Null context with GraphQL DataLoader

Question

In my project that uses graphql-dotnet, I unexpectedly started getting errors saying the DataLoader context was null:

Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[1]
An unhandled exception has occurred while executing the request.
System.ArgumentNullException: Value cannot be null.
Parameter name: context
  at GraphQL.DataLoader.DataLoaderContextExtensions.GetOrAddBatchLoader[TKey,T](DataLoaderContext context, String loaderKey, Func`3 fetchFunc, IEqualityComparer`1 keyComparer, T defaultValue)

The context parameter comes from the IDataLoaderContextAccessor that is injected into the constructor of the graph type. Why would it be null?

The entire graphql-dotnet project is an anti-pattern. I wouldn't expect it to actually work well at the current stage of development for it. — DetectivePikachu, Aug 30 '19 at 15:34
@ĴošħWilliard Well that's weird because I know lots of people using it in production, it works very well indeed. — DavidG, Aug 30 '19 at 15:35
That is a scary thought. The whole project is in dire need of refactor. — DetectivePikachu, Aug 30 '19 at 15:36
The entire project is dripping with code smell and anti-patterns. First thing that comes to mind-- its reliance on the service locator anti-pattern. — DetectivePikachu, Aug 30 '19 at 15:39
There are definitely parts I would want to refactor, but it works well enough right now. — Nate Barbettini, Aug 30 '19 at 17:59

score 3 · Answer 1 · answered Aug 30 '19 at 15:32

The context parameter will be null if you forget to add the DataLoaderDocumentListener before executing the query (like I did):

public GraphQLController(
    ISchema schema,
    IDocumentExecuter documentExecuter,
    DataLoaderDocumentListener dataLoaderListener)
{
    _schema = schema;
    _documentExecuter = documentExecuter;
    _dataLoaderListener = dataLoaderListener;
}

[HttpPost]
public async Task<IActionResult> Post([FromBody]BaseGraphQuery query)
{
    var inputs = query.Variables.ToInputs();
    var executionOptions = new ExecutionOptions
    {
        Schema = _schema,
        Query = query.Query,
        Inputs = inputs,
    };

    // Important!
    executionOptions.Listeners.Add(_dataLoaderListener);

    // (Execute the query)
}

And DataLoaderDocumentListener must be present in the service container:

public void ConfigureServices(IServiceCollection services)
{
    // (other service configuration)

    services.AddSingleton<GraphQL.DataLoader.IDataLoaderContextAccessor, GraphQL.DataLoader.DataLoaderContextAccessor>();
    services.AddSingleton<GraphQL.DataLoader.DataLoaderDocumentListener>();
    services.AddSingleton<GraphQL.IDocumentExecuter, GraphQL.DocumentExecuter>();
    services.AddSingleton<GraphQL.Types.ISchema, RootSchema>();
}

The accurate but vague error message confused me until I remembered I had "refactored" some code and removed the Listeners.Add line by mistake.

score -1 · Answer 2 · answered Mar 25 '20 at 13:20

-1

Also you need to make sure services.AddMvc(); is before all other stuff in startup.ConfigureServices()

answered Mar 25 '20 at 13:20

flieks

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The order of calls in `ConfigureServices` doesn't matter. It _does_ matter in `Configure`, because that is setting up a response pipeline. – Nate Barbettini Mar 26 '20 at 18:12
@NateBarbettini it is what i changed to get my Context filled because it was null before that change.. So not sure about that – flieks Mar 26 '20 at 22:22
Hmm, that is weird. Are you sure it was `ConfigureServices`? Simply changing the position of `AddMvc` caused a difference? My understanding of it could be wrong. – Nate Barbettini Mar 27 '20 at 00:55
Not certain because i changed maybe one other small thing which i think is not related. I know that the order of things in ConfigureServices can make a difference though – flieks Apr 01 '20 at 12:42

Null context with GraphQL DataLoader

2 Answers2