GraphQL in .NetCore 3.1 WebApi giving "Synchronous operations are disallowed" error

Question

I was following a Plural Site course "Building GraphQL APIs with ASP.NET Core" but seem to have come unstuck. When running the "playground", the browser loads with:

• Message "Server can not be reached"
• Schema can not load
• And the following text in the browser's right-hand pane:

{ "error": "Unexpected token S in JSON at position 0" }

If I run this as the application (so not IISExpress) then in the console window I see the following failure:

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1] An unhandled exception has occurred while executing the request. System.InvalidOperationException: Synchronous operations are disallowed.

I found a few posts that said to add the AllowSynchronousIO option, which I did but to no avail. I tried running in IISExpress too...no difference.

Here are some details of my setup:

.Net Core I created an ASP.NET Core 3.1 (3.1.6) Web API project.

NuGet packages

• GraphQL (2.4.0)
• GraphQL.Server.Transports.AspNetCore (3.4.0)
• GraphQL.Server.Ui.Playground (3.4.0)

There is a disclaimer on the github site that the latest stable version has "many known issues" which have probably been fixed in a preview release, but I tried those and couldn't even get it to compile....

Startup.cs

Using statements

using global::GraphQL;
using global::GraphQL.Server;
using global::GraphQL.Server.Ui.Playground;
using global::GraphQL.Types;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using WebApiGQL.GraphQL.Schema;
using WebApiGQL.Repository;

ConfigureServices

services.AddCors();

services.AddScoped<IProductRepository, ProductRepository>();
services.AddScoped<IDependencyResolver>(s => new FuncDependencyResolver(s.GetRequiredService));
services.AddScoped<ProductSchema>();
services
    .AddGraphQL(o => { o.ExposeExceptions = true; })
    .AddGraphTypes(ServiceLifetime.Scoped);
services.Configure<IISServerOptions>(options =>
{
    options.AllowSynchronousIO = true;
});

Configure

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
}

app.UseCors(o => o.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader());
app.UseGraphQL<ISchema>();
app.UseGraphQLPlayground(new GraphQLPlaygroundOptions());

[Note: the app.UseCors(...) will eventually go into the if (env.IsDevelopment()) block.]

I tend not to develop Core apps in IISExpress because I think you get better diagnostics from the command window that pops up.

GraphQL looks so powerful, I'd love to get this up and running....but just can't see what I'm missing.

---

Edit 3rd Aug: response to Yongqing Yu

So this is a really "Hello World" application that does NOT use the Entity Framework. The eventual application (if we go this way) would pull data from a series of end-points (e.g. SQL, other APIs, etc, etc), merge that data together and then return the merged details.

ProductSchema.cs

public class ProductSchema : Schema
{
    public ProductSchema(IDependencyResolver resolver) :
        base(resolver)
    {
        Query = resolver.Resolve<ProductQuery>();
    }
}

IProductRepository

public interface IProductRepository
{
    List<Product> GetAll();

    //List<Product> GetList(List<string> skus);

    //Product Get(string sku);
}

My intention would be to only have the two methods that are commented out, however I went back to basics and just had a "GetAll" method in an attempt to fix this.

ProductRepository

public class ProductRepository : IProductRepository
{
    public List<Product> GetAll()
    {
        return this.AllProducts;
    }

    //public List<Product> GetList(List<string> skus)
    //{
    //    return this.AllProducts.Where(p => skus.Contains(p.Sku)).ToList();
    //}

    //public Product Get(string sku)
    //{
    //    return this.AllProducts.FirstOrDefault(p => p.Sku.Equals(sku));
    //}

    private readonly List<Product> AllProducts = new List<Product>
        {
            new Product
            {
                Sku = "ABC12345"
            },

            new Product
            {
                Sku = "ABC12345"
            }
        }
    }

ProductType.cs

public class ProductType : ObjectGraphType<Product>
{
    public ProductType()
    {
        Field(p => p.Sku);
    }
}

Product.cs I started off with this as a complex beast where the properties included:

• List
• Enums
• strings, integers, bools and decimals

But, to simplify things I cut this down to:

public class Product
{
    public string Sku { get; set; }
}

---

Answer 1

Finally....solved this.

I had:

services.Configure<IISServerOptions>(options =>
{
    options.AllowSynchronousIO = true;
});

But needed:

services.Configure<KestrelServerOptions>(options =>
{
    options.AllowSynchronousIO = true;
});