代码如下,当表示层调用时就报错了,void无法隐式转换string.
using System;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;
namespace ConsoleApplication1
{
    class Program
    {
        public  void codeNumber(string source, string num)

        {
            int n1 = source.Length;
            int n2 = num.Length;
            string str1 = source.Substring(n1 - n2, n2);
            try
            { int str2 = Convert.ToInt32(str1);
              int num2 = Convert.ToInt32(num);
              int sum = str2 + num2-1;
              int n3 = sum.ToString().Length;
              if (n3 > n2)
              {
                  str1 = source.Substring(n1 - n2 - 1, n2 + 1);
                  str2 = Convert.ToInt32(str1);
                  sum = str2 + num2-1;
              }
              string str4 = source.Substring(0, n1 - n2) + sum.ToString();

              Console.WriteLine(str4);
          }

            catch
            {
                Console.WriteLine("输入有误");
            }

        }   
        static void Main(string[] args)
        {
            Program c = new Program();
            c.codeNumber("AB00230001", "70000");
         
        }
    }
}

codeNumber方法另作一个class

using System; 
using System.Collections.Generic; 
using System.Text; 
using System.Text.RegularExpressions; 
 
namespace ConsoleApplication1 
{ 
    class Program 
    { 
        class test 
        { 
            public void codeNumber(string source, string num) 
            { 
                int n1 = source.Length; 
                int n2 = num.Length; 
                string str1 = source.Substring(n1 - n2, n2); 
                try 
                { 
                    int str2 = Convert.ToInt32(str1); 
                    int num2 = Convert.ToInt32(num); 
                    int sum = str2 + num2 - 1; 
                    int n3 = sum.ToString().Length; 
                    if (n3 > n2) 
                    { 
                        str1 = source.Substring(n1 - n2 - 1, n2 + 1); 
                        str2 = Convert.ToInt32(str1); 
                        sum = str2 + num2 - 1; 
                    } 
                    string str4 = source.Substring(0, n1 - n2) + sum.ToString(); 
 
                    Console.WriteLine(str4); 
                } 
 
                catch 
                { 
                    Console.WriteLine("输入有误"); 
                } 
 
            } 
        } 
 
        static void Main(string[] args) 
        { 
            test c = new test(); 
 
            c.codeNumber("AB00230001", "70000"); 
 
        } 
    } 
}

那个，不知道为什么，我执行你的函数，是正确的。。

public  void codeNumber(string source, string num)
这什么不直接改成
public  void codeNumber(string source, int num)