Custom Parser can return null (#2140)

Author: adamsitnik

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -9,6 +9,7 @@
 using System.Linq;
 using System.Threading.Tasks;
 using Xunit;
+using System.Net;
 
 namespace System.CommandLine.Tests
 {
@@ -620,6 +621,29 @@ public void Custom_parser_can_pass_on_remaining_tokens(string commandLine)
                                                     options => options.WithStrictOrdering());
             }
 
+            [Fact]
+            public void Custom_parser_can_return_null()
+            {
+                CliOption<IPAddress> option = new("-ip")
+                {
+                    CustomParser = (argumentResult) =>
+                    {
+                        string value = argumentResult.Tokens.Last().Value;
+                        if (IPAddress.TryParse(value, out var address))
+                        {
+                            return address;
+                        }
+
+                        argumentResult.AddError($"'{value}' is not a valid value");
+                        return null;
+                    }
+                };
+
+                ParseResult parseResult = new CliRootCommand() { option }.Parse("-ip a.b.c.d");
+
+                parseResult.Errors.Should().Contain(error => error.Message == "'a.b.c.d' is not a valid value");
+            }
+
             [Fact]
             public void When_tokens_are_passed_on_by_custom_parser_on_last_argument_then_they_become_unmatched_tokens()
             {

src/System.CommandLine/Argument{T}.cs

@@ -11,7 +11,7 @@ namespace System.CommandLine
     /// <inheritdoc cref="CliArgument" />
     public class CliArgument<T> : CliArgument
     {
-        private Func<ArgumentResult, T>? _customParser;
+        private Func<ArgumentResult, T?>? _customParser;
 
         /// <summary>
         /// Initializes a new instance of the Argument class.
@@ -39,7 +39,7 @@ public CliArgument(string name) : base(name)
         /// The same instance can be set as <see cref="DefaultValueFactory"/>, in such case
         /// the delegate is also invoked when no input was provided.
         /// </remarks>
-        public Func<ArgumentResult, T>? CustomParser
+        public Func<ArgumentResult, T?>? CustomParser
         {
             get => _customParser;
             set

src/System.CommandLine/Option{T}.cs

@@ -42,7 +42,7 @@ public Func<ArgumentResult, T>? DefaultValueFactory
         }
 
         /// <inheritdoc cref="CliArgument{T}.CustomParser" />
-        public Func<ArgumentResult, T>? CustomParser
+        public Func<ArgumentResult, T?>? CustomParser
         {
             get => _argument.CustomParser;
             set => _argument.CustomParser = value;