Fix IndexOutOfRangeException when applying a naming convention to an empty string

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);

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);