Question

I’m trying to combine the following expressions into a single expression: item => item.sub, sub => sub.key to become item => item.sub.key. I need to do this so I can create an OrderBy method which takes the item selector separately to the key selector. This can be accomplished using one of the overloads on OrderBy and providing an IComparer<T>, but it won’t translate to SQL.

Following is a method signature to further clarify what I am trying to achive, along with an implementation that doesn’t work, but should illustrate the point.

    public static IOrderedQueryable<TEntity> OrderBy<TEntity, TSubEntity, TKey>(         this IQueryable<TEntity> source,          Expression<Func<TEntity, TSubEntity>> selectItem,          Expression<Func<TSubEntity, TKey>> selectKey)         where TEntity : class         where TSubEntity : class      {         var parameterItem = Expression.Parameter(typeof(TEntity), 'item');         ...         some magic         ...         var selector = Expression.Lambda(magic, parameterItem);         return (IOrderedQueryable<TEntity>)source.Provider.CreateQuery(             Expression.Call(typeof(Queryable), 'OrderBy', new Type[] { source.ElementType, selector.Body.Type },                  source.Expression, selector                  ));     }  

which would be called as:

.OrderBy(item => item.Sub, sub => sub.Key) 

Is this possible? Is there a better way? The reason I want an OrderBy method that works this way is to support a complex key selection expression that applies to many entities, though they are exposed in different ways. Also, I’m aware of a way to do this using String representations of deep properties, but I’m trying to keep it strongly typed.

Answer 1

Since this is LINQ-to-SQL, you can usually use Expression.Invoke to bring a sub-expression into play. I’ll see if I can come up with an example (update: done). Note, however, that EF doesn’t support this – you’d need to rebuild the expression from scratch. I have some code to do this, but it is quite lengthy…

The expression code (using Invoke) is quite simple:

var param = Expression.Parameter(typeof(TEntity), 'item'); var item = Expression.Invoke(selectItem, param); var key = Expression.Invoke(selectKey, item); var lambda = Expression.Lambda<Func<TEntity, TKey>>(key, param); return source.OrderBy(lambda); 

Here’s example usage on Northwind:

using(var ctx = new MyDataContext()) {     ctx.Log = Console.Out;     var rows = ctx.Orders.OrderBy(order => order.Customer,         customer => customer.CompanyName).Take(20).ToArray(); } 

With TSQL (reformatted to fit):

SELECT TOP (20) [t0].[OrderID], -- snip FROM [dbo].[Orders] AS [t0] LEFT OUTER JOIN [dbo].[Customers] AS [t1]   ON [t1].[CustomerID] = [t0].[CustomerID] ORDER BY [t1].[CompanyName]