property_helper.cs

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Linq.Expressions;
using System.Web;

namespace Admin.Helpers
{
    public class IQueryablePropertyHelper
    {
        /**
         * Eksempel på kall til denne funksjonen
         * 
         * #Felt
         * string field = "Kommunenavn";
         * 
         * #Tabell
         * var entities = database.Set<Entities.DeklarasjonerRapport>(); 
         * 
         * #Utvalg
         * var query = entities.Where(x => x.ID > 3); //Set spørring
         * 
         * #Til liste
         * List<string> records = Helpers.IQueryablePropertyHelper.SelectProperty<Entities.DeklarasjonerRapport>(query, field);
         * 
         */
        public static IQueryable SelectProperty<TEntity>(IQueryable query, string propertyName) where TEntity : class
        {
            var parameter = Expression.Parameter(typeof(TEntity), "instance");
            var propertyAccess = Expression.Property(parameter, propertyName);
            var projection = Expression.Lambda(propertyAccess, parameter);

            var selectExpression = Expression.Call(
                typeof(Queryable).GetMethods()
                                 .First(x => x.Name == "Select")
                                 .MakeGenericMethod(new[] { typeof(TEntity), propertyAccess.Type }),
                query.Expression,
                projection);

            var distinctExpression = Expression.Call(
                typeof(Queryable).GetMethods()
                                 .First(x => x.Name == "Distinct")
                                 .MakeGenericMethod(new[] { propertyAccess.Type }),
                selectExpression);

            var result = query.Provider.CreateQuery(distinctExpression);

            return result;
        }
    }
}