stephen776 stephen776 - 14 days ago 9
C# Question

How to call a method in the where clause of a LINQ query on a IQueryable object

I have an IQueryable of MyType obtained via EF 4.1.

I am applying filters via linq in the form of a where clause, One of which will filter based on distance from a given zip code.

MyType has a ZipCode property and I need to call a method which computes the distance between the MyType zip codes and my given zip code.

I have tried the following, which compiles, but throws an error at runtime.

myTypes = myTypes.Where(x => GetDistance(x.Zip, givenZip) < 10);


How can I accompish this?

EDIT

My Distance method returns a double that represents the distance in miles

public double Distance(Position position1, Position position2)
{

}


Position is a struct containing doubles for lat and long

Answer

This should work in Linq to Objects if GetDistance() returns a boolean - it will not work with Linq to Entities since it will try to map your method to a SQL equivalent, which of course there is none.

As a crude workaround you could use AsEnumerable() but that would materialize all your types so is not recommended if your table is larger:

myTypes = myTypes.AsEnumerable()
                 .Where(x => GetDistance(x.Zip, givenZip) < 10);

Another way would be to map Zip codes to geographic locations in the database and use those locations directly with the soon to be supported spatial data types - this is probably the best approach in my opinion, but not production-ready. Of course if you are restricted to just SQL Server you could just use a store query directly to use geo-locations - but that would work around EF.