The problem I kept having was that at some point you’ll need to convert the GridView.SortExpression from your ASP.NET GridView to a Lamba Expression for your Queryable.OrderBy.

The challenge

You’ll get the string.

“OrderDate DESC”

And you have to translate that to:

ObjectContext.Orders.OrderByDescending(order => order.OrderDate)

In march/may I had spent a half day researching possibilities to solve this automatically. At that point I couldn’t find a clean solution. So I went with a generated solution (using T4 templates) which generated a big switch statement per entity.

A nice solution

We jump to november and I was browsing a little bit on StackOverflow to see if their were some interesting questions. My eye caught the question “How do I apply OrderBy on an IQueryable using a string column name within a generic extension method?”, and it got me started to solve the sorting challenge. I had some hours available so I thought I’d give it another try.

There are a few other’s who have created a solution using the Expression building method, and I’ve combined it and refactored it to make it match more with the current Entity Framework. I’ve also added support for an infinite amount of child entities.

/***** BEGIN LICENSE BLOCK *****
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 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
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 * The Original Code is LINQExtensions.StringFieldNameSortingSupport.
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 * The Initial Developer of the Original Code is
 * Davy Landman.
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using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace LINQExtensions
{
    public static class StringFieldNameSortingSupport
    {
        #region Private expression tree helpers

        private static LambdaExpression GenerateSelector<TEntity>(String propertyName, out Type resultType) where TEntity : class
        {
            // Create a parameter to pass into the Lambda expression (Entity => Entity.OrderByField).
            var parameter = Expression.Parameter(typeof(TEntity), "Entity");
            //  create the selector part, but support child properties
            PropertyInfo property;
            Expression propertyAccess;
            if (propertyName.Contains('.'))
            {
                // support to be sorted on child fields.
                String[] childProperties = propertyName.Split('.');
                property = typeof(TEntity).GetProperty(childProperties[0], BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
                propertyAccess = Expression.MakeMemberAccess(parameter, property);
                for (int i = 1; i < childProperties.Length; i++)
                {
                    property = property.PropertyType.GetProperty(childProperties[i], BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
                    propertyAccess = Expression.MakeMemberAccess(propertyAccess, property);
                }
            }
            else
            {
                property = typeof(TEntity).GetProperty(propertyName, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
                propertyAccess = Expression.MakeMemberAccess(parameter, property);
            }
            resultType = property.PropertyType;
            // Create the order by expression.
            return Expression.Lambda(propertyAccess, parameter);
        }
        private static MethodCallExpression GenerateMethodCall<TEntity>(IQueryable<TEntity> source, string methodName, String fieldName) where TEntity : class
        {
            Type type = typeof(TEntity);
            Type selectorResultType;
            LambdaExpression selector = GenerateSelector<TEntity>(fieldName, out selectorResultType);
            MethodCallExpression resultExp = Expression.Call(typeof(Queryable), methodName,
                            new Type[] { type, selectorResultType },
                            source.Expression, Expression.Quote(selector));
            return resultExp;
        }
        #endregion
        public static IOrderedQueryable<TEntity> OrderBy<TEntity>(this IQueryable<TEntity> source, string fieldName) where TEntity : class
        {
            MethodCallExpression resultExp = GenerateMethodCall<TEntity>(source, "OrderBy", fieldName);
            return source.Provider.CreateQuery<TEntity>(resultExp) as IOrderedQueryable<TEntity>;
        }

        public static IOrderedQueryable<TEntity> OrderByDescending<TEntity>(this IQueryable<TEntity> source, string fieldName) where TEntity : class
        {
            MethodCallExpression resultExp = GenerateMethodCall<TEntity>(source, "OrderByDescending", fieldName);
            return source.Provider.CreateQuery<TEntity>(resultExp) as IOrderedQueryable<TEntity>;
        }
        public static IOrderedQueryable<TEntity> ThenBy<TEntity>(this IOrderedQueryable<TEntity> source, string fieldName) where TEntity : class
        {
            MethodCallExpression resultExp = GenerateMethodCall<TEntity>(source, "ThenBy", fieldName);
            return source.Provider.CreateQuery<TEntity>(resultExp) as IOrderedQueryable<TEntity>;
        }
        public static IOrderedQueryable<TEntity> ThenByDescending<TEntity>(this IOrderedQueryable<TEntity> source, string fieldName) where TEntity : class
        {
            MethodCallExpression resultExp = GenerateMethodCall<TEntity>(source, "ThenByDescending", fieldName);
            return source.Provider.CreateQuery<TEntity>(resultExp) as IOrderedQueryable<TEntity>;
        }
        public static IOrderedQueryable<TEntity> OrderUsingSortExpression<TEntity>(this IQueryable<TEntity> source, string sortExpression) where TEntity : class
        {
            String[] orderFields = sortExpression.Split(',');
            IOrderedQueryable<TEntity> result = null;
            for (int currentFieldIndex = 0; currentFieldIndex < orderFields.Length; currentFieldIndex++)
            {
                String[] expressionPart = orderFields[currentFieldIndex].Trim().Split(' ');
                String sortField = expressionPart[0];
                Boolean sortDescending = (expressionPart.Length == 2) && (expressionPart[1].Equals("DESC", StringComparison.OrdinalIgnoreCase));
                if (sortDescending)
                {
                    result = currentFieldIndex == 0 ? source.OrderByDescending(sortField) : result.ThenByDescending(sortField);
                }
                else
                {
                    result = currentFieldIndex == 0 ? source.OrderBy(sortField) : result.ThenBy(sortField);
                }
            }
            return result;
        }
    }
}

You can also download this file. All you have to do to use these extensions methods is add a using LINQExtensions;".

It's now very simple to write queries like this

var result = queryableOrders.OrderBy("Employee.LastName").ThenByDescending("Freight").ToList();

I’ve deliberately kept the parsing of the SortExpression out of these extension methods. In my opinion it should be kept outside of the string field names support. I’ve added a new extension method just for this purpose.

var result = queryableOrders.OrderUsingSortExpression("Employee.LastName, Freight DESC").ToList();

Bummer

When researching for writing this article I found the article Dynamic LINQ (Part 1: Using the LINQ Dynamic Query Library) by the Scott Guthrie, which does exactly as I developed. The only reason I still post this code is that it’s less heavy and not under de Microsoft Permissive License (but under a very flexible triple license).

So decide for yourself, use this code directly or download the samples pack and locate the DynamicLinq folder and use that library (which has some very cool stuff in it).

Tags: c#, Entity Framework, LINQ