Fix #1726

System.CommandLine.v3.ncrunchsolution

@@ -2,8 +2,8 @@
   <Settings>
     <AllowParallelTestExecution>True</AllowParallelTestExecution>
     <CustomBuildProperties>
-      <Value>TargetFrameworks = net8.0</Value>
-      <Value>TargetFramework = net8.0</Value>
+      <Value>TargetFrameworks = net10.0</Value>
+      <Value>TargetFramework = net10.0</Value>
     </CustomBuildProperties>
     <EnableRDI>False</EnableRDI>
     <RdiConfigured>True</RdiConfigured>

src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt

@@ -35,6 +35,7 @@
     public static Argument<T> AcceptLegalFileNamesOnly<T>(this Argument<T> argument)
     public static Argument<T> AcceptLegalFilePathsOnly<T>(this Argument<T> argument)
     public static Argument<T> AcceptOnlyFromAmong<T>(this Argument<T> argument, System.String[] values)
+    public static Argument<T> AcceptOnlyFromAmong<T>(this Argument<T> argument, System.StringComparer comparer, System.String[] values)
   public class Command : Symbol, System.Collections.IEnumerable
     .ctor(System.String name, System.String description = null)
     public System.CommandLine.Invocation.CommandLineAction Action { get; set; }
@@ -99,6 +100,7 @@
     public Option<T> AcceptLegalFileNamesOnly()
     public Option<T> AcceptLegalFilePathsOnly()
     public Option<T> AcceptOnlyFromAmong(System.String[] values)
+    public Option<T> AcceptOnlyFromAmong(System.StringComparer comparer, System.String[] values)
   public static class OptionValidation
     public static Option<System.IO.FileInfo> AcceptExistingOnly(this Option<System.IO.FileInfo> option)
     public static Option<System.IO.DirectoryInfo> AcceptExistingOnly(this Option<System.IO.DirectoryInfo> option)

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -107,4 +107,39 @@ public void Argument_of_enum_can_limit_enum_members_as_valid_values()
               .Should()
               .BeEquivalentTo(new[] { $"Argument 'Fuschia' not recognized. Must be one of:\n\t'Red'\n\t'Green'" });
     }
+
+    [Fact]
+    public void AcceptOnlyFromAmong_with_comparer_is_case_insensitive()
+    {
+        var argument = new Argument<string>("name");
+        argument.AcceptOnlyFromAmong(StringComparer.OrdinalIgnoreCase, "NAME1", "NAME2");
+
+        Command command = new("run")
+        {
+            argument
+        };
+
+        var result = command.Parse("run name1");
+
+        result.Errors.Should().BeEmpty();
+    }
+
+    [Fact]
+    public void AcceptOnlyFromAmong_with_comparer_rejects_invalid_values()
+    {
+        var argument = new Argument<string>("name");
+        argument.AcceptOnlyFromAmong(StringComparer.OrdinalIgnoreCase, "NAME1", "NAME2");
+
+        Command command = new("run")
+        {
+            argument
+        };
+
+        var result = command.Parse("run NAME3");
+
+        result.Errors
+              .Select(e => e.Message)
+              .Should()
+              .BeEquivalentTo(new[] { $"Argument 'NAME3' not recognized. Must be one of:\n\t'NAME1'\n\t'NAME2'" });
+    }
 }

src/System.CommandLine.Tests/OptionTests.cs

@@ -408,6 +408,31 @@ public void Option_of_enum_can_limit_enum_members_as_valid_values()
                   .BeEquivalentTo(new[] { $"Argument 'Fuschia' not recognized. Must be one of:\n\t'Red'\n\t'Green'" });
         }
 
+        [Fact]
+        public void AcceptOnlyFromAmong_with_comparer_is_case_insensitive()
+        {
+            Option<string> option = new("--name");
+            option.AcceptOnlyFromAmong(StringComparer.OrdinalIgnoreCase, "NAME1", "NAME2");
+
+            var result = new RootCommand { option }.Parse("--name name1");
+
+            result.Errors.Should().BeEmpty();
+        }
+
+        [Fact]
+        public void AcceptOnlyFromAmong_with_comparer_rejects_invalid_values()
+        {
+            Option<string> option = new("--name");
+            option.AcceptOnlyFromAmong(StringComparer.OrdinalIgnoreCase, "NAME1", "NAME2");
+
+            var result = new RootCommand { option }.Parse("--name NAME3");
+
+            result.Errors
+                  .Select(e => e.Message)
+                  .Should()
+                  .BeEquivalentTo(new[] { $"Argument 'NAME3' not recognized. Must be one of:\n\t'NAME1'\n\t'NAME2'" });
+        }
+
         [Fact]
         public void Option_result_provides_identifier_token_if_name_was_provided()
         {

src/System.CommandLine/ArgumentValidation.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.CommandLine.Parsing;
 using System.IO;
+using System.Linq;
 
 namespace System.CommandLine
 {
@@ -131,7 +132,21 @@ public static Argument<T> AcceptOnlyFromAmong<T>(
             this Argument<T> argument, 
             params string[] values)
         {
-            if (values is not null && values.Length > 0)
+            return AcceptOnlyFromAmong(argument, StringComparer.Ordinal, values);
+        }
+
+        /// <summary>
+        /// Configures the argument to accept only the specified values using the specified comparer, and to suggest them as command line completions.
+        /// </summary>
+        /// <param name="argument">The argument to configure.</param>
+        /// <param name="comparer">The comparer used to match argument values against the allowed values.</param>
+        /// <param name="values">The values that are allowed for the argument.</param>
+        public static Argument<T> AcceptOnlyFromAmong<T>(
+            this Argument<T> argument,
+            StringComparer comparer,
+            params string[] values)
+        {
+            if (values?.Length > 0)
             {
                 argument.Validators.Clear();
                 argument.Validators.Add(UnrecognizedArgumentError);
@@ -140,7 +155,7 @@ public static Argument<T> AcceptOnlyFromAmong<T>(
             }
 
             return argument;
-            
+
             void UnrecognizedArgumentError(ArgumentResult argumentResult)
             {
                 for (var i = 0; i < argumentResult.Tokens.Count; i++)
@@ -149,7 +164,7 @@ void UnrecognizedArgumentError(ArgumentResult argumentResult)
 
                     if (token.Symbol is null || token.Symbol == argument)
                     {
-                        if (Array.IndexOf(values, token.Value) < 0)
+                        if (!values.Contains(token.Value, comparer))
                         {
                             argumentResult.AddError(LocalizationResources.UnrecognizedArgument(token.Value, values));
                         }

src/System.CommandLine/Option{T}.cs

@@ -57,6 +57,17 @@ public Option<T> AcceptOnlyFromAmong(params string[] values)
             return this;
         }
 
+        /// <summary>
+        /// Configures the option to accept only the specified values using the specified comparer, and to suggest them as command line completions.
+        /// </summary>
+        /// <param name="comparer">The comparer used to match argument values against the allowed values.</param>
+        /// <param name="values">The values that are allowed for the option.</param>
+        public Option<T> AcceptOnlyFromAmong(StringComparer comparer, params string[] values)
+        {
+            _argument.AcceptOnlyFromAmong(comparer, values);
+            return this;
+        }
+
         /// <summary>
         /// Configures the option to accept only values representing legal file paths.
         /// </summary>