As the title says I'm trying to build an expression tree for source.OrderByDescending(this source, expression, comparer)
This is my code for generating the expression tree:
var orderByDescendingMethod = typeof(Queryable).GetMethods(BindingFlags.Static | BindingFlags.Public).First(m => m.Name == "OrderByDescending" && m.GetParameters().Count() == 3).MakeGenericMethod(typeof(Books), typeof(string));
var comparer = Expression.New(typeof(NumericStringComparer));
var orderByFilter = GenerateOrderByPropertyExpression<string>(propertyName);
var comparison = Expression.Call(orderByDescendingMethod, Expression.Constant(books), orderByFilter, comparer);
return Expression.Lambda(comparison);
And the GenerateOrderByPropertyExpression method:
private static Expression<Func<Books, T>> GenerateOrderByPropertyExpression<T>(string propertyName)
{
var parameter = Expression.Parameter(typeof(Books), "b");
var property = Expression.Property(parameter, propertyName);
var toStringMethod = typeof(object).GetMethod("ToString");
var objectString = Expression.Call(property, toStringMethod);
return Expression.Lambda<Func<Books, T>>(objectString, parameter);
}
But whenever I call lambda.Compile().DynamicInvoke(); and inspect the result, I get the following error:
LINQ to Entities does not recognize the method 'System.Linq.IOrderedQueryable`1[DataAccess.Plusbog.Books] OrderByDescending[Books,String](System.Linq.IQueryable`1[DataAccess.Plusbog.Books], System.Linq.Expressions.Expression`1[System.Func`2[DataAccess.Plusbog.Books,System.String]], System.Collections.Generic.IComparer`1[System.String])' method, and this method cannot be translated into a store expression.
Any idea what I'm doing wrong? I feel like I'm pretty close.
An excerpt from the Supported and Unsupported LINQ Methods (LINQ to Entities) - Ordering Methods section:
Most of the LINQ ordering methods are supported in LINQ to Entities, with the exception of those that accept an IComparer<T>, because the comparer cannot be translated to the data source.