PetaPoco的映射需要继承IMapper接口，该接口提供了四个方法：

• TableInfo GetTableInfo(Type pocoType);
• ColumnInfo GetColumnInfo(PropertyInfo pocoProperty);
• Func<object, object> GetFromDbConverter(PropertyInfo TargetProperty, Type SourceType);
• Func<object, object> GetToDbConverter(PropertyInfo SourceProperty);

Brad提供了一个StandardMapper类来实现这个接口，完成PetaPoco中的默认映射。

这个类实现了两个方法：GetTableInfo 和GetColumnInfo，后两个转换的方法均返回null值。

/// 
        /// Constructs a TableInfo for a POCO by reading its attribute data        /// 
        /// The POCO Type        /// 
             public TableInfo GetTableInfo(Type pocoType)        {            return TableInfo.FromPoco(pocoType);        }        /// 
        /// Constructs a ColumnInfo for a POCO property by reading its attribute data        /// 
        ///         /// 
             public ColumnInfo GetColumnInfo(PropertyInfo pocoProperty)        {            return ColumnInfo.FromProperty(pocoProperty);        }

通过调用TableInfo的FromPoco静态方法生成一个新的TableInfo类，和通过调用ColumnInfo的FromProperty静态方法生成一个新的ColumnInfo类。接下来我们看看这两个方法：

TableInfo.FromPoco

/// 
        /// Creates and populates a TableInfo from the attributes of a POCO        /// 
        /// The POCO type        /// 
     
      A TableInfo instance
             public static TableInfo FromPoco(Type t)        {            TableInfo ti = new TableInfo();            // Get the table name            var a = t.GetCustomAttributes(typeof(TableNameAttribute), true);            ti.TableName = a.Length == 0 ? t.Name : (a[0] as TableNameAttribute).Value;            // Get the primary key            a = t.GetCustomAttributes(typeof(PrimaryKeyAttribute), true);            ti.PrimaryKey = a.Length == 0 ? "ID" : (a[0] as PrimaryKeyAttribute).Value;            ti.SequenceName = a.Length == 0 ? null : (a[0] as PrimaryKeyAttribute).sequenceName;            ti.AutoIncrement = a.Length == 0 ? false : (a[0] as PrimaryKeyAttribute).autoIncrement;            return ti;        }

这个方法通过反射，得到TableInfo的TableName、PrimaryKey、AutoIncrement等信息。

ColumnInfo.FromProperty

/// 
        /// Creates and populates a ColumnInfo from the attributes of a POCO property.        /// 
        /// The property whose column info is required        /// 
     
      A ColumnInfo instance
             public static ColumnInfo FromProperty(PropertyInfo pi)        {            // Check if declaring poco has [Explicit] attribute            bool ExplicitColumns = pi.DeclaringType.GetCustomAttributes(typeof(ExplicitColumnsAttribute), true).Length > 0;            // Check for [Column]/[Ignore] Attributes            var ColAttrs = pi.GetCustomAttributes(typeof(ColumnAttribute), true);            if (ExplicitColumns)            {                if (ColAttrs.Length == 0)                    return null;            }            else            {                if (pi.GetCustomAttributes(typeof(IgnoreAttribute), true).Length != 0)                    return null;            }            ColumnInfo ci = new ColumnInfo();            // Read attribute            if (ColAttrs.Length > 0)            {                var colattr = (ColumnAttribute)ColAttrs[0];                ci.ColumnName = colattr.Name==null ? pi.Name : colattr.Name;                ci.ForceToUtc = colattr.ForceToUtc;                if ((colattr as ResultColumnAttribute) != null)                    ci.ResultColumn = true;            }            else            {                ci.ColumnName = pi.Name;                ci.ForceToUtc = false;                ci.ResultColumn = false;            }            return ci;        }

该方法通过反射得到一个属性所对应的字段的信息。

通过这两个方法能够得到实体到数据表的映射关系，在类PocoData和PocoColumn中，将实际的数据通过映射得到具体的实体。