In-Process vs Isolated Process Azure Functions: What’s the Difference?

 When building Azure Functions using .NET, one of the most important architectural choices you’ll face is deciding between In-Process and Isolated Process (Out-of-Process) hosting models.

This decision impacts everything from startup time to dependency control and middleware capabilities. So let’s break it down.

What is In-Process (InProc) Hosting?

In the In-Process model:

  • Your function code runs inside the same process as the Azure Functions runtime.

  • It uses the same AppDomain and host environment.

Benefits:

  • Better performance (lower cold start time).

  • Full access to features in the Microsoft.Azure.WebJobs SDK (like bindings, triggers, and logging).

  • Easier to use existing .NET libraries and tools.

Limitations:

  • Tightly coupled to the Azure Functions runtime version.

  • Less flexibility in middleware and custom startup configuration.

  • Potential for dependency conflicts if runtime and your code use different versions of libraries.

What is Isolated Process (Out-of-Process) Hosting?

In the Isolated Process model:

  • Your function runs in a completely separate process from the Azure Functions runtime.

  • Uses a new worker model introduced with .NET 5+ and required for .NET 7+.

Benefits:

  • Full control over the application startup and dependency injection.

  • No conflicts between runtime dependencies and your code.

  • Supports modern .NET versions and is the path forward for future development.

Limitations:

  • Slightly higher cold start times.

  • Some advanced triggers and bindings are not yet fully supported (though the gap is closing).

  • You need to manually configure things like logging, DI, and middleware (more boilerplate).

Key Differences at a Glance:

FeatureIn-ProcessIsolated Process
Runtime ModelShared with Azure Functions runtimeSeparate from runtime
PerformanceFaster cold startsSlightly slower cold starts
Dependency IsolationShared environmentFull isolation
Middleware SupportLimitedFull control
Latest .NET SupportLimited (up to .NET 6 LTS)Required for .NET 7/8+
DI and Startup ConfigurationLimitedFull customization

Which One Should You Use?

  • Choose In-Process if:

    • You're using .NET Core 3.1 or .NET 6.

    • You want simpler setup and are fine with using the Azure Functions SDK as-is.

  • Choose Isolated Process if:

    • You’re targeting .NET 7+ or want to future-proof your architecture.

    • You need flexible startup logic, full DI control, or advanced logging.

    • You want clean separation from the Azure runtime.

Final Thoughts

The InProc model is great for fast, simple solutions with less configuration. But the Isolated Process model offers modern capabilities, more control, and is the direction Microsoft is investing in for the future of .NET Azure Functions.

If you're starting a new project—especially with .NET 7 or .NET 8—Isolated Process is the recommended choice.

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