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 query = context.Customers;
                query = Simplified(query, "CustomerID", "ALFKI");
                var list = query.ToList();
            }
        }

        static IQueryable Simplified(IQueryable query, string propertyName, string propertyValue)
        {
            PropertyInfo propertyInfo = typeof(T).GetProperty(propertyName);
            return Simplified(query, propertyInfo, propertyValue);
        }

        static IQueryable Simplified(IQueryable 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> lambda = Expression.Lambda>(b, e);
            return query.Where(lambda);
        }
    }
}