Generic query method in Entity Framework

Question

In my DB I have tables who have an attribute int DeleteState. I want a generic method to query those tables. In other words a method who does this: Context.Table.Where(x => x.DeleteState == 0).

I thought I could do this:

public static class Extensions
{
  public static IQueryable<T> Exists<T>(this IQueryable<T> qry) where T : IDeletable
  {
    return qry.Where(x => x.DeleteState == 0);
  }
}

Where IDeletable is this:

public interface IDeletable
{
    int DeleteState { get; set; }
}

Now I only have to add the IDeletable in the EF model:

public partial class Table : EntityObject, IDeletable { ... }

I did this with the templating mechanism.

Unfortunately, it doesn't work :( It compiles fine, but throws at runtime:

Unable to cast the type 'Table' to type 'IDeletable'. LINQ to Entities only supports casting Entity Data Model primitive types

if I call it like that:

Context.Table.Exists();

How can I solve this problem? Could you think of a fix or a different method to achieve similar results? Thx

Wouter de Kort · Accepted Answer · 2011-12-16 09:44:34Z

0

The problem you have is that the Entity Framework can only work with an Expression Tree. Your function executes a query directly instead of building an Expression Tree.

A simpler solution would be to add a Model Defined Function.

A model defined function can be called directly on an instance of your context.

answered Dec 16, 2011 at 9:44

Wouter de Kort♦

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Comments

Vince · Accepted Answer · 2011-12-16 10:07:57Z

0

Maybe:

public static IQueryable<T> Exists<T>(this IQueryable<T> qry)
{
    return qry.Where(x => (!typeof(IDeletable).IsAssignableFrom(x.GetType()) || typeof(IDeletable).IsAssignableFrom(x.GetType()) && ((IDeletable)x).DeleteState == 0));
}

answered Dec 16, 2011 at 10:07

Vince

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Comments

Community · Accepted Answer · 2017-05-23 11:55:31Z

0

Tsss, this is the answer: Linq Entity Framework generic filter method

I forgot about the class here:

... where T : class, IDeletable

edited May 23, 2017 at 11:55

CommunityBot

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answered Dec 16, 2011 at 10:13

duedl0r

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3 Comments

Vince Over a year ago

If your solution works error you had at runtime does not seems good to me.

duedl0r Over a year ago

@vince: please elaborate. Exceptions aren't good in general :)

Vince Over a year ago

Does your solution work? did you try mine? I just said that if it works by addind class to generic constraint, the error "LINQ to Entities only supports casting Entity Data Model primitive types" does not have sense. It does not realy matter, it won't be the first exception without ^^

James · Accepted Answer · 2011-12-16 10:15:47Z

0

Have you tried converting your objects to IDeletable before you actually query? e.g.

public static IQueryable<T> Exists<T>(this IQueryable<T> qry)
{     
    return qry.Select<T, IDeletable>(x => x).Where(x => x.DeleteState == 0).Cast<T>();
}

I haven't tested this code, however, the error rings a bell and I remember I had to do something similar.

edited Dec 16, 2011 at 10:15

answered Dec 16, 2011 at 9:46

James

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2 Comments

duedl0r Over a year ago

Hmm, I get the same runtime exception. Also a downside to this is that I get a different type from the select. It's not T anymore but IDeletable, right?

James Over a year ago

@duedl0r - Yeah but you can always just cast it back Where(x => x.DeleteState == 0).Cast<T>(). See my updated source, you will still be getting the issue if your still force T to be IDeletable in the method declaration.

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