fix #2257

Jon Sequeira · jonsequitur · commit 0b4618bc8603 · 2025-08-08T12:00:44.000-07:00
diff --git a/src/System.CommandLine.Tests/OptionTests.cs b/src/System.CommandLine.Tests/OptionTests.cs
@@ -512,5 +512,24 @@ public void Custom_parser_can_be_used_to_implement_int_binding_based_on_token_co
 
             result.GetValue(option).Should().Be(3);
         }
+
+        [Fact] // https://github.com/dotnet/command-line-api/issues/2257
+        public void Default_value_is_used_when_option_with_ZeroOrOne_arity_is_parsed_without_an_argument()
+        {
+            var option = new Option<int>("-o")
+            {
+                Arity = ArgumentArity.ZeroOrOne,
+                DefaultValueFactory = _ => 42
+            };
+
+            var rootCommand = new RootCommand
+            {
+                option
+            };
+
+            var parseResult = rootCommand.Parse("-o");
+
+            parseResult.GetValue(option).Should().Be(42);
+        }
     }
 }
diff --git a/src/System.CommandLine/Parsing/ArgumentResult.cs b/src/System.CommandLine/Parsing/ArgumentResult.cs
@@ -153,7 +153,7 @@ private ArgumentConversionResult ValidateAndConvert(bool useValidators)
                 }
             }
 
-            if (Parent!.UseDefaultValueFor(this))
+            if (Argument.HasDefaultValue && Parent!.UseDefaultValueFor(this))
             {
                 var defaultValue = Argument.GetDefaultValue(this);
 
diff --git a/src/System.CommandLine/Parsing/OptionResult.cs b/src/System.CommandLine/Parsing/OptionResult.cs
@@ -64,6 +64,16 @@ internal bool IsArgumentLimitReached
         internal ArgumentConversionResult ArgumentConversionResult
             => _argumentConversionResult ??= GetResult(Option.Argument)!.GetArgumentConversionResult();
 
-        internal override bool UseDefaultValueFor(ArgumentResult argument) => Implicit;
+        internal override bool UseDefaultValueFor(ArgumentResult argumentResult)
+        {
+            if (Implicit)
+            {
+                return true;
+            }
+
+            return Tokens.Count is 0 &&
+                   Option.Arity is { MinimumNumberOfValues: 0, MaximumNumberOfValues: > 0 } &&
+                   !argumentResult.Argument.IsBoolean();
+        }
     }
 }

Original file line number	Diff line number	Diff line change
`@@ -153,7 +153,7 @@ private ArgumentConversionResult ValidateAndConvert(bool useValidators)`
`153`	`153`	`}`
`154`	`154`	`}`
`155`	`155`
`156`		`- if (Parent!.UseDefaultValueFor(this))`
	`156`	`+ if (Argument.HasDefaultValue && Parent!.UseDefaultValueFor(this))`
`157`	`157`	`{`
`158`	`158`	`var defaultValue = Argument.GetDefaultValue(this);`
`159`	`159`