vb.net - 帮助使用 Linq 和泛型。在查询中使用 GetValue

Question

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。

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。

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？

在 C#/VB 中发布您的答案。选择让您感觉更舒适的那一款。

谢谢

编辑 : 我也试过这个代码，结果相同

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

Answer 1

您需要将代码转换为表达式树。

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);
        }
    }
}