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Solving the "Synchronous operations are disallowed" Exception in ASP.NET Core

After making a small change to an ASP.NET core application, I got the following runtime error:

System.InvalidOperationException: 'Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.'

Here is the code that caused the error:

Let's explore why this happens, when it occurs, and most importantly, how to fix it properly.

Why does this exception occur?

Starting with ASP.NET Core 3.0, Microsoft made a significant architectural change: synchronous I/O operations are disabled by default on the server. This decision was made to improve application performance and prevent thread pool starvation.

In traditional synchronous I/O operations, threads are blocked while waiting for data to be read or written. In a web server handling hundreds or thousands of concurrent requests, this can quickly exhaust the thread pool, leading to:

  • Poor scalability
  • Increased response times
  • Potential deadlocks
  • Resource exhaustion

Asynchronous I/O operations release threads back to the thread pool while waiting for I/O operations to complete, allowing the server to handle more concurrent requests with fewer resources avoiding all the issues above.

Solutions: The right way vs. The quick fix

The right way to fix this is by switching to an async version of our code:

If an async alternative is not available and you really want to use synchronous operations, you can enable this globally or at the request level:

More information

Breaking changes in .NET Core 3.0 - .NET | Microsoft Learn

Configure options for the ASP.NET Core Kestrel web server | Microsoft Learn

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