请用C#语言实现，最小公共字符串

class Program
    {
        public Program(string str1,string str2)
        {
            StringBuilder sb = new StringBuilder();
            sb.Append(str1);
            sb.Append(str2);
            for (int i = 0; i < sb.Length; i++)
            {
                for (int j = i + 1; j < sb.Length; j++)
                {
                    if (sb[i] == sb[j])
                    {
                        sb.Remove(j, 1);
                    }
                }
            }
            Console.WriteLine("最小公共字符串为：");
            Console.WriteLine(sb.ToString());
        }

        static void Main(string[] args)
        {
            Console.WriteLine("请输入第一个字符串:");
            string str1 = Console.ReadLine();
            Console.WriteLine("请输入第二个字符串:");
            string str2 = Console.ReadLine();

            new Program(str1,str2);
            Console.ReadLine();
        }
    }