C＃ 字符串首字符大写

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace 字符串首字母大写
{

    static class Program
    {
        public static string FirstCharToUpper(string input)
        {//两个字符串
            if (String.IsNullOrEmpty(input))
                throw new ArgumentException("ARGH!");
            return input.First().ToString().ToUpper() + input.Substring(1);
        }

        public static string FirstLetterToUpper(string str)
        {//两个字符串
            if (str == null)
                return null;

            if (str.Length > 1)
                return char.ToUpper(str[0]) + str.Substring(1);

            return str.ToUpper();
        }

        public static unsafe string ToUpperFirst(this string str)
        {//允许不安全代码
            if (str == null) return null;
            string ret = string.Copy(str);
            fixed (char* ptr = ret)
                *ptr = char.ToUpper(*ptr);
            return ret;
        }



        static void Main(string[] args)
        {
            string s1 = "as";
            string s11 = FirstCharToUpper(s1);

            string s2 = "sd";
            string s22 = FirstLetterToUpper(s2);

            //只需一个 比较好方法
            string s3 = "a";
            string s33 = CultureInfo.CurrentCulture.TextInfo.ToTitleCase(s3.ToLower());

            //只需一个
            string s44 = new CultureInfo("en-US").TextInfo.ToTitleCase("r");

            //需要两个  拼接  
            string s5 = "gs";
            string s55 = s5.Remove(1).ToUpper() + s5.Substring(1);

            //只需一个  性能比较好
            string s6 = "g";
            char[] s66 = s6.ToCharArray();
            s66[0] = char.ToUpper(s66[0]);
            string s666 = new string(s66);

            //只需一个
            string s7 = "r";
            string s77 = System.Text.RegularExpressions.Regex.Replace(s7, "^[a-z]", m => m.Value.ToUpper());

            string s8 = "p";
            string s88 = Regex.Replace(s8, @"^\w", t => t.Value.ToUpper());

            //选中“允许不安全代码”复选框。
            //最好的速度
            string s9 = "l";
            string s99 = s9.ToUpperFirst();
        }
    }
}