使用泛型程序实现输出转换

    当获取了作为输出的SqlDataReader对象后，下一步是将输出内容转换为可发送到业务逻辑层的泛型集合。
GetProductCategories()方法利用辅助类DataAccessHelper（如示例2所示）将SqlDataReader对象内容转换为对象。

    示例2：实现DataAccessHelper类

using System; 
using System.Data; 
using System.Data.SqlClient; 
using System.Configuration; 
using System.Data.Common; 
using System.Reflection; 
using System.Collections; 

namespace AdventureWorksTraderDataAccess 
{ 
public class DataAccessHelper 
{ 
static public object PopulateEntity(object entity, DataColumnMapping[] mappings, IDataReader reader) 
{ 
foreach (DataColumnMapping mapping in mappings) 
{ 
int ordinalPosition = 0; 
try 
{ 
ordinalPosition = reader.GetOrdinal(mapping.SourceColumn); 
} 
catch (IndexOutOfRangeException ex) 
{ 
throw new PropertyColumnMappingException(mapping.SourceColumn + 
" is not a valid SourceColumn", ex); 
} 
object propertyValue = reader.GetValue(ordinalPosition); 
if (propertyValue != DBNull.Value) 
{ 
if (mapping.DataSetColumn == "ID") 
{ 
Nullable<int> tempValue = (int)propertyValue; 
propertyValue = tempValue; 
} 
object[] param = { propertyValue }; 
try 
{ 
entity.GetType().InvokeMember(mapping.DataSetColumn, BindingFlags.SetProperty | BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Static, null, entity, param); 
} 
catch (Exception e) 
{ 
throw new PropertyColumnMappingException 
(GetPropertyColumnMappingExceptionMessage(mapping), e); 
} 
} 
} 
return entity; 
} 

private static string GetPropertyColumnMappingExceptionMessage 
(DataColumnMapping mapping) 
{ 
return "Could not populate " + mapping.DataSetColumn + 
" property from the " + mapping.SourceColumn + " database column"; 
} 
} 
} 

    在GetProductCategories()方法内部，首先声明了一个DataColumnMapping数组，该数组中包括ProductCategory表列与ProductCategory对象属性之间的所有映射信息：

private DataColumnMapping[] mappings = new DataColumnMapping[] { 
new DataColumnMapping("ProductCategoryID", "ProductCategoryID"), 
new DataColumnMapping("Name", "Name"), 
new DataColumnMapping("rowguid", "Rowguid"), 
new DataColumnMapping("ModifiedDate", "ModifiedDate")}; 

    当调用PopulateEntity()时，需要提供ProductCategory对象实例，DataColumnMapping数组以及包括执行存储过程所得输出的SqlDataReader对象：

ProductCategory category = (ProductCategory)DataAccessHelper.PopulateEntity(temp, mappings, reader);

    PopulateEntity()方法将SqlDataReader对象的每一行转换为具有适当属性值的ProductCategory对象。