Azure Functions Startup Class Explained for Developers

The Azure Functions startup class is a crucial part of any Azure Functions project. It's responsible for initializing the function app and setting up the environment for your code to run in.

Azure Functions provides a built-in startup class called FunctionHostBuilder, which is used to configure and build the function app. This class is the default startup class for Azure Functions projects.

You can also create a custom startup class by implementing the IHostBuilder interface. This allows you to add custom configuration and initialization logic to your function app.

A custom startup class is useful when you need to perform specific setup tasks or inject custom services into your function app.

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To create an Azure Functions startup class, you need to inherit from FunctionsStartup, which is available through the Microsoft.Azure.Functions.Extensions NuGet package. This class lives in the Microsoft.Azure.Functions.Extensions.DependencyInjection namespace.

The built-in Microsoft.Extensions.DependencyInjection is already available, allowing you to use the .AddSingleton method. You can inject services as Singletons, making them available to all Function instances.

Here's a basic example of what the class can look like, with the Configure method overridden to inject services as Singletons: Microsoft.Azure.Functions.Extensions and Microsoft.Azure.Functions.Extensions.DependencyInjection.

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To create a function in Azure, you first need to create a FunctionStartup class. This class inherits from FunctionsStartup and can be found in the Microsoft.Azure.Functions.Extensions.DependencyInjection namespace.

You can install the required NuGet package, Microsoft.Azure.Functions.Extensions, to use this class.

The FunctionStartup class has a Configure method that you can override. In this method, you can inject services as Singletons, making them available to all function instances.

The Microsoft.Extensions.DependencyInjection package is already available, which enables you to use the .AddSingleton method.

Here's a simple example of what the FunctionStartup class can look like:

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This example shows how to create a FunctionStartup class and inject services as Singletons.

Function Apps can be configured using Azure App Configuration, a fully managed service for storing and retrieving application settings.

App Configuration allows you to manage application settings in a centralized location, making it easier to update and share settings across multiple environments.

Discover more: Azure Function Local Settings Json

You can store settings in a hierarchical structure, using keys and values to organize your configuration.

Settings can be stored as strings, numbers, booleans, or even complex objects.

Function Apps can be configured to use App Configuration to retrieve settings at runtime, making it easy to switch between different environments.

App Configuration supports a range of data types, including strings, numbers, and booleans, making it a versatile tool for managing application settings.

Settings can be retrieved using a simple API, or using the Azure App Configuration SDK in your Function App code.

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Dependency Injection is a crucial aspect of Azure Functions, and it's built on the .NET Core Dependency Injection features. It begins with Azure Functions 2.x.

You can implement Dependency Injection with Azure Functions using the Microsoft.Azure.Functions.Extensions package. This package provides the FunctionsStartup class, which you can inherit from to create your own Startup class.

To configure Dependency Injection, you'll need to create a Startup class that inherits from FunctionsStartup. This class will be used to register services and configure Dependency Injection. The Microsoft.Extensions.DependencyInjection package should already be available, which enables you to use the .AddSingleton method.

For another approach, see: Azure Startup Credits

The FunctionsStartup class lives in the Microsoft.Azure.Functions.Extensions.DependencyInjection namespace. You can inject services as Singletons, which means they're available to all Function instances.

Here's a brief overview of the core steps to implement Dependency Injection with Azure Functions:

By following these steps, you can decouple your code and make it more modular, maintainable, and scalable. For example, you can remove message compose logic from an HTTP function to a dedicated service, like the MessageService.

The startup process in Azure Functions has become more straightforward with the removal of the need for a separate Startup class. We can now directly access the host instance through the Program.cs file.

The host instance is created using a new HostBuilder object that returns a IHost instance, which runs asynchronously to start the Function. This setup also allows for integration with Azure Functions logging and default gRPC support.

We can add the configuration we need for our Function App using the ConfigureAppConfiguration method. This is useful for local debugging, where we can use the local.settings.json file.

The ConfigureServices method is where we can inject the services we need in our application. This is where we can add a Singleton instance of our Cosmos Client, for example.

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