/// <summary>
/// 首字母小寫寫
/// </summary>
/// <param name="input"></param>
/// <returns></returns>
public static string FirstCharToLower(this string input)
{
    if (String.IsNullOrEmpty(input))
        return input;
    string str = input.First().ToString().ToLower() + input.Substring(1);
    return str;
}
 
/// <summary>
/// 首字母大寫
/// </summary>
/// <param name="input"></param>
/// <returns></returns>
public static string FirstCharToUpper(this string input)
{
    if (String.IsNullOrEmpty(input))
        return input;
    string str = input.First().ToString().ToUpper() + input.Substring(1);
    return str;
}
 

不要拓展可以把方法引數裡的this去掉