第一次出现此错误-可访问性不一致

这是我第一次使用接口和适当的名称空间结构。当我编译时,我得到了下面的错误,我不知道它是什么意思。我做的任何搜索都会显示私人的公共问题,但两者都是公共的。有什么想法吗?

可访问性不一致错误:返回类型“System.Collections.Generic.List”的可访问性低于方法“Webtext.ApplicationEntities.implementationals.AdditionalEntities.UrlBuilderO2.GetUrlRequests” (string,string,string,string)'C:\Users\Laptop\documents\visual studio 2010\Projects\Webtext\Webtext\applicationenties\Implementations\AdditionalEntities\UrlBuilderO2.cs 19 39 Webtext

代码文件是:

namespace Webtext.ApplicationEntities.Interfaces
{
    interface IUrlBuilder
    {
       List<IOperatorRequest> GetUrlRequests(string UserName, string Password, string MessageRecipient, string Message);
    }
}

以及:

namespace Webtext.ApplicationEntities.Implementations.AdditionalEntities
{
    public class UrlBuilderO2: IUrlBuilder
    {

        public List<IOperatorRequest> GetUrlRequests(string UserName, string Password, string MessageRecipient, string Message)
        {
            throw new NotImplementedException();
        }
    }
}

命名空间结构

Webtext

ApplicationEntities

     Interfaces
          ##### INTERFACE IS HERE

     Implementations

          AdditionalEntities
               ##### URL BUILDER IS HERE