Error Converting Enum to String in LINQ Query

Question

I have the following query, which is using Entity Framework.

Analytic firstSent = (from a in Repository.Query<Analytic>()
                      where a.EntityType == "Proposal" &&
                      a.EntityId == Program.ActiveProposal.Id &&
                      a.Marker == AnalyticMarker.EmailProposalUrl.ToString()
                      orderby a.TimestampUtc
                      select a).FirstOrDefault();

At run time, I get the following error:

LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.

a.Marker is a string column, and AnalyticMarker.EmailProposalUrl is an enum value, and I want to compare that column against the name of that enum.

I understand that the conversion from an enum to a string is not supported by SQL, but why won't it resolve the value of this string and then pass the resulting string to SQL? That should work just fine.

Answer 1

Try this:

var emailProposalUrl = AnalyticMarker.EmailProposalUrl.ToString();
Analytic firstSent = (from a in Repository.Query<Analytic>()
                      where a.EntityType == "Proposal" &&
                      a.EntityId == Program.ActiveProposal.Id &&
                      a.Marker == emailProposalUrl
                      orderby a.TimestampUtc
                      select a).FirstOrDefault();

This other answer is explaining the reason why this could work as well..

The problem arises because ToString() isn't really executed, it is turned into a MethodGroup and then parsed and translated to SQL. Since there is no ToString() equivalent, the expression fails.