Azure Functions Middleware Examples — .NET 6 and .NET 8 Isolated Worker (C#)

Runnable C# examples of Azure Functions middleware with IFunctionsWorkerMiddleware. They show how to intercept an HTTP-triggered function in the .NET isolated worker model, transform the JSON request body before the function runs, share data through FunctionContext.Items, and turn unhandled exceptions into a structured ProblemDetails error response.

There are two versions: a .NET 6 comparison of the same function written with and without middleware, and a .NET 8 project that combines request-transforming middleware with exception-handling middleware.

Structure

Folder Target Hosting model Description
Dotnet6/WithoutMiddleware .NET 6, Functions v4 In-process (Microsoft.NET.Sdk.Functions) Baseline: the function reads the name from the request itself, with no middleware
Dotnet6/WithMiddleware .NET 6, Functions v4 Isolated worker UppercaseNameMiddleware parses and uppercases the name before the function runs
Dotnet8/Middleware .NET 8, Functions v4 Isolated worker (ASP.NET Core integration) UppercaseNameMiddleware plus ExceptionHandlerMiddleware and a custom error-response utility

Middleware is only available in the isolated worker model, which is why the .NET 6 baseline uses the in-process model for comparison.

What Is Azure Functions Middleware?

In the isolated worker model, middleware is a class that implements IFunctionsWorkerMiddleware and is registered in Program.cs. It runs for every function invocation and can execute code before and after the function handler. Middleware can read and modify the FunctionContext, read the HTTP request body, add values to context.Items, and catch exceptions thrown further down the pipeline.

The .NET 8 project registers its middleware like this (Program.cs):

var host = new HostBuilder()
    .ConfigureAppConfiguration(builder =>
        builder.AddEnvironmentVariables())
    .ConfigureFunctionsWebApplication(worker =>
    {
        // Middleware Sequence
        worker.UseMiddleware<UppercaseNameMiddleware>();
        worker.UseMiddleware<ExceptionHandlerMiddleware>();
    })
    // ...
    .Build();

How the .NET 8 Example Works

Middleware runs in the order it is registered:

  1. UppercaseNameMiddleware copies the request body to a MemoryStream, parses it as JSON, uppercases the name property (or sets it to "name not found" when it is missing), stores the result in context.Items["updated_body"], and calls the next step.
  2. ExceptionHandlerMiddleware wraps the function call in try/catch. If the function throws, it logs the error and writes an HTTP 400 response containing a ProblemDetails object.
  3. SendMessageFunction (HTTP POST) reads updated_body from FunctionContext.Items. It returns Hello {NAME}. as plain text, or throws InvalidProcessException when no name was supplied — which the exception middleware converts into the 400 response.

Because ExceptionHandlerMiddleware is registered after UppercaseNameMiddleware, it handles exceptions from the function, not exceptions raised inside UppercaseNameMiddleware itself (for example, a request body that is not valid JSON).

Requirements

Quick Start

From the repository root:

# .NET 6 demo (isolated worker with middleware)
cd Middleware/Dotnet6/WithMiddleware
func start --port 7052

# .NET 8 demo
cd Middleware/Dotnet8/Middleware
func start --port 7256

The ports match each project’s launchSettings.json, so running from Visual Studio uses the same addresses. Send a JSON body with a name property in a POST request to the function URL that func start prints.

See each sub-folder’s readme (.NET 6, .NET 8) for detailed usage and sample requests.

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