Question

How can I do the following in linq to entities for entity framework::

public static Expression<Func<T, bool>> IsOk<T>(long? entityId)
{
  return x => (!entityId.HasValue || entityId.Value == x.GetEntityId());
 }

The x.GetEntityId() returns an exception since it doesn’t recognize the method.

Answer 1

You cannot – your database cannot run GetEntityId() (since it does not understand it) hence your Linq provider cannot translate it.

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UPDATE

So how can you solve it:

• Id must be a property on the entity hence mapped to a column in the database. This way it can be easily used in an expression
• If Id is calculated (there is logic in GetEntityId()) then this needs to be done at the client: return all rows and do the filtering in C# subsequently. Of course it is needless to say what it is gonna do to your performance.