From 98219f17f5d584aed8115764c396312e79607d56 Mon Sep 17 00:00:00 2001
From: Jon Sequeira <jonsequeira@gmail.com>
Date: Wed, 30 Jul 2025 18:32:44 -0700
Subject: [PATCH 1/2] fix #2640

---
 .../HelpOptionTests.cs                        | 36 +++++++++++++++++++
 src/System.CommandLine/Argument.cs            |  6 +++-
 .../Parsing/SymbolResult.cs                   |  2 +-
 .../Parsing/SymbolResultTree.cs               | 10 +++---
 4 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/src/System.CommandLine.Tests/HelpOptionTests.cs b/src/System.CommandLine.Tests/HelpOptionTests.cs
index 27552bf8ac..fa9ca7df1f 100644
--- a/src/System.CommandLine.Tests/HelpOptionTests.cs
+++ b/src/System.CommandLine.Tests/HelpOptionTests.cs
@@ -6,6 +6,7 @@
 using System.CommandLine.Invocation;
 using System.CommandLine.Tests.Utility;
 using System.IO;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Xunit;
@@ -255,6 +256,41 @@ public void The_users_can_set_max_width()
         output.ToString().Should().Contain($"  {secondPart}{Environment.NewLine}");
     }
 
+    [Fact] // https://github.com/dotnet/command-line-api/issues/2640
+    public void DefaultValueFactory_does_not_throw_when_help_is_invoked()
+    {
+        var invocationConfiguration = new InvocationConfiguration
+        {
+            Output = new StringWriter(),
+            Error = new StringWriter()
+        };
+
+        Command subcommand = new("do")
+        {
+            new Option<DirectoryInfo>("-x")
+            {
+                DefaultValueFactory = result =>
+                {
+                    result.AddError("Oops!");
+                    return null;
+                    
+                }
+            }
+        };
+        subcommand.SetAction(_ =>
+        {
+        });
+        RootCommand rootCommand = new()
+        {
+            subcommand
+        };
+
+        rootCommand.Parse("do --help").Invoke(invocationConfiguration);
+
+        invocationConfiguration.Error.ToString().Should().Be("");
+    }
+
+
     private sealed class CustomizedHelpAction : SynchronousCommandLineAction
     {
         internal const string CustomUsageText = "This is custom command usage example.";
diff --git a/src/System.CommandLine/Argument.cs b/src/System.CommandLine/Argument.cs
index 7df55ce93f..1640e883df 100644
--- a/src/System.CommandLine/Argument.cs
+++ b/src/System.CommandLine/Argument.cs
@@ -112,7 +112,11 @@ public List<Func<CompletionContext, IEnumerable<CompletionItem>>> CompletionSour
         /// <returns>Returns the default value for the argument, if defined. Null otherwise.</returns>
         public object? GetDefaultValue()
         {
-            return GetDefaultValue(new ArgumentResult(this, null!, null));
+            var command = Parents.FlattenBreadthFirst(x => x.Parents)
+                                 .OfType<Command>()
+                                 .FirstOrDefault();
+
+            return GetDefaultValue(new ArgumentResult(this, new SymbolResultTree(command), null));
         }
 
         internal abstract object? GetDefaultValue(ArgumentResult argumentResult);
diff --git a/src/System.CommandLine/Parsing/SymbolResult.cs b/src/System.CommandLine/Parsing/SymbolResult.cs
index aed8a3ac4b..0016375754 100644
--- a/src/System.CommandLine/Parsing/SymbolResult.cs
+++ b/src/System.CommandLine/Parsing/SymbolResult.cs
@@ -15,7 +15,7 @@ public abstract class SymbolResult
 
         private protected SymbolResult(SymbolResultTree symbolResultTree, SymbolResult? parent)
         {
-            SymbolResultTree = symbolResultTree;
+            SymbolResultTree = symbolResultTree ?? throw new ArgumentNullException(nameof(symbolResultTree));
             Parent = parent;
         }
 
diff --git a/src/System.CommandLine/Parsing/SymbolResultTree.cs b/src/System.CommandLine/Parsing/SymbolResultTree.cs
index 49abc6be86..22731d82cf 100644
--- a/src/System.CommandLine/Parsing/SymbolResultTree.cs
+++ b/src/System.CommandLine/Parsing/SymbolResultTree.cs
@@ -7,14 +7,14 @@ namespace System.CommandLine.Parsing
 {
     internal sealed class SymbolResultTree : Dictionary<Symbol, SymbolResult>
     {
-        private readonly Command _rootCommand;
+        private readonly Command? _rootCommand;
         internal List<ParseError>? Errors;
         internal List<Token>? UnmatchedTokens;
         private Dictionary<string, SymbolNode>? _symbolsByName;
 
         internal SymbolResultTree(
-            Command rootCommand, 
-            List<string>? tokenizeErrors)
+            Command? rootCommand = null, 
+            List<string>? tokenizeErrors = null)
         {
             _rootCommand = rootCommand;
 
@@ -78,13 +78,13 @@ internal void AddUnmatchedToken(Token token, CommandResult commandResult, Comman
 
         public SymbolResult? GetResult(string name)
         {
-            if (_symbolsByName is null)
+            if (_symbolsByName is null && _rootCommand is not null)
             {
                 _symbolsByName = new();  
                 PopulateSymbolsByName(_rootCommand);
             }
           
-            if (!_symbolsByName.TryGetValue(name, out SymbolNode? node))
+            if (!_symbolsByName!.TryGetValue(name, out SymbolNode? node))
             {
                 throw new ArgumentException($"No symbol result found with name \"{name}\".");
             }

From 0858f5000c55c737d9bfed3392d8c0cff7f8139e Mon Sep 17 00:00:00 2001
From: Jon Sequeira <jonsequeira@gmail.com>
Date: Thu, 31 Jul 2025 07:24:16 -0700
Subject: [PATCH 2/2] cleanup

---
 src/System.CommandLine.Tests/HelpOptionTests.cs | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/src/System.CommandLine.Tests/HelpOptionTests.cs b/src/System.CommandLine.Tests/HelpOptionTests.cs
index fa9ca7df1f..922fc159e2 100644
--- a/src/System.CommandLine.Tests/HelpOptionTests.cs
+++ b/src/System.CommandLine.Tests/HelpOptionTests.cs
@@ -6,7 +6,6 @@
 using System.CommandLine.Invocation;
 using System.CommandLine.Tests.Utility;
 using System.IO;
-using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Xunit;
@@ -273,13 +272,10 @@ public void DefaultValueFactory_does_not_throw_when_help_is_invoked()
                 {
                     result.AddError("Oops!");
                     return null;
-                    
                 }
             }
         };
-        subcommand.SetAction(_ =>
-        {
-        });
+        subcommand.SetAction(_ => { });
         RootCommand rootCommand = new()
         {
             subcommand