帮助使用 Linq 和泛型.在查询中使用 GetValue [英] Help with Linq and Generics. Using GetValue inside a Query
问题描述
我正在尝试创建一个函数,该函数将一个where"子句添加到基于属性和值的查询中.这是我的函数的一个非常简单的版本.
I'm triying to make a function that add a 'where' clause to a query based in a property and a value. This is a very simplefied version of my function.
Private Function simplified(ByVal query As IQueryable(Of T), ByVal PValue As Long, ByVal p As PropertyInfo) As ObjectQuery(Of T)
query = query.Where(Function(c) DirectCast(p.GetValue(c, Nothing), Long) = PValue)
Dim t = query.ToList 'this line is only for testing, and here is the error raise
Return query
End Function
错误消息是:LINQ to Entities 无法识别方法 'System.Object CompareObjectEqual(System.Object, System.Object, Boolean)' 方法,并且此方法无法转换为存储表达式.
The error message is: LINQ to Entities does not recognize the method 'System.Object CompareObjectEqual(System.Object, System.Object, Boolean)' method, and this method cannot be translated into a store expression.
看起来不能在 linq 查询中使用 GetValue.我可以通过其他方式实现这一目标吗?
Looks like a can't use GetValue inside a linq query. Can I achieve this in other way?
在 C#/VB 中发布您的答案.选择让您感觉更舒适的那一款.
Post your answer in C#/VB. Chose the one that make you feel more confortable.
谢谢
EDIT:我也用相同的结果尝试了这段代码
EDIT: I also tried this code with the same results
Private Function simplified2(ByVal query As IQueryable(Of T))
query = From q In query
Where q.GetType.GetProperty("Id").GetValue(q, Nothing).Equals(1)
Select q
Dim t = query.ToList
Return query
End Function
推荐答案
您需要将代码转换为表达式树.
You need to convert the code to an expression tree.
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
namespace WindowsFormsApplication1
{
static class Program
{
[STAThread]
static void Main()
{
using (var context = new NorthwindEntities())
{
IQueryable<Customer> query = context.Customers;
query = Simplified<Customer>(query, "CustomerID", "ALFKI");
var list = query.ToList();
}
}
static IQueryable<T> Simplified<T>(IQueryable<T> query, string propertyName, string propertyValue)
{
PropertyInfo propertyInfo = typeof(T).GetProperty(propertyName);
return Simplified<T>(query, propertyInfo, propertyValue);
}
static IQueryable<T> Simplified<T>(IQueryable<T> query, PropertyInfo propertyInfo, string propertyValue)
{
ParameterExpression e = Expression.Parameter(typeof(T), "e");
MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
ConstantExpression c = Expression.Constant(propertyValue, propertyValue.GetType());
BinaryExpression b = Expression.Equal(m, c);
Expression<Func<T, bool>> lambda = Expression.Lambda<Func<T, bool>>(b, e);
return query.Where(lambda);
}
}
}
这篇关于帮助使用 Linq 和泛型.在查询中使用 GetValue的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!