以下代码总是报错， 不知道是哪里的问题， 单独使用console.writeline, 读出来的文件地址是正确的；但若调用ta.Gettalltext 就报错

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace TestVS
{

class Program
{
    class Ta
    {
        private string txtFullName;
        public string TxtFullNamE
        {
            set
            {
                txtFullName = value;
            }
            get
            { return txtFullName; }

        }

        public void GetAlltext()//方法：读取文件中的所有内容
        {
            string str1 = File.ReadAllText(txtFullName);
            Console.WriteLine(str1);
        }


    }
     static void Main(string[] args)
    {
        var ta = new Ta();
        string fullName = System.Diagnostics.Process.GetCurrentProcess().MainModule.FileName;
        string[] path = fullName.Split('\\');
        string fullpath = null;
        for (int i = 0; i < path.Length - 2; i++)
        {
            fullpath = fullpath + path[i] + @"\";
        }
        ta.TxtFullNamE = fullpath + @"a.txt";


        ta.GetAlltext();
   
        Console.ReadKey();
    }
}

}