cref="AddEntity"/>
     /// <seealso cref="TryAddEntity"/>
     public TEntity Extend<TEntity>()
@@ -204,21 +204,18 @@ public TEntity Extend<TEntity>()
 
     /// <summary>
     ///     Extends the TypeGraph to include a new entity.
-    ///     Throws an exception if the entity would be a duplicate or have unmet dependencies.
+    ///     If the entity already exists in the graph, then the existing instance is re-used.
+    ///     Throws an exception if the entity would have unmet dependencies.
     /// </summary>
-    /// <param name="entity"></param>
-    /// <typeparam name="TEntity"></typeparam>
-    /// <returns></returns>
-    /// <exception cref="InvalidOperationException">If the entity type already exists in the graph</exception>
     /// <exception cref="InvalidOperationException">If the entity requires another entity that is missing from the graph</exception>
-    /// <see cref="Extend{TEntity}()"/>
+    /// <seealso cref="Extend{TEntity}()"/>
     /// <seealso cref="AddEntity"/>
     /// <seealso cref="TryAddEntity"/>
     public TEntity Extend<TEntity>(TEntity entity)
         where TEntity : ASEntity
     {
-        if (IsEntity<TEntity>())
-            throw new InvalidOperationException($"Cannot extend the graph with entity {typeof(TEntity)}: that type already exists in the graph");
+        if (IsEntity<TEntity>(out var existingEntity))
+            return existingEntity;
 
         // Check dependencies - this is a workaround to avoid the risk case described in TryAdd().
         if (entity.BaseTypeName != null && !AllASTypes.Contains(entity.BaseTypeName))

Test/ActivityPub.Types.Tests/Unit/AS/APActorTests.cs

@@ -0,0 +1,52 @@
+// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
+// If a copy of the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+using ActivityPub.Types.AS;
+using ActivityPub.Types.AS.Extended.Actor;
+using ActivityPub.Types.AS.Extended.Object;
+
+namespace ActivityPub.Types.Tests.Unit.AS;
+
+public abstract class APActorTests
+{
+    public class ExtendConstructorShould : APActorTests
+    {
+        [Fact]
+        public void ExtendRegularObject()
+        {
+            var note = new NoteObject()
+            {
+                Id = "https://example.com/object",
+                Content = "content"
+            };
+
+            var actor = new APActor(note)
+            {
+                Inbox = "https://example.com/inbox",
+                Outbox = "https://example.com/outbox"
+            };
+
+            actor.Is<NoteObject>().Should().BeTrue();
+            note.Is<APActor>().Should().BeTrue();
+        }
+
+        [Fact]
+        public void NoOpWhenWrappingAnotherActor()
+        {
+            var person = new PersonActor()
+            {
+                Inbox = "https://example.com/inbox",
+                Outbox = "https://example.com/outbox"
+            };
+            var actor = new APActor(person)
+            {
+                Inbox = person.Inbox,
+                Outbox = person.Outbox
+            };
+
+            person.Inbox = "https://example.com/changed";
+
+            actor.Inbox.Should().Be(person.Inbox);
+        }
+    }
+}