Fix ChildRules with hierarchies #2165

src/FluentValidation.Tests/ChildRulesTests.cs

@@ -20,6 +20,7 @@
 
 namespace FluentValidation.Tests;
 
+using System;
 using System.Collections.Generic;
 using Xunit;
 
@@ -121,6 +122,14 @@ public class ChildRulesTests {
 		result.IsValid.ShouldBeFalse();
 	}
 
+	[Fact]
+	public void Doesnt_throw_InvalidCastException() {
+		// See https://github.com/FluentValidation/FluentValidation/issues/2165
+		var validator = new RootValidator();
+		var result = validator.Validate(new Root { Data = new() { Value = -1 }});
+		result.Errors.Count.ShouldEqual(1);
+	}
+
 	private class RulesetChildRulesValidator : AbstractValidator<Person> {
 		public RulesetChildRulesValidator() {
 			RuleSet("testing", () => {
@@ -163,4 +172,34 @@ public class Baz {
 		}
 	}
 
+	public class Root {
+		public Bar Data {get; set;} = null;
+	}
+
+	public class Base {
+		public int Value {get; set;}
+	}
+
+	public class Bar : Base  {
+		public int BarValue {get; set;}
+	}
+
+	public class RootValidator : AbstractValidator<Root> {
+		public RootValidator() {
+			RuleFor(x => x).ChildRules(RootRules());
+		}
+
+		public static Action<InlineValidator<Base>> BaseRules() {
+			return rules => {
+				rules.RuleFor(x => x.Value).NotEqual(-1);
+			};
+		}
+
+		public static Action<InlineValidator<Root>> RootRules() {
+			return rules => {
+				rules.RuleFor(x => x.Data).ChildRules(BaseRules());
+			};
+		}
+	}
+
 }

src/FluentValidation/DefaultValidatorExtensions.cs

@@ -1204,7 +1204,7 @@ public static partial class DefaultValidatorExtensions {
 	public static IRuleBuilderOptions<T, TProperty> ChildRules<T, TProperty>(this IRuleBuilder<T, TProperty> ruleBuilder, Action<InlineValidator<TProperty>> action) {
 		if (action == null) throw new ArgumentNullException(nameof(action));
 		var validator = new ChildRulesContainer<TProperty>();
-		var parentValidator = ((RuleBuilder<T, TProperty>) ruleBuilder).ParentValidator;
+		var parentValidator = ((IRuleBuilderInternal<T>) ruleBuilder).ParentValidator;
 
 		string[] ruleSets;
 

src/FluentValidation/Syntax.cs

@@ -113,6 +113,10 @@ public interface IConditionBuilder {
 	void Otherwise(Action action);
 }
 
-internal interface IRuleBuilderInternal<T, out TProperty> {
+internal interface IRuleBuilderInternal<T, out TProperty> : IRuleBuilderInternal<T> {
 	IValidationRule<T, TProperty> Rule { get; }
 }
+
+internal interface IRuleBuilderInternal<T> {
+	AbstractValidator<T> ParentValidator { get; }
+}