微型项目实践（8）：数据访问的实现

这个代码比较长，我们可以把它分为三部分来看，第一部分是实现了一个Singelton模式，用于构造 Linq的外部映射文件，涉及的代码如下：

1:  //定义内嵌XML映射资源文件的位置
   2:  private const string 

MAPPING_SOURCE_RESOURCE_NAME = "DongBlog.DataAccess.MappingSource.Xml";
   3:  

//定义XML映射源
   4:  private static MappingSource _MappingSource;
   5:  //定义锁


   6:  private static object _Locker = new object();
   7:
   8:  //私有的构造函数


   9:  private Database(string connectionString, MappingSource mappingSource)
  10:   

   : base(connectionString, mappingSource) { }
  11:
  12:  //构造Linq映射源
  13:  

private static MappingSource getMappingSource()
  14:  {
  15:      if (_MappingSource == null)
  16:      {
  17:          lock (_Locker)
  18:          {
  19:       

       if (_MappingSource == null)
  20:              {
  21:                  Stream mapingSourceStream = Assembly.GetAssembly(typeof(Database))
                          

.GetManifestResourceStream(MAPPING_SOURCE_RESOURCE_NAME);
  22:                  if 

(mapingSourceStream == null)
  23:                      throw new System.IO
           

                    .InvalidDataException(			        "映射文件不存在

！您确定已经将其编译属性设置为\"嵌入资源（Embedded Resource）\"?");
  24:  

                _MappingSource = XmlMappingSource.FromStream(mapingSourceStream);
  25:   

           }
  26:          }
  27:      }
  28:      return _MappingSource;
  

29:  }
  30:
  31:  /// <summary>
  3