How to Mock Azure Event Hub in .Net Unit tests

There is no one-size-fits-all solution for EventHub integration in Azure Functions.

In this article, we will delve into different approaches for using EventHub in Azure Functions and demonstrate how to effectively test them using NUnit, a popular unit testing framework for .NET.

The sample Azure function for each variation will be a simple cloud function with a TimerTrigger. It runs every 5 minutes (specified by the Cron expression “0 */5 * * * *”). The function sends two events (“Event1” and “Event2”) to an Azure Event Hub using an EventHubProducerClient. If any exceptions occur during the event-sending process, they are caught and added to a list.

First approach — using EventHubProducerClient binding.

This function uses an EventHubProducerClient as the output parameter to send events to the Azure Event Hub.

Second approach — using IAsyncCollector<EventData> parameter.

This function uses IAsyncCollector<EventData> as the output parameter to collect and send events to the Azure Event Hub.

Third approach — using custom EventHubService implementation.

The third implementation follows the dependency injection pattern. It takes an IEventHubService<string> as a constructor parameter, allowing the injection of an external service to handle Event Hub-related operations.

And EventHubServiceimplementation is below:

This class encapsulates the event-sending functionality for Azure Event Hubs, making it reusable and easy to manage for different types of messages. In general, it is beneficial to apply for complex use cases with custom logic.

How to configure the event hub for Azure functions and how to use different bindings can be found here.

Unit tests

1. EventHubProducerClient provides a mock-friendly design as all its public members are virtual or settable. The class is not sealed and offers a parameterless constructor, making it compatible with popular mocking libraries like Moq or FakeItEasy.

   
   

Thus, we can use a mocking library to set the behavior of SendAsync function. Then we can compare the EventData using the extensions method.

Extension method mentioned above: It will return output in deserialized format from the event body.

2. To mock IAsyncCollector<EventData> we will be using a custom implementation AsyncCollector<EventData> that captures the events sent by the function.

Then, we will be able to verify that the events captured in the AsyncCollector match the expected events, effectively allowing the test to check the correct behavior of the Azure Function without actually sending events to the Event Hub during testing.

Async data collector mocked the implementation

3. This test implementation uses a stub of IEventHubService<T> to mock the Event Hub service’s behavior and capture the messages sent during the test. By using this approach, the test verifies the correct behavior of AzureEventHubFunction3 without interacting with the actual Event Hub, ensuring the test’s isolation and reliability.

EventHubServiceStubimplementation

That's all. The source code for the described approaches can be found in the GitHub repo: https://github.com/FairyFox5700/AzureEventHubProcessorUnitTests.

Cheers 😉 🍷.