From 5ac95019bbb5bdeb3a35c68368d9c2045cc48e44 Mon Sep 17 00:00:00 2001
From: Vijay Misal <misalvijay153@gmail.com>
Date: Mon, 10 Aug 2026 21:39:42 +0530
Subject: [PATCH] Fix IndexOutOfRangeException when applying naming convention
 to empty string

INamingConvention.Apply("") throws System.IndexOutOfRangeException for the
camel/pascal/hyphenated/underscored conventions because
ToCamelOrPascalCase and FromCamelCase in StringExtensions.cs index the
first character (text[0] / str[0]) without checking for an empty input.

Add a short-circuit that returns the input unchanged when it is null or
empty, matching the suggested fix in the issue report. Added a theory
test covering all built-in naming conventions with an empty string
input.

Fixes #1114
---
 .../Serialization/NamingConventionTests.cs      | 17 +++++++++++++++++
 .../Serialization/Utilities/StringExtensions.cs | 10 ++++++++++
 2 files changed, 27 insertions(+)

diff --git a/YamlDotNet.Test/Serialization/NamingConventionTests.cs b/YamlDotNet.Test/Serialization/NamingConventionTests.cs
index 9f22c60a1..dca5f434b 100644
--- a/YamlDotNet.Test/Serialization/NamingConventionTests.cs
+++ b/YamlDotNet.Test/Serialization/NamingConventionTests.cs
@@ -19,6 +19,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+using System.Collections.Generic;
 using FluentAssertions;
 using Xunit;
 using YamlDotNet.Serialization;
@@ -77,6 +78,22 @@ public void AppliesLowerCaseConvention(string expectedName, string input)
             ShouldApplyConventionGiven(input, expectedName, LowerCaseNamingConvention.Instance);
         }
 
+        public static IEnumerable<object[]> AllNamingConventions()
+        {
+            yield return new object[] { CamelCaseNamingConvention.Instance };
+            yield return new object[] { PascalCaseNamingConvention.Instance };
+            yield return new object[] { HyphenatedNamingConvention.Instance };
+            yield return new object[] { UnderscoredNamingConvention.Instance };
+            yield return new object[] { LowerCaseNamingConvention.Instance };
+        }
+
+        [Theory]
+        [MemberData(nameof(AllNamingConventions))]
+        public void ApplyingConventionToEmptyStringReturnsEmptyString(INamingConvention convention)
+        {
+            convention.Apply(string.Empty).Should().Be(string.Empty);
+        }
+
         private void ShouldApplyConventionGiven(string input, string expectedName, INamingConvention convention)
         {
             convention.Apply(input).Should().Be(expectedName);
diff --git a/YamlDotNet/Serialization/Utilities/StringExtensions.cs b/YamlDotNet/Serialization/Utilities/StringExtensions.cs
index 4fcf9dd5c..75276660c 100644
--- a/YamlDotNet/Serialization/Utilities/StringExtensions.cs
+++ b/YamlDotNet/Serialization/Utilities/StringExtensions.cs
@@ -32,6 +32,11 @@ internal static class StringExtensions
     {
         private static string ToCamelOrPascalCase(string str, Func<char, char> firstLetterTransform)
         {
+            if (string.IsNullOrEmpty(str))
+            {
+                return str;
+            }
+
             var text = Regex.Replace(str, "([_\\-])(?<char>[a-z])", match => match.Groups["char"].Value.ToUpperInvariant(), RegexOptions.IgnoreCase);
             return firstLetterTransform(text[0]) + text.Substring(1);
         }
@@ -70,6 +75,11 @@ public static string ToPascalCase(this string str)
         /// <returns>Converted string</returns>
         public static string FromCamelCase(this string str, string separator)
         {
+            if (string.IsNullOrEmpty(str))
+            {
+                return str;
+            }
+
             // Ensure first letter is always lowercase
             str = char.ToLower(str[0], CultureInfo.InvariantCulture) + str.Substring(1);