Why Does My LINQ JOIN Fail When Using an In-Memory Collection?

Troubleshooting LINQ Joins and In-Memory Collections

The error "Unable to create a constant value of type... Only primitive types or enumeration types are supported in this context" often appears when attempting to incorporate an in-memory collection (like ppCombined in this example) directly into a database LINQ query. This is because database queries operate solely on database-resident data; joining with in-memory data isn't directly supported.

The problem typically manifests within the Select clause of the Join statement. The attempt to create a PersonDTO object, including a property populated from the in-memory collection, causes the conflict.

The Root Cause of LINQ JOIN Failure

The error arises from the attempt to perform a filtering operation (Where clause) on the ppCombined collection within the database query's Select statement. The database engine can't translate this in-memory operation into its own equivalent.

Solution: Separate Database and In-Memory Operations

The solution is to separate the database query from the in-memory processing. This involves executing the database query first, then performing the filtering and data manipulation on the resulting in-memory dataset. The AsEnumerable() method is key here. It forces the execution of the database query, returning the results as an IEnumerable object, allowing subsequent operations to occur entirely in memory.

Here's the corrected code:

<code class="language-csharp">var persons = db.Favorites
    .Where(f => f.userId == userId)
    .Join(db.Person, f => f.personId, p => p.personId, (f, p) => new {
            personId = p.personId,
            addressId = p.addressId,
            favoriteId = f.favoriteId
        })
    .AsEnumerable() // Database query completes here
    .Select(x => new PersonDTO {
            personId = x.personId,
            addressId = x.addressId,
            favoriteId = x.favoriteId,
            personProtocol = ppCombined
                .Where(p => p.personId == x.personId)
                .Select(p => new PersonProtocol {
                    personProtocolId = p.personProtocolId,
                    activateDt = p.activateDt,
                    personId = p.personId
                })
                .ToList()
        });</code>

By using AsEnumerable(), the database query is finalized, and the subsequent Select statement operates solely on the data retrieved from the database, resolving the incompatibility with the in-memory ppCombined collection. This ensures the query executes successfully.

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