C# Linq to Entities: The type of one of the expressions in the join clause is incorrect. Type inference failed

Question

I am trying to join two tables together using C# linq to entities.

One table has clients, the other table has licences. The tables are joined by Client_ID and ClientLicence_ClientID. For some reason (they are not my tables) the Client_ID is an int, and the ClientLicence_ClientID is a string.

The code I have is:

var licences = (from client in entities.Clients
                            join licence in entities.ClientLicences
                            on new { clientId = SqlFunctions.StringConvert((double)client.Client_ID) } equals
                            new { licence.ClientLicence_ClientID }
                            into clientGroup
                            where (filter.clientId == 0 || client.Client_ID == filter.clientId)
                            select licence);

When I try to compile this I get an error: The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'GroupJoin'.

If I comment out the where statement then it works (albeit without filtering results).

The filter variable is a class that is passed in to the function. It only has one propery: int clientId.

Answer 1

Look at your join:

join licence in entities.ClientLicences on
new { clientId = SqlFunctions.StringConvert((double)client.Client_ID) }
equals
new { licence.ClientLicence_ClientID }
into clientGroup

The key types are:

• An anonymous type with a property called clientId
• An anonymous type with a property called ClientLicence_ClientID

Those two anonymous types can't be compared for equality. I suspect you don't need anonymous types at all here, actually. I'd expect this to work:

join licence in entities.ClientLicences on
SqlFunctions.StringConvert((double)client.Client_ID)
equals
licence.ClientLicence_ClientID
into clientGroup

(Obviously you don't need all the new lines - I was just trying to separate out the various bits for clarity.)

You claim that without the where clause it works - but that would be very surprising, given that it's the join that's a problem.