Can a LINQ query with a where clause on the key to an OData service be done with filter query option instead of a canonical URL? - linq

Question

The problem
I'm trying to query data from an OData V4 service. This is done with a C# client generated by the OData Connected Service extension for Visual Studio. The query is done with a LINQ expression with a where clause. The where clause contains criteria for the key fields of the entity being queried.
The query results in a DataServiceQueryException ("An error occurred while processing this request") with an inner DataServiceClientException ("NotFound"), both from the Microsoft.OData.Client namespace. See below for the full stack trace.
Analysis
Using Fiddler I can see that the request being sent is using a canonical URL (also called a by-key request). If the criteria values do not match any existing data, the response has the code 404 Not Found. This code seems to cause the exception.
When the where clause is changed to also include non-key fields, the request is sent using a $filter query option. In this case, if the criteria values do not match any existing data, the response has the code 200 OK. This does not cause an exception and returns null as result of the LINQ query.
Another workaround is to not use LINQ and instead specify explicitely that a filter query option should be used.
Comparison with the OData reference service TripPin showed that the 404 response does not seem to be the correct response in this case. TripPin instead returns 204 No Content. While the OData specification has several indications that this seems the correct response in this case, I could not find an explicit statement to that effect. In any case, this point is moot since I don't have control over the OData service and can't change its behavior.
Repro details
Unfortunately, the OData service in question is not publicly available. It may be possible to mock such a service or find a public service that shows the same behavior. I have not looked into this since I found a solution (see my answer).
Nevertheless, here is the code that causes the exception:
static void GetData()
{
Uri odataUri = new Uri("https://the-odata-service", UriKind.Absolute);
// Resources is a class generated by the OData Connected Service extension
// and extends Microsoft.OData.Client.DataServiceContext
Resources context = new Resources(odataUri);
var entity = context.Entities.Where(x => x.Key == 1).SingleOrDefault();
}
Producing this request and response:
GET https://the-odata-service/entities(1) HTTP/1.1
HTTP/1.1 404 Not Found
The exception:
Unhandled exception. Microsoft.OData.Client.DataServiceQueryException: An error occurred while processing this request.
---> Microsoft.OData.Client.DataServiceClientException: NotFound
at Microsoft.OData.Client.QueryResult.ExecuteQuery()
at Microsoft.OData.Client.DataServiceRequest.Execute[TElement](DataServiceContext context, QueryComponents queryComponents)
--- End of inner exception stack trace ---
at Microsoft.OData.Client.DataServiceRequest.Execute[TElement](DataServiceContext context, QueryComponents queryComponents)
at Microsoft.OData.Client.DataServiceQuery`1.GetEnumerator()
at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Boolean& found)
at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
at Microsoft.OData.Client.DataServiceQueryProvider.ReturnSingleton[TElement](Expression expression)
at System.Linq.Queryable.SingleOrDefault[TSource](IQueryable`1 source)
at <my test program in the GetData method in the line of the SingleOrDefault call>
If I change the LINQ to
var entity = context.Entities
.Where(x =>
x.Key == 1
&& x.AnotherNonKeyField == "2")
.SingleOrDefault();
I get
GET https://the-odata-service/Entities?$filter=Key%20eq%201%20and%20AnotherNonKeyField%20eq%20'2'&$top=2 HTTP/1.1
HTTP/1.1 200 OK
{
"#odata.context":"https://the-odata-service/$metadata#Entities","value":[
]
}
which does not result in an exception, but entity being null.
The question
To sum up, while there are workarounds, I would prefer if I could query the odata service with LINQ and without having to add dummy criteria (which would not always be possible). Is there a way to do that?

Answer 1

TLDR
The KeyComparisonGeneratesFilterQuery property of the DataServiceContext can be used to generate a $filter query option.
Some more background
I spent some time researching this issue in context of LINQ and the client that was generated. In hindsight, it is obvious that the Microsoft OData Client library would have been a better place to start, since it throws the exception. But who has time to read a stack trace when instead you can furiously google and debug for a few hours *sigh* ?
Eventually I found my way to issue #851 DataServiceQuery makes a "by key" request when Where clause compares just the ID, causing exception instead of empty result if the entity is not found. and pull request #1762 Enable Where clause to generate $filter query options for key predicates. Especially the later does a much better job of explaining the purpose and how to use the KeyComparisonGeneratesFilterQuery property than the documentation.
With that, the above code can be fixed like this:
static void GetData()
{
Uri odataUri = new Uri("https://the-odata-service", UriKind.Absolute);
// Resources is a class generated by the OData Connected Service extension
// and extends Microsoft.OData.Client.DataServiceContext
Resources context = new Resources(odataUri);
context.KeyComparisonGeneratesFilterQuery = true;
var entity = context.Entities.Where(x => x.Key == 1).SingleOrDefault();
}
Which produces
GET https://the-odata-service/Entities?$filter=Key%20eq%201&$top=2 HTTP/1.1
HTTP/1.1 200 OK
{
"#odata.context":"https://the-odata-service/$metadata#Entities","value":[
]
}