看代码

    internal class Program
    {
        static void Main(string[] args)
        {
            try
            {
                var abc = EnvironmentVarialbeHelper.Get("abc");
                if (!string.IsNullOrEmpty(abc))
                {
                    Console.WriteLine(abc);
                }
                else
                {
                    EnvironmentVarialbeHelper.Set("abc", DateTime.Now.ToString());
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }

            Console.ReadLine();
        }
    }
    public static class EnvironmentVarialbeHelper
    {
        public static string Get(string name)
        {
            return Environment.GetEnvironmentVariable(name, EnvironmentVariableTarget.Machine);
        }
        /// <summary>
        /// (设置系统级环境变量)需要管理员权限
        /// </summary>
        /// <param name="name"></param>
        /// <param name="value"></param>
        public static void Set(string name, string value)
        {
            Environment.SetEnvironmentVariable(name, value, EnvironmentVariableTarget.Machine);
        }
    }

需要注意

写入需要管理员权限。其他没了