Guide for running C# Azure Functions in an isolated process

This article is an introduction to using C# to develop .NET isolated process functions, which run out-of-process in Azure Functions. Running out-of-process lets you decouple your function code from the Azure Functions runtime. Isolated process C# functions run on both .NET 5.0 and .NET 6.0. In-process C# class library functions aren't supported on .NET 5.0.

Getting started Concepts Samples

Why .NET isolated process?

Previously Azure Functions has only supported a tightly integrated mode for .NET functions, which run as a class library in the same process as the host. This mode provides deep integration between the host process and the functions. For example, .NET class library functions can share binding APIs and types. However, this integration also requires a tighter coupling between the host process and the .NET function. For example, .NET functions running in-process are required to run on the same version of .NET as the Functions runtime. To enable you to run outside these constraints, you can now choose to run in an isolated process. This process isolation also lets you develop functions that use current .NET releases (such as .NET 5.0), not natively supported by the Functions runtime. Both isolated process and in-process C# class library functions run on .NET 6.0. To learn more, see Supported versions.

Because these functions run in a separate process, there are some feature and functionality differences between .NET isolated function apps and .NET class library function apps.

Benefits of running out-of-process

When running out-of-process, your .NET functions can take advantage of the following benefits:

  • Fewer conflicts: because the functions run in a separate process, assemblies used in your app won't conflict with different version of the same assemblies used by the host process.
  • Full control of the process: you control the start-up of the app and can control the configurations used and the middleware started.
  • Dependency injection: because you have full control of the process, you can use current .NET behaviors for dependency injection and incorporating middleware into your function app.

Supported versions

Versions of the Functions runtime work with specific versions of .NET. To learn more about Functions versions, see Azure Functions runtime versions overview. Version support depends on whether your functions run in-process or out-of-process (isolated).

The following table shows the highest level of .NET Core or .NET Framework that can be used with a specific version of Functions.

Functions runtime version In-process
(.NET class library)
(.NET Isolated)
Functions 4.x .NET 6.0 .NET 6.0
Functions 3.x .NET Core 3.1 .NET 5.01
Functions 2.x .NET Core 2.12 n/a
Functions 1.x .NET Framework 4.8 n/a

1 Build process also requires .NET Core 3.1 SDK.
2 For details, see Functions v2.x considerations.

For the latest news about Azure Functions releases, including the removal of specific older minor versions, monitor Azure App Service announcements.

.NET isolated project

A .NET isolated function project is basically a .NET console app project that targets a supported .NET runtime. The following are the basic files required in any .NET isolated project:

  • host.json file.
  • local.settings.json file.
  • C# project file (.csproj) that defines the project and dependencies.
  • Program.cs file that's the entry point for the app.

Package references

When running out-of-process, your .NET project uses a unique set of packages, which implement both core functionality and binding extensions.

Core packages

The following packages are required to run your .NET functions in an isolated process:

Extension packages

Because functions that run in a .NET isolated process use different binding types, they require a unique set of binding extension packages.

You'll find these extension packages under Microsoft.Azure.Functions.Worker.Extensions.

Start-up and configuration

When using .NET isolated functions, you have access to the start-up of your function app, which is usually in Program.cs. You're responsible for creating and starting your own host instance. As such, you also have direct access to the configuration pipeline for your app. When running out-of-process, you can much more easily add configurations, inject dependencies, and run your own middleware.

The following code shows an example of a HostBuilder pipeline:

var host = new HostBuilder()
    .ConfigureServices(s =>
        s.AddSingleton<IHttpResponderService, DefaultHttpResponderService>();

This code requires using Microsoft.Extensions.DependencyInjection;.

A HostBuilder is used to build and return a fully initialized IHost instance, which you run asynchronously to start your function app.

await host.RunAsync();


The ConfigureFunctionsWorkerDefaults method is used to add the settings required for the function app to run out-of-process, which includes the following functionality:

  • Default set of converters.
  • Set the default JsonSerializerOptions to ignore casing on property names.
  • Integrate with Azure Functions logging.
  • Output binding middleware and features.
  • Function execution middleware.
  • Default gRPC support.

Having access to the host builder pipeline means that you can also set any app-specific configurations during initialization. You can call the ConfigureAppConfiguration method on HostBuilder one or more times to add the configurations required by your function app. To learn more about app configuration, see Configuration in ASP.NET Core.

These configurations apply to your function app running in a separate process. To make changes to the functions host or trigger and binding configuration, you'll still need to use the host.json file.

Dependency injection

Dependency injection is simplified, compared to .NET class libraries. Rather than having to create a startup class to register services, you just have to call ConfigureServices on the host builder and use the extension methods on IServiceCollection to inject specific services.

The following example injects a singleton service dependency:

.ConfigureServices(s =>
    s.AddSingleton<IHttpResponderService, DefaultHttpResponderService>();

This code requires using Microsoft.Extensions.DependencyInjection;. To learn more, see Dependency injection in ASP.NET Core.


.NET isolated also supports middleware registration, again by using a model similar to what exists in ASP.NET. This model gives you the ability to inject logic into the invocation pipeline, and before and after functions execute.

The ConfigureFunctionsWorkerDefaults extension method has an overload that lets you register your own middleware, as you can see in the following example.

var host = new HostBuilder()
    .ConfigureFunctionsWorkerDefaults(workerApplication =>
        // Register our custom middleware with the worker

For a more complete example of using custom middleware in your function app, see the custom middleware reference sample.

Execution context

.NET isolated passes a FunctionContext object to your function methods. This object lets you get an ILogger instance to write to the logs by calling the GetLogger method and supplying a categoryName string. To learn more, see Logging.


