Dynamic LINQ OrderBy using String Names

Question

Hi,

I need to create a dynamic order by expression from a field name. I try to perform something like explained at this post:

http://www.singingeels.com/Articles/Self_Sorting_GridView_with_LINQ_Expression_Trees.aspx

That is:

 public Expression<Func<KEntity, object>> SortExpression
 {
  get
  {
  if (string.IsNullOrEmpty(this.SortFieldName))
  {
   return null;
  }
  else
  {
   ParameterExpression pe = Expression.Parameter(typeof(KEntity), "p");
   var property = Expression.Property(pe, this.SortFieldName);
   Expression expression = Expression.Convert(property, typeof(object));
   return Expression.Lambda<Func<KEntity, object>>(expression, pe);
  }
  }
 }

But when I try to sort a DateTime field I still have this exception:

Unable to cast the type 'System.DateTime' to type 'System.Object'. LINQ to Entities only supports casting Entity Data Model primitive types.

I know this could be solved using a generic returned type instead an object, but I don't know how to pass this type dynamically.

Any ideas about how to solve this problem?

Thanks in advance,

JAReyes.

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Answer 1

See: http://forums.asp.net/p/1580414/3981470.aspx#3983399 ...

public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> query, string memberName)
{
 ParameterExpression[] typeParams = new ParameterExpression[] { Expression.Parameter(typeof(T), "") };

 System.Reflection.PropertyInfo pi = typeof(T).GetProperty(memberName);

 return (IOrderedQueryable<T>)query.Provider.CreateQuery(
 Expression.Call(
  typeof(Queryable),
  "OrderBy",
  new Type[] { typeof(T), pi.PropertyType },
  query.Expression,
  Expression.Lambda(Expression.Property(typeParams[0], pi), typeParams))
 );
}

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Answer 2

Hi Kristofer,

I will prefer a function returning an expression like you propose first:

public static Expression<Func<T, TKey>> OrderExpression<T, TKey>(string memberName)
{
    ParameterExpression[] typeParams = new ParameterExpression[] { Expression.Parameter(typeof(T), "") };

    Expression<Func<T, TKey>> orderByExpression
        = (Expression<Func<T, TKey>>)Expression.Lambda(
            Expression.Property(typeParams[0], memberName),
            typeParams
          );

    return orderByExpression;
}

I know I can use generics, but I can't know this type because it is dynamic.

If I perform an OrderBy extension, I will need to extend also OrderByDescending, ThenBy and ThenByDescending methods. Besides perform changes at my repositories...

---

Please remember to Vote & "Mark As Answer" if this post is helpful to you.
Por favor, recuerda Votar y "Marcar como respuesta" si la solucion de esta pregunta te ha sido útil.