Question

I want to pass dynamic lambda expressions to the function below, but I’m not sure how to define the .Take() or .OrderByDescending() on the expression object.
If I want to call the function below, then I want to be able to do this:

dbprovider.Query = (x => x.ConfigurationReference == "172.16.59.175")
                   .Take(100)
                   .OrderByDescending(x.Date)
FindEntities(db, dbprovider.Query)

But I can’t (this syntax is invalid). Any ideas?

public static List<T> FindEntities<T>(TrackingDataContext dataContext, System.Linq.Expressions.Expression<Func<T, bool>> find) where T : class
{
    try
    {
        var val = dataContext.GetTable<T>().Where(find).ToList<T>();
        return val;
    }
    catch (Exception ex)
    {
        throw ex;
    }
}

Answer 1

The parameter is of type:

System.Linq.Expressions.Expression<Func<T, bool>> find

That means it can take a predicate (the “where” clause), and only a predicate. Thus the only bit you can pass in there is the filter:

x => x.ConfigurationReference == "172.16.59.175"

To do what you want, you would need to add the rest of the code in FindEntities, so that it becomes:

var val = dataContext.GetTable<T>().Where(find)
              .OrderByDescending(x => x.Date).Take(100).ToList<T>();

(note also that the Take should really be after the OrderByDescending)

One way you could do that would be:

public static List<T> FindEntities<T>(TrackingDataContext dataContext,
    System.Linq.Expressions.Expression<Func<T, bool>> find,
    Func<IQueryable<T>, IQueryable<T>> additonalProcessing = null
) where T : class
{
    var query = dataContext.GetTable<T>().Where(find);
    if(additonalProcessing != null) query = additonalProcessing(query);
    return query.ToList<T>();
}

and call:

var data = FindEntities(db, x => x.ConfigurationReference == "172.16.58.175",
    q => q.OrderByDescending(x => x.Date).Take(100));

However, frankly I’m not sure what the point of this would be… the caller could do all of that themselves locally more conveniently, without using FindEntities at all. Just:

var data = db.GetTable<T>()
             .Where(x => x.ConfigurationReference == "172.16.58.175")
             .OrderByDescending(x => x.Date).Take(100).ToList(); 

or even:

var data = db.SomeTable
             .Where(x => x.ConfigurationReference == "172.16.58.175")
             .OrderByDescending(x => x.Date).Take(100).ToList();

or just:

var data = (from row in db.SomeTable
            where row.ConfigurationReference == "172.16.58.175"
            orderby row.Date descending
            select row).Take(100).ToList();