c# 查询字符串内有多少个指定字符!

C#计算字符串中有多少个“a”

using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
  
namespace ConsoleApplication1  
{  
    class Program  
    {  
        static void Main(string[] args)  
        {  
            Console.WriteLine("这是个计算字符串中含有多少/"a/"的程序");  
            Console.WriteLine("请输入字符串");  
  
            string s1 = Console.ReadLine();  
            //int x = s1.IndexOf("a");  
            int count = 0;  
            int startIndex = 0;  
            while (true)  
            {  
                int y = s1.IndexOf("a", startIndex);  
                if (y != -1)  
                {  
                    count++;  
                    startIndex = y + 1;  
                }  
                else  
                {  
                    break;  
                }  
            }  
            Console.WriteLine("/"a/"在字符串中出现了{0}次", count);  
            Console.ReadLine();  
        }  
    }  
}  

first:

using System.Text;
using System.Text.RegularExpressions;

   string a = "adfdf:adfa:afafwer:";
            Regex rg = new Regex(":");
            MatchCollection mc = rg.Matches(a);

            Console.WriteLine(string.Format("Count of @a :{0}", mc.Count));