FinalCaseConverter.cs

using System;
using System.Collections.Generic;
using System.Text;

using ConverterFunction = System.Func<string[], string>;

class Angel
{
    static string camelConverter(string[] words)
    {

        var result = new StringBuilder();

        // First word is lower
        result.Append(
            words[0]
                .Trim()
                .ToLower()
        );

        // Subsequent words are Proper case
        for (var offset = 1;
             offset < words.Length;
             offset++)
        {
            // First letter is uppercase
            result.Append(
                words[offset]
                    .Substring(0, 1)
                    .ToUpper());

            // Rest of the letters are lowercase
            result.Append(
                words[offset].
                    Substring(1).
                    ToLower());
        }

        return result.ToString();
    }

    static string lowerConverter(params string[] words)
    {
        // Accumulate result in StringBuilder
        var result = new StringBuilder();

        // Every word is converted to lower
        // And concatenated
        foreach (var word in words)
        {
            result.Append(
                word
                    .Trim()
                    .ToLower());
        }
        return result.ToString();
    }

    Dictionary<string, ConverterFunction> converters =
        new Dictionary<string, ConverterFunction>
        {
            {"lower", lowerConverter},
            {"camel", camelConverter}
        };

    public string changeCase(string caseType, params string[] words)
    {
        // Sanitize the input
        caseType = caseType.Trim().ToLower();

        ConverterFunction converter;
        converters.TryGetValue(caseType, out converter);
        if (converter != null)
            return converter(words);
        return null;
    }

    public void AddConverter(string caseType, ConverterFunction func)
    {
        converters[caseType] = func;
    }
}

class ClientClass
{
    static string snakeConverter(params string[] words)
    {
        // snake_case_is_separated_by_underscore
        var result = new StringBuilder();

        // Convert each word to lower 
        // and separate using underscore
        for (var offset = 0;
             offset < words.Length-1;
             offset++)
        {
            result.Append(
                words[offset]
                   .Trim()
                   .ToLower()
            );

            result.Append("_");
        }

        // Add the last word without _
        result.Append(
            words[words.Length - 1]
                .Trim()
                .ToLower()
        );

        return result.ToString();
    }

    static void Main(string[] args)
    {
        var obj = new Angel();
        obj.AddConverter("snake", snakeConverter);
        Console.WriteLine(obj.changeCase("snake", "Two", "Words"));
    }
}