Extract base class for LinkList

Author: Matthias Gessinger

src/Abstractions/src/Asp.Versioning.Abstractions/LinkList.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+
+namespace Asp.Versioning;
+
+using System.Collections.ObjectModel;
+
+internal abstract class LinkList : Collection<LinkHeaderValue>
+{
+    public LinkList() { }
+
+    protected override void InsertItem( int index, LinkHeaderValue item )
+    {
+        EnsureRelationType( item );
+        base.InsertItem( index, item );
+    }
+
+    protected override void SetItem( int index, LinkHeaderValue item )
+    {
+        EnsureRelationType( item );
+        base.SetItem( index, item );
+    }
+
+    protected abstract void EnsureRelationType( LinkHeaderValue item );
+}

src/Abstractions/src/Asp.Versioning.Abstractions/SunsetPolicy.cs

@@ -2,40 +2,46 @@
 
 namespace Asp.Versioning;
 
-using System.Collections.ObjectModel;
-
 /// <summary>
 /// Represents an API version sunset policy.
 /// </summary>
 public class SunsetPolicy
 {
-    private SunsetLinkList? links;
+    private readonly LinkList links;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="SunsetPolicy"/> class.
     /// </summary>
-    public SunsetPolicy() { }
+    public SunsetPolicy()
+    {
+        links = new SunsetLinkList();
+    }
 
     /// <summary>
     /// Initializes a new instance of the <see cref="SunsetPolicy"/> class.
     /// </summary>
     /// <param name="date">The date and time when the API version will be sunset.</param>
     /// <param name="link">The optional link which provides information about the sunset policy.</param>
     public SunsetPolicy( DateTimeOffset date, LinkHeaderValue? link = default )
+        : this()
     {
         Date = date;
 
         if ( link is not null )
         {
-            links = new() { link };
+            links.Add( link );
         }
     }
 
     /// <summary>
     /// Initializes a new instance of the <see cref="SunsetPolicy"/> class.
     /// </summary>
     /// <param name="link">The link which provides information about the sunset policy.</param>
-    public SunsetPolicy( LinkHeaderValue link ) => links = new() { link };
+    public SunsetPolicy( LinkHeaderValue link )
+        : this()
+    {
+        links.Add( link );
+    }
 
     /// <summary>
     /// Gets the date and time when the API version will be sunset.
@@ -47,7 +53,7 @@ public SunsetPolicy() { }
     /// Gets a value indicating whether the sunset policy has any associated links.
     /// </summary>
     /// <value>True if the sunset policy has associated links; otherwise, false.</value>
-    public bool HasLinks => links is not null && links.Count > 0;
+    public bool HasLinks => links.Count > 0;
 
     /// <summary>
     /// Gets a read-only list of links that provide information about the sunset policy.
@@ -56,23 +62,11 @@ public SunsetPolicy() { }
     /// <remarks>If a link is provided, generally only one link is necessary; however, additional
     /// links might be provided for different languages or different formats such as a HTML page
     /// or a JSON file.</remarks>
-    public IList<LinkHeaderValue> Links => links ??= new();
+    public IList<LinkHeaderValue> Links => links;
 
-    private sealed class SunsetLinkList : Collection<LinkHeaderValue>
+    internal sealed class SunsetLinkList : LinkList
     {
-        protected override void InsertItem( int index, LinkHeaderValue item )
-        {
-            base.InsertItem( index, item );
-            EnsureRelationType( item );
-        }
-
-        protected override void SetItem( int index, LinkHeaderValue item )
-        {
-            base.SetItem( index, item );
-            EnsureRelationType( item );
-        }
-
-        private static void EnsureRelationType( LinkHeaderValue item )
+        protected override void EnsureRelationType( LinkHeaderValue item )
         {
             if ( !item.RelationType.Equals( "sunset", StringComparison.OrdinalIgnoreCase ) )
             {