帮助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

我正在试图创建一个函数，为基于属性和值的查询添加'where'子句。这是一个非常简单的函数版本。

pre $ 私有函数简化（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'这一行仅用于测试，这里是错误raise
返回查询

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，并且此方法不能转换成商店表达式。

看起来像一个linq查询中不能使用GetValue。我可以通过其他方式实现吗？

在C＃/ VB中发布您的答案。选择让你觉得更舒适的那个。

感谢

编辑：

  Private Function simplified2（ByVal query As IQueryable（Of T））
 
 query = From q查询
其中q.GetType.GetProperty（Id）.GetValue（q，Nothing）.Equals（1）
选择q 
 Dim t = query.ToList 
返回查询
 End Function 
  

解决方案

您需要将代码转换为表达式树。

  using System; 
使用System.Linq; 
使用System.Linq.Expressions; 
使用System.Reflection; 
 
命名空间WindowsFormsApplication1 
 {
静态类程序
 {
 [STAThread] 
 static void Main（）
 {
 using（var context = new NorthwindEntities（））
 {
 IQueryable< Customer> query = context.Customers; 
 query = Simplified&Customer>（查询，CustomerID，ALFKI）; 
 var list = query.ToList（）; 
} 
} 
 
静态IQueryable< T>简化< T>（IQueryable< T>查询，字符串propertyName，字符串propertyValue）
 {
 PropertyInfo propertyInfo = typeof（T）.GetProperty（propertyName）; 
 return简化< T>（query，propertyInfo，propertyValue）; 
} 
 
静态IQueryable< T>简化< 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）; 
 
表达式< Func< T，bool>> lambda = Expression.Lambda< Func< T，bool>>（b，e）; 
返回query.Where（lambda）; 
} 
} 
} 
  

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

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.

Looks like a can't use GetValue inside a linq query. Can I achieve this in other way?

Post your answer in C#/VB. Chose the one that make you feel more confortable.

Thanks

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

解决方案

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);
        }
    }
}

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