Simple C# extension method to convert a camel case string to underscore notation without any regex

underscoreCase.cs

public static class ExtensionMethods {
    public static string ToUnderscoreCase(this string str) {
        return string.Concat(str.Select((x, i) => i > 0 && char.IsUpper(x) ? "_" + x.ToString() : x.ToString())).ToLower();
    }
}

This improved method solved the mentioned problems in the comments:

public static class ExtensionMethods
{
    
    public static string ToUnderscoreCase(this string str) =>
        string.Concat(str.Select((x, i) => (i > 0 && char.IsUpper(x) && (char.IsLower(str[i - 1]) || char.IsLower(str[i + 1])))
            ? "_" + x.ToString() : x.ToString())).ToLower();
}

public static string ToUnderscoreCase(this string str) =>
string.Concat(str.Select((x, i) => (i > 0 && char.IsUpper(x) && (char.IsLower(str[i - 1]) || char.IsLower(str[i + 1])))
? "_" + x.ToString() : x.ToString())).ToLower();

This causes an exception with "abcDeFG".

Here the corrected method:

static string ToUnderscoreCase(this string str) =>
    string.Concat(
        str.Select((x, i) =>
            i > 0 && char.IsUpper(x) && (char.IsLower(str[i - 1]) || i < str.Length - 1 && char.IsLower(str[i + 1]))
                ? "_" + x
                : x.ToString())).ToLowerInvariant();

Easy on eyes solution with custom separator:

public static string ToLowerBySep(this string source, char sep)
{
    var result = new List<char>();
    var chars = source.Replace(" ", "").ToCharArray();

    for (var i = 0; i < chars.Length; i++)
    {
        if (i != 0 && char.IsUpper(chars[i]))
        {
            result.Add(sep);
        }

        result.Add(chars[i]);
    }

    return string.Concat(result).ToLowerInvariant();
}