我完全理解这个错误,但有没有一种方法可以实现一些代码,使linq-to实体明白我的自定义类型应该被视为decimal
基元类型?
我创建了一个自定义的Money
类型,它接受一个可为null的decimal
作为其构造函数重载之一。它有隐式运算符重载和所有其他类型的酷爵士乐。我想在linq中使用这种自定义类型来进行实体投影,但遗憾的是标题中出现了错误。
有没有一个属性可以在我的Money
类型上声明,它说:"嘿,为了linq投影的目的,我是基元类型decimal
"?
自定义表达式树/访问者是否可以在这里帮助声明所有类型的Money
都将被视为linq投影的decimal
基元?
[HeyImADecimalForLinqProjections] // custom attribute
public class Money : IComparable<Money>
{
public Money()
{
Value = decimal.Zero;
}
public Money(decimal? value)
{
if (value.HasValue)
Value = value.Value;
else
Value = decimal.Zero;
}
public static implicit operator Money(decimal value)
{
return new Money(value);
}
public static implicit operator Money(decimal? value)
{
return new Money(value.Value);
}
...
}
示例投影:
var items = orderQuery.Where(a => a.OrderId = 345)
.Select(a => new OrderModel()
{
OrderId = a.OrderId,
SubtotalIncludingTax = a.SubtotalIncludingTax, // no dice, SubtotalIncludingTax is of type Money - Unable to cast the type 'System.Decimal' to type 'Money'. LINQ to Entities only supports casting EDM primitive or enumeration types.
SubtotalIncludingTaxRaw = a.SubtotalIncludingTax, // this is fine because SubtotalIncludingTaxRaw is of type decimal
})
.ToList();
我目前的解决方案是可行的,但我讨厌它。我先投影到一个匿名类型,然后将匿名类型投影到我的实际模型。这个例子被淡化了,这个双投影解决方案导致了很多重复,我宁愿避免。
// first get everything as anonymous projection objects in linq to entities
var anonymousItems = orderQuery.Where(a => a.OrderId = 345)
.Select(a =>
{
OrderId = a.OrderId,
SubtotalIncludingTax = a.SubtotalIncludingTax // decimal to decimal
.ToList();
// now cast to our actual model since we are back in .Net world
var items = anonymousItems
.Select(a => new OrderModel()
{
OrderId = a.OrderId,
SubtotalIncludingTax = a.SubtotalIncludingTax // decimal to Money
.ToList();
此时实体框架不支持这样的强制转换。您可以使用映射属性和未映射属性来解决问题,例如:
public decimal RawTotalIncludingTax {get;set;}
public Money TotalIncludingTax
{
get { return RawTotalIncludingtax; }
set { RawTotalIncludingTax = value; }
}
这可能会将额外的匿名类型保存到具体的类型选择中,但仍然会留下类似的问题。更有可能有帮助的解决方案是一种扩展方法,可以执行以下操作:
using System;
public static class MoneyExtensions
{
public static string Format(this decimal val){
if(val < 0)
return "(" + Math.Abs(val).ToString("N2") + ")";
else
return val.ToString("N2");
}
}
public class Program
{
public static void Main()
{
Console.WriteLine(2.2m.Format());
Console.WriteLine((-12.42m).Format());
}
}
这样,您就不需要自定义类型,但仍然可以使用您想要的MoneyExtensions和语法!