make error on missing symbol in SetHandler more helpful (#1558)

* make error on missing symbol in SetHandler more helpful

* improve error message

Author: jonsequitur

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

@@ -9,6 +9,7 @@
 using System.Threading.Tasks;
 using FluentAssertions;
 using Xunit;
+using static System.Environment;
 
 namespace System.CommandLine.Tests.Binding
 {
@@ -160,6 +161,62 @@ public void If_service_is_not_found_then_an_error_results()
             exitCode.Should().Be(1);
         }
 
+        [Fact]
+        public void If_no_symbol_was_passed_for_binding_then_the_error_message_suggests_a_fix_for_the_first_missing_symbol()
+        {
+            var boolOption = new Option<bool>("-o");
+            var stringArg = new Argument<string>("value");
+
+            var subcommand = new Command("TheCommand")
+            {
+                boolOption,
+                stringArg
+            };
+
+            var command = new RootCommand
+            {
+                subcommand
+            };
+
+            subcommand.SetHandler((bool boolValue, string stringValue) => { });
+
+            var console = new TestConsole();
+
+            command.Invoke("TheCommand -o hi", console);
+
+            console.Error.ToString().Should()
+                   .Contain(
+                       $"The SetHandler call for command 'TheCommand' is missing an Argument or Option for the parameter at position 0. Did you mean to pass one of these?{NewLine}Option<Boolean> -o");
+        }
+
+        [Fact]
+        public void If_no_symbol_was_passed_for_binding_subsequent_parameter_then_the_error_message_suggests_a_fix_for_the_first_missing_symbol()
+        {
+            var boolOption = new Option<bool>("-o");
+            var stringArg = new Argument<string>("value");
+
+            var subcommand = new Command("TheCommand")
+            {
+                boolOption,
+                stringArg
+            };
+
+            var command = new RootCommand
+            {
+                subcommand
+            };
+
+            subcommand.SetHandler((bool boolValue, string stringValue) => { }, boolOption);
+
+            var console = new TestConsole();
+
+            command.Invoke("TheCommand -o hi", console);
+
+            console.Error.ToString().Should()
+                   .Contain(
+                       $"The SetHandler call for command 'TheCommand' is missing an Argument or Option for the parameter at position 1. Did you mean to pass one of these?{NewLine}Argument<String> value");
+        }
+
         [Fact]
         public void Custom_types_can_be_bound()
         {

src/System.CommandLine/Handler.cs

@@ -3,6 +3,9 @@
 
 using System.CommandLine.Binding;
 using System.CommandLine.Invocation;
+using System.CommandLine.Parsing;
+using System.Linq;
+using static System.Environment;
 
 namespace System.CommandLine;
 
@@ -34,6 +37,33 @@ public static partial class Handler
 
         if (service is null)
         {
+            var candidates = context.ParseResult
+                                    .RootCommandResult
+                                    .AllSymbolResults()
+                                    .Where(r => r.Symbol.Parents.All(p => p is not Option))
+                                    .Select(r => r.Symbol)
+                                    .OfType<IValueDescriptor>()
+                                    .ToArray();
+
+            if (candidates.Any())
+            {
+                var candidatesDescription = string.Join(
+                    NewLine,
+                    candidates
+                        .Where(c => typeof(T).IsAssignableFrom(c.ValueType))
+                        .Select(c => c switch
+                        {
+                            Argument<T> argument => $"{nameof(Argument)}<{argument.ValueType.Name}> {argument.Name}",
+                            Argument argument => $"{nameof(Argument)} {argument.Name}",
+                            Option<T> option => $"{nameof(Option)}<{option.ValueType.Name}> {option.Aliases.First()}",
+                            Option option => $"{nameof(Option)} {option.Aliases.First()}",
+                            _ => throw new ArgumentOutOfRangeException(nameof(c))
+                        }));
+
+                throw new ArgumentException(
+                    $"The {nameof(SetHandler)} call for command '{context.ParseResult.CommandResult.Command.Name}' is missing an {nameof(Argument)} or {nameof(Option)} for the parameter at position {index}. Did you mean to pass one of these?{NewLine}{candidatesDescription}");
+            }
+
             throw new ArgumentException($"Service not found for type {typeof(T)}.");
         }