代码如下
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace 任务一
{
    class Program
    {
        
        static void Main(string[] args)
        {
          string s;
            Console.WriteLine("请输入一个字符串");
            s= Console.ReadLine();
            Console.WriteLine(getstring(s));
            Console.ReadKey();
            }
        static string getstring(string s)
        {
            string[] a = new string[] { "a", "e", "i", "o", "u" };
           
            for (int i = 0; i < a.Length; i++)
            {
                if (s==a[i])
                {
                    Console.WriteLine("您输入了一个元音");
                }
              else
                {
                    Console.WriteLine("这不是一个元音");
                }

            }
            return null;
           
         
        }
        
    }
   
}

s[i]就是字符串s中的数组元素，s[i]是一个char类型。例如string s="abcdefg",那么s[0]='a',s[1]='b',s[2]='c'.....

恩这个我明白但是我想得到数组中的元素再进行判断

static bool getstring(string s)
        {
            string[] a = new string[] { "a", "e", "i", "o", "u" };

            for (int i = 0; i < a.Length; i++)
            {
                if (s == a[i])
                {
                    return true;
                }
            }
            return false;
        }

.net 3.5以上可以直接用
bool c = a.Contains("a");
bool d = a.Contains("x");

结果是我想要的谢了啊呵呵

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

namespace Inherit
{
    class Test
    {
        private static string a = "aeiou";
      
         public static void Main(string[] args)
        {
            string s = "";
            Console.WriteLine("请输入一个字符串");
            s = Console.ReadLine();
            Solve(s);
        }
        public static void Solve(string s)
        {
            char[] arr = a.ToCharArray();
            char[] arrs = s.ToCharArray();
            for (int i = 0; i < arr.Length; i++)
            {
                for (int j = 0; j < arrs.Length; j++)
                {
                    if (arrs[j] == arr[i])
                    {
                        Console.WriteLine("您输入的字符串中含有元音");
                    }
                    else
                    {
                        Console.WriteLine("您所输入的字符串中不含有元音");
                    }
                }
            }


        }
    }
}