Support parsing nullable enums

Author: natemcmaster

src/CommandLineUtils/Internal/ValueParserProvider.cs

@@ -48,6 +48,12 @@ public IValueParser GetParser(Type type)
             if (typeInfo.IsGenericType && typeInfo.GetGenericTypeDefinition() == typeof(Nullable<>))
             {
                 var wrappedType = type.GetTypeInfo().GetGenericArguments().First();
+
+                if (wrappedType.GetTypeInfo().IsEnum)
+                {
+                    return new NullableValueParser(new EnumParser(wrappedType));
+                }
+
                 if (_parsers.TryGetValue(wrappedType, out parser))
                 {
                     return new NullableValueParser(parser);

src/CommandLineUtils/Properties/Strings.cs

@@ -41,19 +41,19 @@ public static string DuplicateSubcommandName(string commandName)
             => $"The subcommand name '{commandName}' has already been been specified. Subcommand names are case insensitive and must be unique.";
 
         public static string BothOptionAndArgumentAttributesCannotBeSpecified(PropertyInfo prop)
-            => $"Cannot specify both {nameof(OptionAttribute)} and {nameof(ArgumentAttribute)} on a property {prop.DeclaringType.Name}.{prop.Name}.";
+            => $"Cannot specify both {nameof(OptionAttribute)} and {nameof(ArgumentAttribute)} on property {prop.DeclaringType.Name}.{prop.Name}.";
 
         public static string BothOptionAndHelpOptionAttributesCannotBeSpecified(PropertyInfo prop)
-            => $"Cannot specify both {nameof(OptionAttribute)} and {nameof(HelpOptionAttribute)} on a property {prop.DeclaringType.Name}.{prop.Name}.";
+            => $"Cannot specify both {nameof(OptionAttribute)} and {nameof(HelpOptionAttribute)} on property {prop.DeclaringType.Name}.{prop.Name}.";
 
         public static string BothOptionAndVersionOptionAttributesCannotBeSpecified(PropertyInfo prop)
-            => $"Cannot specify both {nameof(OptionAttribute)} and {nameof(VersionOptionAttribute)} on a property {prop.DeclaringType.Name}.{prop.Name}.";
+            => $"Cannot specify both {nameof(OptionAttribute)} and {nameof(VersionOptionAttribute)} on property {prop.DeclaringType.Name}.{prop.Name}.";
 
         internal static string UnsupportedParameterTypeOnMethod(string methodName, ParameterInfo methodParam)
             => $"Unsupported type on {methodName} '{methodParam.ParameterType.FullName}' on parameter {methodParam.Name}";
 
         public static string BothHelpOptionAndVersionOptionAttributesCannotBeSpecified(PropertyInfo prop)
-            => $"Cannot specify both {nameof(HelpOptionAttribute)} and {nameof(VersionOptionAttribute)} on a property {prop.DeclaringType.Name}.{prop.Name}.";
+            => $"Cannot specify both {nameof(HelpOptionAttribute)} and {nameof(VersionOptionAttribute)} on property {prop.DeclaringType.Name}.{prop.Name}.";
 
         public static string DuplicateArgumentPosition(int order, PropertyInfo first, PropertyInfo second)
             => $"Duplicate value for argument order. Both {first.DeclaringType.FullName}.{first.Name} and {second.DeclaringType.FullName}.{second.Name} have set Order = {order}";
@@ -62,7 +62,7 @@ public static string OnlyLastArgumentCanAllowMultipleValues(string lastArgName)
             => $"The last argument '{lastArgName}' accepts multiple values. No more argument can be added.";
 
         public static string CannotDetermineParserType(PropertyInfo prop)
-            => $"Could not automatically determine how to convert string values into {prop.PropertyType.FullName}.";
+            => $"Could not automatically determine how to convert string values into {prop.PropertyType.FullName} on property {prop.DeclaringType.Name}.{prop.Name}.";
 
         public static string MultipleValuesArgumentShouldBeCollection
             = "ArgumentAttribute.MultipleValues should be true if the property type is an array or collection.";

test/CommandLineUtils.Tests/ValueParserProviderTests.cs

@@ -77,7 +77,10 @@ private class Program
             public ISet<string> StringSet { get; }
 
             [Option("--color")]
-            public Color ColorOption { get; }
+            public Color Color { get; }
+
+            [Option("--color-opt")]
+            public Color? ColorOpt { get; }
         }
 
         public static IEnumerable<object[]> GetFloatingPointSymbolsData()
@@ -296,7 +299,18 @@ public void ParsesStringSet()
         public void ParsesEnum(Color color)
         {
             var parsed = CommandLineParser.ParseArgs<Program>("--color", color.ToString().ToLowerInvariant());
-            Assert.Equal(color, parsed.ColorOption);
+            Assert.Equal(color, parsed.Color);
+        }
+
+        [Theory]
+        [InlineData(Color.Blue)]
+        [InlineData(Color.Red)]
+        [InlineData(Color.Green)]
+        public void ParsesNullableEnum(Color? color)
+        {
+            var parsed = CommandLineParser.ParseArgs<Program>("--color-opt", color.ToString().ToLowerInvariant());
+            Assert.True(parsed.ColorOpt.HasValue, "Option should have value");
+            Assert.Equal(color, parsed.ColorOpt);
         }
 
         private class ArgumentProgram