fix #1135

src/System.CommandLine.Tests/SuggestionTests.cs

@@ -987,7 +987,7 @@ public void Enum_suggestions_can_be_configured_without_list_clear()
             {
                 var command = new Command("the-command")
                 {
-                    new Argument<DayOfWeek?>()
+                    new Argument<DayOfWeek?>
                     {
                         Suggestions = { "mon", "tues", "wed", "thur", "fri", "sat", "sun" }
                     }

src/System.CommandLine/Argument.cs

@@ -19,7 +19,7 @@ public class Argument : Symbol, IArgument
         private IArgumentArity? _arity;
         private TryConvertArgument? _convertArguments;
         private Type _argumentType = typeof(string);
-        private List<ISuggestionSource>? _suggestions = null;
+        private SuggestionSourceList? _suggestions = null;
 
         /// <summary>
         /// Initializes a new instance of the Argument class.
@@ -93,13 +93,13 @@ internal TryConvertArgument? ConvertArguments
         /// <summary>
         /// Gets the list of suggestion sources for the argument.
         /// </summary>
-        public List<ISuggestionSource> Suggestions
+        public SuggestionSourceList Suggestions
         { 
             get
             {
                 if (_suggestions is null)
                 {
-                    _suggestions = new List<ISuggestionSource>
+                    _suggestions = new SuggestionSourceList
                     {
                         SuggestionSource.ForType(ArgumentType)
                     };

src/System.CommandLine/Builder/CommandLineBuilderExtensions.cs

@@ -10,7 +10,6 @@
 using System.IO;
 using System.Linq;
 using System.Reflection;
-using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using static System.Environment;

src/System.CommandLine/SuggestionSourceExtensions.cs

@@ -1,15 +1,14 @@
 // Copyright (c) .NET Foundation and contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using System.Collections.Generic;
 using System.CommandLine.Suggestions;
 
 namespace System.CommandLine
 {
     public static class SuggestionSourceExtensions
     {
         public static void Add(
-            this List<ISuggestionSource> suggestionSources,
+            this SuggestionSourceList suggestionSources,
             SuggestDelegate suggest)
         {
             if (suggestionSources is null)
@@ -26,7 +25,7 @@ public static void Add(
         }
 
         public static void Add(
-            this List<ISuggestionSource> suggestionSources,
+            this SuggestionSourceList suggestionSources,
             params string[] suggestions)
         {
             if (suggestionSources is null)

src/System.CommandLine/SuggestionSourceList.cs

@@ -0,0 +1,32 @@
+// // Copyright (c) .NET Foundation and contributors. All rights reserved.
+// // Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Collections;
+using System.Collections.Generic;
+using System.CommandLine.Suggestions;
+
+namespace System.CommandLine
+{
+    public class SuggestionSourceList : IReadOnlyList<ISuggestionSource>
+    {
+        private readonly List<ISuggestionSource> _sources = new List<ISuggestionSource>();
+
+        public void Add(ISuggestionSource source)
+        {
+            _sources.Add(source);
+        }
+
+        public IEnumerator<ISuggestionSource> GetEnumerator() => _sources.GetEnumerator();
+
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+        public void Clear()
+        {
+            _sources.Clear();
+        }
+
+        public int Count => _sources.Count;
+
+        public ISuggestionSource this[int index] => _sources[index];
+    }
+}