Reversing IQueryable based on passed property for sorting logic - linq

Question

I am implementing sort based on parameter passed to ascending or descending OrderBy method
else if (showGrid.Sortdir == "DESC")
{
alerts = DB.Incidents.OfType<Alert>().Where(
a =>
a.IncidentStatusID == (int)AlertStatusType.New ||
a.IncidentStatusID == (int)AlertStatusType.Assigned ||
a.IncidentStatusID == (int)AlertStatusType.Watching)
.OrderByDescending(a => showGrid.Sort);
}
else
{
alerts = DB.Incidents.OfType<Alert>().Where(
a =>
a.IncidentStatusID == (int)AlertStatusType.New ||
a.IncidentStatusID == (int)AlertStatusType.Assigned ||
a.IncidentStatusID == (int)AlertStatusType.Watching)
.OrderBy(a => showGrid.Sort);
}
In case of ascending order sorting it works fine but for descending order sorting doesn't work. I debugged the code and I found that list is not revered its same as ascending order. Please help me

Answer 1

Ok. I've written a small test. It is funny, but your code can actually compile and work, but very differently from what you expect :)
Obviously showGrid is not of type Alert, it is an instance of some other class, that incidentally have the same propery as Alert, called Sort.
First I was confused, because expected this code to fail to compile.
// The signature of OrderBy
public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector)
// In your case it will result in
public static IOrderedQueryable<Alert> OrderBy<Alert, string>(this IQueryable<Alert> source, Expression<Func<Alert, string>> keySelector)
//when you call it like you do
DB.Incidents.OfType<Alert>().OrderByDescending(a => showGrid.Sort);
// You supply a property from object of type different from your entity.
// This is incorrect usage, the only object you can use here is the
// "a" argument. Like this:
DB.Incidents.OfType<Alert>().OrderByDescending(a => a.Sort);
// Because anything else does not make any sense to entity provider.
So your order by simply does not work.
As far as I understood, what you want is to perform sorting based on selection in UI. This is not easily achieved in strongly-typed LINQ. Because as I showed above, you send a property, not a value to the OrderBy. It does not care about the value inside the prop. So there are several solutions to the problem:
Write a big switch, that will check every possible Sort value, and will append appropriate 'OrderBy(a => a.YouPropToSort)' to the query. This is straitforward, and you should begin with this. Of course this is a static way, and will require to change code everytime you want new columns to be added for sorting.
Create argument for your OrderBy using 'LINQ Expression Trees'. For you case it should not be very hard to do. Look for the term, you will find a lot of examples.
Try to use Dynamic LINQ. I did not not use it myself, just looked at the docs. This seems to be an extension to the normal LINQ which allows you to write parts of queries as strings, to overcome limitations like the current one with dynamic sorting.

Answer 2

Here's my solution to sorting based on user selections:
Create your base query
var query = DB.Incidents.OfType<Alert>.Where(
a =>
a.IncidentStatusID == (int)AlertStatusType.New ||
a.IncidentStatusID == (int)AlertStatusType.Assigned ||
a.IncidentStatusID == (int)AlertStatusType.Watching);
and then apply your sort using a case statement
bool desc = showGrid.SortDir = "DESC";
switch(showGrid.Sort)
{
case "col1":
query = desc ? query.OrderByDescending( a => a.Col1 ) : query.OrderBy( a => a.Col1 );
break;
case "col2":
query = desc ? query.OrderByDescending( a => a.Col2 ) : query.OrderBy( a => a.Col2 );
break;
...
}
var results = query.ToList();