After upgrading a .NET Core application to .NET Core 3.1, my GraphQL endpoint started to fail with the following error message:
System.InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.
at Microsoft.AspNetCore.Server.IIS.Core.HttpRequestStream.Read(Byte[] buffer, Int32 offset, Int32 count)
at Microsoft.AspNetCore.Server.IIS.Core.WrappingStream.Read(Byte[] buffer, Int32 offset, Int32 count)
at System.IO.StreamReader.ReadBuffer(Span`1 userBuffer, Boolean& readToUserBuffer)
at System.IO.StreamReader.ReadSpan(Span`1 buffer)
at System.IO.StreamReader.Read(Char[] buffer, Int32 index, Int32 count)
at Newtonsoft.Json.JsonTextReader.ReadData(Boolean append, Int32 charsRequired)
at Newtonsoft.Json.JsonTextReader.ParseValue()
at Newtonsoft.Json.JsonReader.ReadAndMoveToContent()
at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter)
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
at Newtonsoft.Json.JsonSerializer.Deserialize(JsonReader reader, Type objectType)
at Newtonsoft.Json.JsonSerializer.Deserialize[T](JsonReader reader)
at GraphQL.Server.Transports.AspNetCore.GraphQLHttpMiddleware`1.Deserialize[T](Stream s)
at GraphQL.Server.Transports.AspNetCore.GraphQLHttpMiddleware`1.InvokeAsync(HttpContext context)
at GraphQL.Server.Transports.WebSockets.GraphQLWebSocketsMiddleware`1.InvokeAsync(HttpContext context)
at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
at Microsoft.AspNetCore.ResponseCompression.ResponseCompressionMiddleware.Invoke(HttpContext context)
at Microsoft.AspNetCore.ResponseCaching.ResponseCachingMiddleware.Invoke(HttpContext httpContext)
at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
To fix it you can apply the suggestion as described in the error message:
But there is a good reason why Microsoft changed the default behavior in .NET Core 3.0 and disabled synchronous IO. It can be a source of thread starvation and application hangs.
The GraphQL.NET team is working on a new 3.0 release specifically created for .NET Core 3.0 but this is still in preview.
You can get the latest pre-release packages from the MyGet feed, where you may want to explicitly pull a certain version using -v.
dotnet add package GraphQL.SystemTextJson -v 3.0.0-preview-1448