Building REST APIs with Azure Functions in .NET

In the cloud first era, serverless architecture is no longer a buzzword. it’s the new normal. And if you're working with .NET and want to build lightweight, event-driven, scalable APIs without managing infrastructure, Azure Functions should be in your toolkit.

In this article, we’ll break down how to use Azure Functions as REST API endpoints, the right way.

What Are Azure Functions?

Azure Functions is Microsoft’s serverless compute service that allows you to run small pieces of code (functions) in the cloud without worrying about provisioning servers. It comes under "Function as a Service" (FaaS) azure service model.

It supports triggers like HTTP, Queue, Timer, Cosmos DB, and more. For building APIs, we use the HTTP trigger in this article.

Why Use Azure Functions for APIs?

• Auto-scale on demand (hello, unpredictable traffic).
• Pay only for what you use (per millisecond).
• Quick deployment (ideal for MVPs and microservices).
• Built-in bindings to Azure services (like Blob Storage, Cosmos DB, Queues).

But be warned, this is for lightweight services. If you're building a full-blown enterprise app with complex orchestration, consider Azure App Services or AKS.

Setting Up Azure Functions for REST

Step 1: Install Prerequisites

• .NET SDK (>=6.0)
• Azure Functions Core Tools
• Azure CLI

Step 2: Create a Function Project

`func init MyFunctionApp --worker-runtime dotnet --target-framework net8.0 cd MyFunctionApp
func new --name GetProducts --template "HTTP trigger" --authlevel "anonymous"`

This creates a basic HTTP-triggered function.

Step 3: Write Your REST Endpoint

Update GetProducts.cs:

`public  static  class  GetProducts {
    [FunctionName("GetProducts")] public  static IActionResult Run( [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "products")] HttpRequest req,
        ILogger log)
    { var products = new[]
        { new { Id = 1, Name = "Laptop", Price = 1000 }, new { Id = 2, Name = "Mouse", Price = 25 }
        }; return  new OkObjectResult(products);
    }
}`

You've now got a RESTful GET endpoint at:
https://<your-function-app>.azurewebsites.net/api/products

Step 4: Add More REST Endpoints

Want to handle POST/PUT/DELETE? Create separate functions or reuse one with multiple verbs.

[HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = "products/{id?}")] HttpRequest req

Use req.Method to branch logic inside the function - or better, separate functions per verb for clean separation.

Step 5: Local Testing

Run the azure function locally:

`func start`

Your API will be available at:
http://localhost:7071/api/products

Step 6: Deploy to Azure

Azure CLI command to deploy azure fucntion with required other services by using the below command:

`az login
az functionapp create --resource-group my-rg --consumption-plan-location westus --runtime dotnet --functions-version 4 --name my-api-func --storage-account mystorage
func azure functionapp publish my-api-func`

Done.

Keep in Mind

• Use Azure API Management in front of your functions for throttling, caching, and versioning.
• Keep functions small and single-responsibility.
• Structure your project using Dependency Injection (supported in .NET Functions).
• Use Azure Key Vault or app settings for secrets-not hardcoded values.
• Don't forget OpenAPI (Swagger) support for API documentation.

Bonus: Add Swagger/OpenAPI

Add this NuGet package:

dotnet add package Microsoft.Azure.WebJobs.Extensions.OpenApi

Then decorate your function with [OpenApiOperation], [OpenApiParameter], etc., and hit /api/swagger/ui for the Swagger UI.

When Not to Use Azure Functions for APIs?

• You need low-latency, cold-start-free APIs (consider Premium Plan or App Service).
• You want long-running operations (>5 mins).
• You have complex routing needs or a huge monolith.

Wrapping Up

Azure Functions make it blazingly fast to spin up REST APIs without the devops headache. Whether it’s a microservice, webhook handler, or backend for a mobile app, they give you just enough firepower-without the bloat.

But like any tool, they’re not a silver bullet.

Use wisely. And write clean functions.

Want the GitHub repo for this example or a tutorial on Function DI & Middleware in .NET? Let me know I'll cook that up next.

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