How to make a first letter capital in C#

public static string ToUpperFirstLetter(this string source)
{
    if (string.IsNullOrEmpty(source))
        return string.Empty;
    // convert to char array of the string
    char[] letters = source.ToCharArray();
    // upper case the first char
    letters[0] = char.ToUpper(letters[0]);
    // return the array made of the new char array
    return new string(letters);
}