fix #862

Author: jonsequitur

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -2,8 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System.Collections.Generic;
-using System.CommandLine.Builder;
-using System.CommandLine.Help;
 using System.CommandLine.Invocation;
 using System.CommandLine.Parsing;
 using System.IO;

src/System.CommandLine.Tests/Binding/TypeConversionTests.cs

@@ -7,6 +7,7 @@
 using FluentAssertions;
 using System.Linq;
 using Xunit;
+using System.CommandLine.Invocation;
 
 namespace System.CommandLine.Tests.Binding
 {
@@ -736,6 +737,55 @@ public void Values_can_be_correctly_converted_to_array_of_int_without_the_parser
             value.Should().BeEquivalentTo(1, 2, 3);
         }
 
+        [Theory]
+        [InlineData(0, int.MaxValue, typeof(string[]))]
+        [InlineData(0, 3, typeof(string[]))]
+        [InlineData(0, int.MaxValue, typeof(IEnumerable<string>))]
+        [InlineData(0, 3, typeof(IEnumerable<string>))]
+        [InlineData(0, int.MaxValue, typeof(List<string>))]
+        [InlineData(0, 3, typeof(List<string>))]
+        [InlineData(0, int.MaxValue, typeof(IList<string>))]
+        [InlineData(0, 3, typeof(IList<string>))]
+        [InlineData(0, int.MaxValue, typeof(ICollection<string>))]
+        [InlineData(0, 3, typeof(ICollection<string>))]
+        
+        [InlineData(1, int.MaxValue, typeof(string[]))]
+        [InlineData(1, 3, typeof(string[]))]
+        [InlineData(1, int.MaxValue, typeof(IEnumerable<string>))]
+        [InlineData(1, 3, typeof(IEnumerable<string>))]
+        [InlineData(1, int.MaxValue, typeof(List<string>))]
+        [InlineData(1, 3, typeof(List<string>))]
+        [InlineData(1, int.MaxValue, typeof(IList<string>))]
+        [InlineData(1, 3, typeof(IList<string>))]
+        [InlineData(1, int.MaxValue, typeof(ICollection<string>))]
+        [InlineData(1, 3, typeof(ICollection<string>))]
+        public void Max_arity_greater_than_1_converts_to_enumerable_types(
+            int minArity,
+            int maxArity,
+            Type argumentType)
+        {
+            var argument = new Argument
+            {
+                ArgumentType = argumentType,
+                Arity = new ArgumentArity(minArity, maxArity)
+            };
+
+            var option = new Option("--items")
+            {
+                Argument = argument
+            };
+
+            var command = new RootCommand
+            {
+                option
+            };
+
+            var result = command.Parse("--items one two three");
+
+            result.Errors.Should().BeEmpty();
+            result.FindResultFor(option).GetValueOrDefault().Should().BeAssignableTo(argumentType);
+        }
+
         [Fact]
         public void Values_can_be_correctly_converted_to_List_of_int_without_the_parser_specifying_a_custom_converter()
         {

src/System.CommandLine/Binding/ArgumentConverter.cs

@@ -177,6 +177,11 @@ public static ArgumentConversionResult ConvertStrings(
 
         private static Type GetItemTypeIfEnumerable(Type type)
         {
+            if (type.IsArray)
+            {
+                return type.GetElementType();
+            }
+
             var enumerableInterface =
                 IsEnumerable(type)
                     ? type
@@ -192,10 +197,18 @@ private static Type GetItemTypeIfEnumerable(Type type)
             return enumerableInterface.GenericTypeArguments[0];
         }
 
-        internal static bool IsEnumerable(this Type i)
+        internal static bool IsEnumerable(this Type type)
         {
-            return i.IsGenericType &&
-                   i.GetGenericTypeDefinition() == typeof(IEnumerable<>);
+            if (type == typeof(string))
+            {
+                return false;
+            }
+
+            return 
+                type.IsArray 
+                ||
+                (type.IsGenericType &&
+                 type.GetGenericTypeDefinition() == typeof(IEnumerable<>));
         }
 
         private static bool HasStringTypeConverter(this Type type)

src/System.CommandLine/Builder/CommandLineBuilderExtensions.cs

@@ -296,8 +296,6 @@ internal static CommandLineBuilder UseHelp(
                         await next(context);
                     }
                 }, MiddlewareOrderInternal.HelpOption);
-
-                return builder;
             }
 
             return builder;