using System;

using System.Linq;

using System.Collections.Generic;

using System.Text.RegularExpressions;

public class Program

{   

    private static readonly IEnumerable<(string Text, string Expectation)> Samples = new [] 

    {

        ("TestSC", "test_sc"),

        ("testSC", "test_sc"),

        ("TestSnakeCase", "test_snake_case"),

        ("testSnakeCase", "test_snake_case"),

        ("TestSnakeCase123", "test_snake_case123"),

        ("_testSnakeCase123", "_test_snake_case123"),

        ("test_SC", "test_sc")

    };

    public static void Main()

    {

        Test(x => x.ToSnakeCase());

        Console.WriteLine("------------------------------------");

        Test(x => x.ToSnakeCase2());

    }   

    private static void Test(Func<string, string> testFunc)

    {

        foreach(var sample in Samples)

        {

            var val = testFunc(sample.Text);

            var result = val == sample.Expectation ? "true" : "false";

            Console.WriteLine($"Test:        {sample.Text}\nResult:      {val}\nExpectation: {sample.Expectation}\nTest result: {result}\n");

        }

    }

}

​

public static class StringExtensions

{

    public static string ToSnakeCase(this string str) =>

        string.Concat(

            str.Select(

                (x, i) => i > 0 && char.IsUpper(x)

                    ? "_" + x

                    : x.ToString()

                    )

            ).ToLower();

    public static string ToSnakeCase2(this string text) => Regex.Replace(Regex.Replace(text, "(.)([A-Z][a-z]+)", "$1_$2"), "([a-z0-9])([A-Z])", "$1_$2").ToLower();

}