CODEWITHSUNDEEP
c#linqentity-framework
Florian
Florian

Reputation: 5994

Linq + Where + abstract method = LINQ to Entities does not recognize method

First of all: I know that there were already many questions before about this topic. But I really could not find any solutions mor MY problem. My problem is that I am using an abstract method to select from an dbset. My code looks like this:

var dbe = (from i in dbEntities where IsEqualRecord(me, i) select i);

And this is my abstract method deklaration:

protected abstract bool IsEqualRecord(MEntity modelEntities, DEntity databaseEntity);

MEntity and DEntity are generic-types. I've read that my where Statement can't be translated to an sql statement. But how can I solve that problem? Is there any other approach?

Please don't vote to close this question. I've took a look at nearly every similar question on stackoverflow but I could not find a solution.

Upvotes: 4

Views: 498

Answers (1)

Thomas Levesque
Thomas Levesque

Reputation: 292425

Linq to Entities queries are translated to SQL to be executed by the DBMS, so you can only use code that can be translated to SQL. A custom C# method cannot be translated to SQL, so EF fails to translate the query. The fact that it's abstract is irrelevant.

A possible workaround would be to replace IsEqualRecord by a method that returns an Expression<Func<DEntity, bool>> that can be translated to SQL; you could then do something like that:

var dbe = dbEntities.Where(MakeEqualityPredicate(me));

...


protected abstract Expression<Func<DEntity, bool>> MakeEqualityPredicate(MEntity m);

The MakeEqualityPredicate method should return an Expression that compares a DEntity to the given MEntity. For instance, a derived class could implement it like this:

protected override Expression<Func<DEntity, bool>> MakeEqualityPredicate(MEntity m)
{
    return d => d.Id == m.Id;
}

Upvotes: 5

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