fix #2640

src/System.CommandLine.Tests/HelpOptionTests.cs

@@ -255,6 +255,38 @@ 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.";

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);

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;
         }
 

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}\".");
             }