Model binding should follow default values (#1250)

Author: Keboo

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

@@ -547,6 +547,86 @@ public void Explicit_model_binder_binds_only_to_configured_ctor_parameters()
             instance.StringOption.Should().Be("the default");
         }
 
+        [Fact]
+        public async Task Bound_array_command_arguments_default_to_an_empty_array_when_not_specified()
+        {
+            var rootCommand = new RootCommand("Command")
+            {
+                new Argument<string[]>("names"),
+            };
+            rootCommand.Handler = CommandHandler.Create<string[]>(Handler);
+            string[] passedNames = null;
+            await rootCommand.InvokeAsync("");
+
+            passedNames.Should().BeEmpty();
+
+            int Handler(string[] names)
+            {
+                passedNames = names;
+                return 0;
+            }
+        }
+
+        [Fact]
+        public async Task Bound_enumerable_command_arguments_default_to_an_empty_array_when_not_specified()
+        {
+            var rootCommand = new RootCommand("Command")
+            {
+                new Argument<IEnumerable<string>>("names"),
+            };
+            rootCommand.Handler = CommandHandler.Create<IEnumerable<string>>(Handler);
+            IEnumerable<string> passedNames = null;
+            await rootCommand.InvokeAsync("");
+
+            passedNames.Should().BeEmpty();
+
+            int Handler(IEnumerable<string> names)
+            {
+                passedNames = names;
+                return 0;
+            }
+        }
+
+        [Fact]
+        public async Task Bound_array_options_default_to_an_empty_array_when_not_specified()
+        {
+            var rootCommand = new RootCommand("Command")
+            {
+                new Option<string[]>("--names"),
+            };
+            rootCommand.Handler = CommandHandler.Create<string[]>(Handler);
+            string[] passedNames = null;
+            await rootCommand.InvokeAsync("");
+
+            passedNames.Should().BeEmpty();
+
+            int Handler(string[] names)
+            {
+                passedNames = names;
+                return 0;
+            }
+        }
+
+        [Fact]
+        public async Task Bound_enumerable_options_default_to_an_empty_array_when_not_specified()
+        {
+            var rootCommand = new RootCommand("Command")
+            {
+                new Option<IEnumerable<string>>("--names"),
+            };
+            rootCommand.Handler = CommandHandler.Create<IEnumerable<string>>(Handler);
+            IEnumerable<string> passedNames = null;
+            await rootCommand.InvokeAsync("");
+
+            passedNames.Should().BeEmpty();
+
+            int Handler(IEnumerable<string> names)
+            {
+                passedNames = names;
+                return 0;
+            }
+        }
+
         [Fact]
         public void Custom_ModelBinders_specified_via_BindingContext_can_be_used_for_option_binding()
         {
@@ -628,8 +708,6 @@ public void Default_values_from_options_with_the_same_type_are_bound_and_use_the
             second.Should().Be(2);
         }
 
-      
-
         [Fact]
         public void Binder_does_not_match_on_partial_name()
         {

src/System.CommandLine/Parsing/CommandResultExtensions.cs

@@ -19,7 +19,14 @@ internal static bool TryGetValueForArgument(
 
                 if (valueDescriptor.ValueName.IsMatch(argument.Name))
                 {
-                    value = commandResult.FindResultFor(argument)?.GetValueOrDefault();
+                    if (commandResult.FindResultFor(argument) is { } argumentResult)
+                    {
+                        value = argumentResult.GetValueOrDefault();
+                    }
+                    else
+                    {
+                        value = valueDescriptor.GetDefaultValue();
+                    }
                     return true;
                 }
             }