Bindings are defined by using attributes on methods, parameters, and return types. A function method is a method with a Function attribute and a trigger attribute applied to an input parameter, as shown in the following example:

public static string[] Run([QueueTrigger("input-queue")] Book myQueueItem,

    FunctionContext context)

The trigger attribute specifies the trigger type and binds input data to a method parameter. The previous example function is triggered by a queue message, and the queue message is passed to the method in the myQueueItem parameter.

The Function attribute marks the method as a function entry point. The name must be unique within a project, start with a letter and only contain letters, numbers, _, and -, up to 127 characters in length. Project templates often create a method named Run, but the method name can be any valid C# method name.

Because .NET isolated projects run in a separate worker process, bindings can't take advantage of rich binding classes, such as ICollector<T>, IAsyncCollector<T>, and CloudBlockBlob. There's also no direct support for types inherited from underlying service SDKs, such as DocumentClient and BrokeredMessage. Instead, bindings rely on strings, arrays, and serializable types, such as plain old class objects (POCOs).

For HTTP triggers, you must use HttpRequestData and HttpResponseData to access the request and response data. This is because you don't have access to the original HTTP request and response objects when running out-of-process.

For a complete set of reference samples for using triggers and bindings when running out-of-process, see the binding extensions reference sample.

Input bindings

A function can have zero or more input bindings that can pass data to a function. Like triggers, input bindings are defined by applying a binding attribute to an input parameter. When the function executes, the runtime tries to get data specified in the binding. The data being requested is often dependent on information provided by the trigger using binding parameters.

Output bindings

To write to an output binding, you must apply an output binding attribute to the function method, which defined how to write to the bound service. The value returned by the method is written to the output binding. For example, the following example writes a string value to a message queue named myqueue-output by using an output binding:

public static string[] Run([QueueTrigger("input-queue")] Book myQueueItem,

    FunctionContext context)
    // Use a string array to return more than one message.
    string[] messages = {
        $"Book name = {myQueueItem.Name}",
        $"Book ID = {myQueueItem.Id}"};
    var logger = context.GetLogger("QueueFunction");

    // Queue Output messages
    return messages;

Multiple output bindings

The data written to an output binding is always the return value of the function. If you need to write to more than one output binding, you must create a custom return type. This return type must have the output binding attribute applied to one or more properties of the class. The following example from an HTTP trigger writes to both the HTTP response and a queue output binding:

public static class MultiOutput
    public static MyOutputType Run([HttpTrigger(AuthorizationLevel.Anonymous, "get")] HttpRequestData req,
        FunctionContext context)
        var response = req.CreateResponse(HttpStatusCode.OK);

        string myQueueOutput = "some output";

        return new MyOutputType()
            Name = myQueueOutput,
            HttpResponse = response

public class MyOutputType
    public string Name { get; set; }

    public HttpResponseData HttpResponse { get; set; }

The response from an HTTP trigger is always considered an output, so a return value attribute isn't required.

HTTP trigger

HTTP triggers translates the incoming HTTP request message into an HttpRequestData object that is passed to the function. This object provides data from the request, including Headers, Cookies, Identities, URL, and optional a message Body. This object is a representation of the HTTP request object and not the request itself.

Likewise, the function returns an HttpResponseData object, which provides data used to create the HTTP response, including message StatusCode, Headers, and optionally a message Body.

The following code is an HTTP trigger

public static HttpResponseData Run([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequestData req,
    FunctionContext executionContext)
    var logger = executionContext.GetLogger("HttpFunction");
    logger.LogInformation("message logged");

    var response = req.CreateResponse(HttpStatusCode.OK);
    response.Headers.Add("Date", "Mon, 18 Jul 2016 16:06:00 GMT");
    response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
    response.WriteString("Welcome to .NET 5!!");

    return response;


In .NET isolated, you can write to logs by using an ILogger instance obtained from a FunctionContext object passed to your function. Call the GetLogger method, passing a string value that is the name for the category in which the logs are written. The category is usually the name of the specific function from which the logs are written. To learn more about categories, see the monitoring article.

The following example shows how to get an ILogger and write logs inside a function:

var logger = executionContext.GetLogger("HttpFunction");
logger.LogInformation("message logged");

Use various methods of ILogger to write various log levels, such as LogWarning or LogError. To learn more about log levels, see the monitoring article.

An ILogger is also provided when using dependency injection.

Differences with .NET class library functions

This section describes the current state of the functional and behavioral differences running on out-of-process compared to .NET class library functions running in-process:

Feature/behavior In-process Out-of-process
.NET versions .NET Core 3.1
.NET 6.0
.NET 5.0
.NET 6.0
Core packages Microsoft.NET.Sdk.Functions Microsoft.Azure.Functions.Worker
Binding extension packages Microsoft.Azure.WebJobs.Extensions.* Under Microsoft.Azure.Functions.Worker.Extensions.*
Logging ILogger passed to the function ILogger obtained from FunctionContext
Cancellation tokens Supported Not supported
Output bindings Out parameters Return values
Output binding types IAsyncCollector, DocumentClient, BrokeredMessage, and other client-specific types Simple types, JSON serializable types, and arrays.
Multiple output bindings Supported Supported
HTTP trigger HttpRequest/ObjectResult HttpRequestData/HttpResponseData
Durable Functions Supported Not supported
Imperative bindings Supported Not supported
function.json artifact Generated Not generated
Configuration host.json host.json and custom initialization
Dependency injection Supported Supported
Middleware Not supported Supported
Cold start times Typical Longer, because of just-in-time start-up. Run on Linux instead of Windows to reduce potential delays.
ReadyToRun Supported TBD

Next steps