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

c#, inference, join, linq, where-clause

Solution

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.

Problem

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.

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