/// <summary>
/// 从提供的 DataRecord 对象创建新委托实例。
/// </summary>
/// <param name="RecordInstance">表示一个 DataRecord 实例</param>
/// <returns>从提供的 DataRecord 对象创建新委托实例。</returns>
/// <remarks></remarks>
private static Func<Record, Target> GetInstanceCreator(Record RecordInstance)
{List<MemberBinding> Bindings = new List<MemberBinding>();Type TargetType = typeof(Target);Type SourceType = typeof(Record);ParameterExpression SourceInstance = Expression.Parameter(SourceType, "SourceInstance");MethodInfo GetSourcePropertyMethodExpression = SourceType.GetProperty("Item", new Type[] { typeof(int) }).GetGetMethod();DataTable SchemaTable = ((IDataReader)RecordInstance).GetSchemaTable();//通过在目标属性和字段在记录中的循环检查哪些是匹配的for (int i = 0; i <= RecordInstance.FieldCount - 1; i++){foreach (PropertyInfo TargetProperty in TargetType.GetProperties(BindingFlags.Public | BindingFlags.Instance)) {//如果属性名和字段名称是一样的if (TargetProperty.Name.ToLower() == RecordInstance.GetName(i).ToLower() && TargetProperty.CanWrite) {//获取 RecordField 的类型Type RecordFieldType = RecordInstance.GetFieldType(i);//RecordField 可空类型检查if ((bool)(SchemaTable.Rows[i]["AllowDBNull"]) == true && RecordFieldType.IsValueType) {RecordFieldType = typeof(Nullable<>).MakeGenericType(RecordFieldType);}//为 RecordField 创建一个表达式Expression RecordFieldExpression = Expression.Call(SourceInstance, GetSourcePropertyMethodExpression, Expression.Constant(i, typeof(int)));//获取一个表示 SourceValue 的表达式Expression SourceValueExpression = GetSourceValueExpression(RecordFieldType, RecordFieldExpression);Type TargetPropertyType = TargetProperty.PropertyType;//从 RecordField 到 TargetProperty 类型的值转换Expression ConvertedRecordFieldExpression = GetConvertedRecordFieldExpression(RecordFieldType, SourceValueExpression, TargetPropertyType);MethodInfo TargetPropertySetter = TargetProperty.GetSetMethod();//为属性创建绑定var BindExpression = Expression.Bind(TargetPropertySetter, ConvertedRecordFieldExpression);//将绑定添加到绑定列表
                Bindings.Add(BindExpression);}}}//创建 Target 的新实例并绑定到 DataRecordMemberInitExpression Body = Expression.MemberInit(Expression.New(TargetType), Bindings);return Expression.Lambda<Func<Record, Target>>(Body, SourceInstance).Compile();
}

/// <summary>
/// 获取表示 RecordField 真实值的表达式。
/// </summary>
/// <param name="RecordFieldType">表示 RecordField 的类型。</param>
/// <param name="RecordFieldExpression">表示 RecordField 的表达式。</param>
/// <returns>表示 SourceValue 的表达式。</returns>
private static Expression GetSourceValueExpression(Type RecordFieldType, Expression RecordFieldExpression)
{//首先从 RecordField 取消装箱值，以便我们可以使用它UnaryExpression UnboxedRecordFieldExpression = Expression.Convert(RecordFieldExpression, RecordFieldType);//获取一个检查 SourceField 为 null 值的表达式UnaryExpression NullCheckExpression = Expression.IsFalse(Expression.TypeIs(RecordFieldExpression, typeof(DBNull)));ParameterExpression Value = Expression.Variable(RecordFieldType, "Value");//获取一个设置 TargetProperty 值的表达式Expression SourceValueExpression = Expression.Block(new ParameterExpression[] { Value }, Expression.IfThenElse(NullCheckExpression, Expression.Assign(Value, UnboxedRecordFieldExpression),Expression.Assign(Value, Expression.Constant(GetDefaultValue(RecordFieldType), RecordFieldType))), Expression.Convert(Value, RecordFieldType));return SourceValueExpression;
}

/// <summary>
/// Gets an expression representing the recordField converted to the TargetPropertyType
/// </summary>
/// <param name="RecordFieldType">The Type of the RecordField</param>
/// <param name="UnboxedRecordFieldExpression">An Expression representing the Unboxed RecordField value</param>
/// <param name="TargetPropertyType">The Type of the TargetProperty</param>
/// <returns></returns>
private static Expression GetConvertedRecordFieldExpression(Type RecordFieldType, Expression UnboxedRecordFieldExpression, Type TargetPropertyType)
{Expression ConvertedRecordFieldExpression = default(Expression);if (object.ReferenceEquals(TargetPropertyType, RecordFieldType)){//Just assign the unboxed expressionConvertedRecordFieldExpression = UnboxedRecordFieldExpression;}else if (object.ReferenceEquals(TargetPropertyType, typeof(string))){//There are no casts from primitive types to String.//And Expression.Convert Method (Expression, Type, MethodInfo) only works with static methods.ConvertedRecordFieldExpression = Expression.Call(UnboxedRecordFieldExpression, RecordFieldType.GetMethod("ToString", Type.EmptyTypes));}else{//Using Expression.Convert works wherever you can make an explicit or implicit cast.//But it casts OR unboxes an object, therefore the double cast. First unbox to the SourceType and then cast to the TargetType//It also doesn't convert a numerical type to a String or date, this will throw an exception.ConvertedRecordFieldExpression = Expression.Convert(UnboxedRecordFieldExpression, TargetPropertyType);}return ConvertedRecordFieldExpression;
}

/// <summary>
/// A Singleton construct that returns a precompiled delegate if it exists, otherwise it will create one
/// </summary>
/// <param name="RecordInstance"></param>
/// <returns></returns>
static internal Func<Record, Target> GetCreator(Record RecordInstance)
{if (_Creator == null){lock (SyncRoot){if (_Creator == null){//Get Creator on first access_Creator = GetInstanceCreator(RecordInstance);}}}return _Creator;
}

/// <summary>
/// ExtensionMethod that creates a List<Target> from the supplied IDataReader
/// </summary>
/// <param name="Reader"></param>
/// <returns></returns>
public static List<Target> ToList<Target>(this IDataReader Reader) where Target : class, new()
{List<Target> List = new List<Target>();while (Reader.Read()){List.Add(CreateInstance<Target>(Reader));}return List;
}/// <summary>
/// ExtensionMethod that Creates an instance<Target>) from a DataRecord.
/// </summary>
/// <param name="Record">The DataRecord containing the values to set on new instance</param>
/// <returns>An instance of Target class</returns>
public static Target CreateInstance<Target>( this IDataRecord Record) where Target : class, new()
{return (Mapper<IDataRecord, Target>.GetCreator(Record))(Record);
}