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C# New Added 15 October 2026

ASP.NET Core: Synchronous operations are disallowed. Call ReadAsync or set

Kestrel and IIS block synchronous reads and writes on the request and response bodies, because a blocked thread per request starves the thread pool under load. The exception usually comes from a library rather than your code: an XmlSerializer, an old Newtonsoft.Json formatter, or a StreamReader.ReadToEnd() in middleware. It only shows up when the body is large enough not to be buffered, so small test requests may pass.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix
// The stack trace names the synchronous caller; look below the Kestrel frames

// Your own code: read the body asynchronously
using var reader = new StreamReader(Request.Body);
var body = await reader.ReadToEndAsync();

// A serializer that only works synchronously: buffer the body first
Request.EnableBuffering();
await Request.Body.DrainAsync(CancellationToken.None); // Microsoft.AspNetCore.WebUtilities
Request.Body.Position = 0;

// Last resort, per request, for a library you cannot change
var feature = HttpContext.Features.Get<IHttpBodyControlFeature>();
if (feature != null) feature.AllowSynchronousIO = true;

How to diagnose C# errors

C# errors divide into compile-time and restore-time problems (NuGet resolution, target framework mismatches) and runtime problems dominated by null references and async misuse. The classic .NET production hazard is the sync-over-async deadlock, calling .Result or .Wait() on a task in a context with a synchronisation context, which manifests as a hang rather than an exception and is therefore much harder to spot.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

  1. Run dotnet restore --verbosity detailed to see which feed each package resolved from. Most restore failures are a private feed with expired credentials, not a missing package.
  2. Enable nullable reference types (<Nullable>enable</Nullable>) and treat the warnings as errors. This converts a whole class of production NullReferenceExceptions into compile failures.
  3. For async hangs, search the codebase for .Result, .Wait() and .GetAwaiter().GetResult(). Replace with await all the way up, or use ConfigureAwait(false) in library code.
  4. For EF Core, run dotnet ef migrations list to see which migrations the database believes are applied before generating a new one.
  5. Capture a dump with dotnet-dump and inspect it with dotnet-gcdump / dotnet-counters when the process misbehaves without throwing.

Tools worth reaching for

  • dotnet restore -v detailed
  • dotnet ef migrations list
  • dotnet-counters
  • dotnet-dump

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

learn.microsoft.com learn.microsoft.com

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