Question

I’ve got a situation where I need to use a custom expression in a LINQ to Entities query (because I want to have custom logic that L2E wouldn’t otherwise understand:

var query = db.MyTable.Where(MyPredicateExpression)

But I’d rather use query comprehension syntax:

var query = from x in db.MyTable where [x matches the predicate of MyPredicateExpression]

I know this is possible, because L2E supports it in other places:

var query = from x in db.MyTable where x.Length > 10

How do they make that work?

Edit: I’m using devart’s LinqConnect for Oracle, which may behave somewhat differently than Microsoft L2E.

Answer 1

Entity Framework and LINQ to SQL do not support this scenario, because the translation of MyPredicateExpression should be added to expression tree translator.
I recommend you to create a stored function performing the predicate check and add this function to DataContext. You will be able to use a query like the following in this case:

var query = from x in db.MyTable where context.MyPredicateFunction(x.Field) select x;

Update. Here is the updated query that takes into account your comments:

int[] values = new int[] { 1, 2, 3 };
var query = from x in db.MyTable where values.Contains(x.AuditState) select x;  

Update 2. You can add a Queryable property to your context that will be obtaining the necessary set of MyTable objects as shown in the following example:

public partial class MyDataContext {  
  IQueryable<MyTable> GetSpecialTables {  
    get {   
      int[] values = new int[] { 1, 2, 3 };
      return this.MyTables.Where(x => values.Contains(x.AuditState));  
    }  
  }
}

Replace MyDataContext with the actual name of your context.