Returning HTTP Status Codes from Azure Functions

26 March 2019 (0)

(This post refers to Azure Functions v2)

When creating HTTP-triggered Azure Functions there are a number of ways to indicate results back to the calling client.

Returning HTTP Status Codes Manually

To return a specific status code to the client you can create an instance of one of the …Result classes and return that from the function body.

The following example returns an instance of an OkResult or a BadRequestResult:

[FunctionName("AddActor1")]
public static async Task<IActionResult> AddActor1(
    [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req,
    ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");

    string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
    dynamic data = JsonConvert.DeserializeObject(requestBody);

    string name = data.actorName; // get name from dynamic/JSON object

    if (name == null)
    {
        // Return a 400 bad request  result to the client
        return new BadRequestResult();
    }

    // Do some processing
    char firstLetter = name[0];

    // Return a 200 OK to the client
    return new OkResult();                
}

If you wanted to provide additional success/failure information you could use the OkObjectResult and BadRequestObjectResult classes instead, these allow you to provide additional contextual information to the client:

[FunctionName("AddActor2")]
public static async Task<IActionResult> AddActor2(
    [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req,
    ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");

    string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
    dynamic data = JsonConvert.DeserializeObject(requestBody);

    string name = data.actorName; // get name from dynamic/JSON object

    if (name == null)
    {
        // Return a 400 bad request result to the client with additional information
        return new BadRequestObjectResult("Please pass an actorName in the request body");
    }

    // Do some processing
    char firstLetter = name[0];

    // Return a 200 OK to the client with additional information
    return new OkObjectResult($"Actor {name} was added");
}

Automatically Returning Status Codes

In addition to manually returning status code instances, you can let the functions runtime take care of this for you.

For example, the following code will automatically return a “204 no content” if the function executes without throwing an exception, or a “500 internal server error” if an exception was thrown:

[FunctionName("AddActor3")]
public static async Task AddActor3(
    [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req,
    ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");

    string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
    dynamic data = JsonConvert.DeserializeObject(requestBody);

    string name = data.actorName; // get name from dynamic/JSON object

    // Do some processing
    char firstLetter = name[0]; // 500 internal server error if name is null

    // Auto return a 204 no content if no exception was thrown
}

In the preceding code, if the client fails to provide a actorName in the JSON, rather then getting a more helpful “400 bad request” (with optional additional message), they instead get a less useful “500 internal server error” status code and they have no idea what may have gone wrong or how to resolve it.

In this way, automatic status codes can be helpful if you want to write less code or perhaps use the return value of the function in a binding as in the following example:

[FunctionName("AddActor4")]
[return: Queue("new-actor-first-letter")]
public static async Task<string> AddActor4(
    [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req,
    ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");

    string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
    dynamic data = JsonConvert.DeserializeObject(requestBody);

    string name = data.actorName;

    return name.Substring(0,1); // add a new message to the queue containing the first letter of the name
}

Once again, the preceding function will return a 500 if there is an exception (e.g. actorName not provided in JSON) but will return a “200 OK” if no exception occurs (rather than the “204 no content” in the earlier example).

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