Functional Testing for .NET 8 Isolated Azure Functions

Follow along to learn how to create functional tests using UnitTestEx for your .NET 8 (Isolated) HTTP-triggered functions

Written: February 2025

While working on a project recently, I needed an approach for creating some functional tests for a few HTTP-triggered Azure Functions I was working on. Typically I would use WebApplicationFactory for testing a .NET web application, but this doesn't seem to be available for isolated Azure Functions. I saw a post that mentioned UnitTestEx might offer a way forward and decided to give it a try!

Packages In Use

Package	Version
.NET	8
UnitTestEx	5.3.0
UnitTestEx.Azure.Functions	5.3.0
UnitTestEx.Xunit	5.3.0
Shouldly	4.3.0
xunit	2.9.3

The code under test

This is a straightforward HTTP-triggered Azure function that I've put together which doubles a given number. It takes a request in the format defined by DoubleMyNumberRequest. The request body is deserialized into an object of this type. If the body is not present, then the caller will receive a 400 Bad Request result, but if the request is successfully decoded then the caller will receive a 200 OK response with the doubled number in the response body.

DoubleMyNumberFunction.cs

namespace FunctionalTestingExample;

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;

public class DoubleMyNumberFunction(ILogger<DoubleMyNumberFunction> logger)
{
    [Function(nameof(DoubleMyNumberFunction))]
    public async Task<IActionResult> RunAsync([HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "double-my-number")] HttpRequest req)
    {
        logger.LogInformation("Received a request to double a number...");

        var request = await req.ReadFromJsonAsync<DoubleMyNumberRequest>();
        if (request == null)
        {
            return new BadRequestObjectResult("A request object must be supplied");
        }

        return new OkObjectResult(request.Number * 2);
    }
}

public record DoubleMyNumberRequest(double Number);

The test code

I'm an XUnit fan, so that's what I've used in this example. This is a test fixture which uses XUnit's InlineData feature to supply several different test cases to a single test. See this excellent guide here on creating parameterised tests in XUnit.

Use FunctionTester.Create<T> to create an instance of the function tester. Then, configure that tester to point at your function under test, in our case DoubleMyNumberFunction.

Then, create an HTTP request using the function tester, supplying the HTTP method, API route, body and content type. Execute the request, and then assert the results!

This is a simple example and you may wish to extract / generalise / simplify this for use across several different tests.

WhenDoublingANumber.cs

namespace FunctionalTestingExample.Tests;

using System.Text.Json;
using Microsoft.AspNetCore.Mvc;
using Shouldly;
using UnitTestEx;
using UnitTestEx.Azure.Functions;
using UnitTestEx.Xunit;
using Xunit.Abstractions;

public class WhenDoublingANumber(ITestOutputHelper helper) : UnitTestBase(helper)
{
    [Theory]
    [InlineData(0, 0)]
    [InlineData(-2, -4)]
    [InlineData(3.5, 7)]
    public async Task ShouldDoubleTheNumberAsync(double number, int expectedResult)
    {
        using var functionTester = FunctionTester.Create<Program>();

        var triggerTester = functionTester
            .HttpTrigger<DoubleMyNumberFunction>()
            .WithRouteCheck(RouteCheckOption.None);

        var requestBody = new DoubleMyNumberRequest(number);

        var request = functionTester.CreateHttpRequest(HttpMethod.Post, "api/double-my-number", JsonSerializer.Serialize(requestBody), "application/json");
        var response = await triggerTester.RunAsync(f => f.RunAsync(request));

        response.Result.ShouldBeOfType<OkObjectResult>();

        var responseBody = ((OkObjectResult)response.Result).Value as double?;
        responseBody.ShouldNotBeNull();

        responseBody.Value.ShouldBe(expectedResult);
    }
}

Full Example

Check out a full example in this repository.