Help with Linq and Generics. Using GetValue inside a Query

Question

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.

Pri         


        

Answer 1

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);
        }
    }
}

Answer 2

Have you tried pulling out the DirectCast and reflecting on the lambda parameter instead? Something like this:

query.Where(Function(c) _
    c.GetType().GetProperty(p, GetType("Int64")).GetValue(c, Nothing) = PValue)

I'm not certain you can even do this sort of thing in a lambda, but it's worth a shot.