WebApi2 GET always says error has occurred - entity-framework

Question

I've got a simple GET method in my Web API 2 project which queries my Microsof tSQL database via Entity Framework that always returns an error. If I step through it in the debugger the exception is NOT hit. It actually looks like it's cleanly leaving the method. I'm very confused.
[Route("ar")]
public IHttpActionResult GetAuditArs(int auditId)
{
using (var context = new LabSOREntities()) {
try {
var ars = from r in context.SMBWA_Audit_AR
where r.SMBWA_Audit_Id == auditId
select r;
var ret = Ok(ars.ToArray());
return ret;
} catch (Exception ex) {
return BadRequest($"Something went wrong: {ex.Message}");
}
}
}
There's one row in the database, and I see my ars.ToArray() says that's there's a single element in it. How can I debug this since it's left my method when it blows up?
If I just hit that endpoint via the browser I get:
<Error>
<Message>An error has occurred.</Message>
</Error>

Answer 1

The issue will be that you are returning entities from your API call. Behind the scenes WebAPI has to serialize the data being returned, as it does this it will hit any lazy-load reference properties and attempt to load them. Since you are instantiating a scoped DB Context within a using block, the entities will be orphaned from the context prior to the serialization so EF will be throwing exceptions that the DB Context is not available.
Option to verify the behaviour
- Eager-load all references in your "SMBWA_Audit_AR" class. This should eliminate the error and confirm the lazy load serialization issue.
var ars = context.SMBWA_Audit_AR
.Include(x => x.Reference1)
.Include(x => x.Reference2) // etc. where Reference1/2 are related entites to your audit record. If you have a lot of references, that is a lot of includes...
.Where(x => x.SMBWA_Audit_Id = auditId)
.ToArray();
To avoid issues like this, and the cost/time to eager-load everything I recommend using a POCO DTO/ViewModel to return the details about these audit records. Then you can .Select() the fields needed for the POCO. This avoids the lazy-load serialization issue, plus optimizes your queries from EF to return just the data that is needed, not the entire object graph.
for example:
If you need the Audit #, Name, and a Notes field to display in a list of audit summaries:
public class AuditSummary
{
public int AuditID {get; set;}
public string AuditorName {get; set;}
public string Notes {get; set;}
// You can add whatever fields are needed from the Audit or related entities... Including collections of other DTOs for related entites, or summary details like Counts etc..
}
var ars = context.SMBWA_Audit_AR
.Where(x => x.SMBWA_Audit_Id = auditId)
.Select(x => new AuditSummary
{
AuditId = x.AuditId,
AuditorName = x.AuditedBy.Name, //Example getting reference details..
Notes = x.Notes
}).ToArray();
Return models that reflect what the consumer will need. This avoids issues with EF and ensures your queries are efficient.
Scope the DbContext to the Request using an IoC Container (Unity/Autofac, etc.) This can seem like a viable option but it isn't recommended. While it will avoid the error, as the serializer iterates over your entity, your DbContext will be querying each individual dependency one at a time by ID. You can see this behaviour by running a profiler against the database while the application is running to detect lazy-load calls. It will work in the end, but it will be quite slow.
As a general rule, don't return entities from Web API or MVC controller methods to avoid errors and performance issues with serializers.