本人初学C#，随便写个程序有问题
请教各位高手
using System;
class exercise
{  
    public static void Main() 
    {
    //string input1;
    char ch;
    string input1=Console.ReadLine();
    for(int i=0;i<input1.Length;i++)
        {
         if (i==0)
         {
              ch=input1[input1.Length-1];
         }
        if (i!=0)
        {
            if (input1[i-1]<'x')
            {

              ch=(char)((int)input1[i-1]+3);
            }
            else 
            {
                switch (input1[i-1])
                {
                case 'x': ch='a'; break;
                case 'y': ch='b'; break;
                case 'z': ch='c'; break;
                default : break;
                }
            }
        }
             Console.Write(ch);
        }
    
    //Console.WriteLine(input1.Length);
    Console.ReadLine();
    }
}



错误是：use of unassigned local variable 'ch'