From e9c920f84656668dc3e3d67abffde043c4382567 Mon Sep 17 00:00:00 2001
From: dualfroz <me@dualfroz.com>
Date: Wed, 2 Sep 2026 10:59:11 +0200
Subject: [PATCH] fix: reject inverted range when comparer returns negative
 other than -1

RangeValidator's constructor guarded against an inverted range with
comparer.Compare(to, from) == -1. The IComparer<T>/IComparable<T> contract
only guarantees the sign of the result, not the magnitude, so the guard
silently failed for comparers returning other negatives, e.g. char.CompareTo
('a'.CompareTo('c') == -2) or custom comparers that return x - y. This let
InclusiveBetween('c', 'a') build a validator that can never pass instead of
throwing, unlike the int case which happens to return -1.

Compare against < 0 to honor the comparer contract. Add char-bound throw
tests to the inclusive and exclusive between suites.
---
 .../ExclusiveBetweenValidatorTests.cs                       | 6 ++++++
 .../InclusiveBetweenValidatorTests.cs                       | 6 ++++++
 src/FluentValidation/Validators/RangeValidator.cs           | 3 ++-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/FluentValidation.Tests/ExclusiveBetweenValidatorTests.cs b/src/FluentValidation.Tests/ExclusiveBetweenValidatorTests.cs
index a4ebfa4..75fd95d 100644
--- a/src/FluentValidation.Tests/ExclusiveBetweenValidatorTests.cs
+++ b/src/FluentValidation.Tests/ExclusiveBetweenValidatorTests.cs
@@ -127,6 +127,12 @@ public class ExclusiveBetweenValidatorTests {
 		Assert.Throws<ArgumentOutOfRangeException>(() => RangeValidatorFactory.CreateExclusiveBetween<Person, string>("ccc", "aaa"));
 	}
 
+	[Fact]
+	public void When_the_to_is_smaller_than_the_from_then_the_validator_should_throw_for_chars() {
+		// char.CompareTo returns the difference ('a' vs 'c' is -2), not just -1.
+		Assert.Throws<ArgumentOutOfRangeException>(() => RangeValidatorFactory.CreateExclusiveBetween<Person, char>('c', 'a'));
+	}
+
 	[Fact]
 	public void When_the_validator_fails_the_error_message_should_be_set_for_strings() {
 		var validator = new TestValidator(v => v.RuleFor(x => x.Surname).ExclusiveBetween("bbb", "zzz"));
diff --git a/src/FluentValidation.Tests/InclusiveBetweenValidatorTests.cs b/src/FluentValidation.Tests/InclusiveBetweenValidatorTests.cs
index 8341b18..c10a982 100644
--- a/src/FluentValidation.Tests/InclusiveBetweenValidatorTests.cs
+++ b/src/FluentValidation.Tests/InclusiveBetweenValidatorTests.cs
@@ -122,6 +122,12 @@ public class InclusiveBetweenValidatorTests {
 		Assert.Throws<ArgumentOutOfRangeException>(() => RangeValidatorFactory.CreateInclusiveBetween<Person, string>("ccc", "aaa"));
 	}
 
+	[Fact]
+	public void When_the_to_is_smaller_than_the_from_then_the_validator_should_throw_for_chars() {
+		// char.CompareTo returns the difference ('a' vs 'c' is -2), not just -1.
+		Assert.Throws<ArgumentOutOfRangeException>(() => RangeValidatorFactory.CreateInclusiveBetween<Person, char>('c', 'a'));
+	}
+
 	[Fact]
 	public void When_the_validator_fails_the_error_message_should_be_set_for_strings() {
 		var validator = new TestValidator(v => v.RuleFor(x => x.Surname).InclusiveBetween("bbb", "zzz"));
diff --git a/src/FluentValidation/Validators/RangeValidator.cs b/src/FluentValidation/Validators/RangeValidator.cs
index eafe717..65b00e4 100644
--- a/src/FluentValidation/Validators/RangeValidator.cs
+++ b/src/FluentValidation/Validators/RangeValidator.cs
@@ -34,7 +34,8 @@ public abstract class RangeValidator<T, TProperty> : PropertyValidator<T, TPrope
 
 		_explicitComparer = comparer;
 
-		if (comparer.Compare(to, from) == -1) {
+		// IComparer<T> only guarantees the sign, not the magnitude.
+		if (comparer.Compare(to, from) < 0) {
 			throw new ArgumentOutOfRangeException(nameof(to), "To should be larger than from.");
 		}
 	}