How do I convert PascalCase to kebab-case with C#?

Here is how to do that with a regular expression:

public static class StringExtensions
{
    public static string PascalToKebabCase(this string value)
    {
        if (string.IsNullOrEmpty(value))
            return value;

        return Regex.Replace(
            value,
            "(?<!^)([A-Z][a-z]|(?<=[a-z])[A-Z])",
            "-$1",
            RegexOptions.Compiled)
            .Trim()
            .ToLower();
    }
}

Here's my solution using Microsoft's capitalization conventions. https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/capitalization-conventions

public static class StringExtensions
{
    public static string PascalToKebabCase(this string source)
    {
        if (source is null) return null;

        if (source.Length == 0) return string.Empty;

        StringBuilder builder = new StringBuilder();

        for (var i = 0; i < source.Length; i++)
        {
            if (char.IsLower(source[i])) // if current char is already lowercase
            {
                builder.Append(source[i]);
            }
            else if (i == 0) // if current char is the first char
            {
                builder.Append(char.ToLower(source[i]));
            }
            else if (char.IsDigit(source[i]) && !char.IsDigit(source[i - 1])) // if current char is a number and the previous is not
            {
                builder.Append('-');
                builder.Append(source[i]);
            }
            else if (char.IsDigit(source[i])) // if current char is a number and previous is
            {
                builder.Append(source[i]);
            }
            else if (char.IsLower(source[i - 1])) // if current char is upper and previous char is lower
            {
                builder.Append('-');
                builder.Append(char.ToLower(source[i]));
            }
            else if (i + 1 == source.Length || char.IsUpper(source[i + 1])) // if current char is upper and next char doesn't exist or is upper
            {
                builder.Append(char.ToLower(source[i]));
            }
            else // if current char is upper and next char is lower
            {
                builder.Append('-');
                builder.Append(char.ToLower(source[i]));
            }
        }
        return builder.ToString();
    }
}

Test

string[] stringArray = new[]
{
    null,
    "",
    "I",
    "IO",
    "FileIO",
    "SignalR",
    "IOStream",
    "COMObject",
    "WebAPI",
    "Windows10",
    "WindowsServer2019R2"
};

foreach (var str in stringArray)
{
    Console.WriteLine($"{str} --> {str.PascalToKebabCase()}");
}

// Output:
//  -->
//  -->
// I --> i
// IO --> io
// FileIO --> file-io
// SignalR --> signal-r
// IOStream --> io-stream
// COMObject --> com-object
// WebAPI --> web-api
// Windows10 --> windows-10
// WindowsServer2016R2 --> windows-server-2016-r-2