README.md

HTTP Trigger (.NET 6)

Watch the recording of this lesson on YouTube 🎥.

Goal 🎯

The goal of this lesson is to create your first function (in .NET 6 isolated process) which can be triggered by doing an HTTP GET or POST to the function endpoint.

This lessons consists of the following exercises:

Nr	Exercise
0	Prerequisites
1	Creating a Function App
2	Changing the template for GET requests
3	Adding a BadRequest response
4	Handling POST requests with string data
5	Handling POST requests with JSON data
6	Changing the route for a custom greeting
7	Homework
8	More info
9	Feedback

📝 Tip - If you're stuck at any point you can have a look at the source code in this repository.

📝 Tip - If you have questions or suggestions about this lesson, feel free to create a Lesson Q&A discussion here on GitHub.

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0. Prerequisites

Prerequisite	Exercise
Azure Functions Core Tools	1-6
VS Code with Azure Functions extension	1-6
REST Client for VS Code or Postman	1-6

See .NET 6 prerequisites for more details.

1. Creating a Function App

In this exercise, you'll be creating a Function App with the default HTTPTrigger and review the generated files & code.

Steps

1. In VSCode, create the Function App by running AzureFunctions: Create New Project in the Command Palette (CTRL+SHIFT+P).

2. Browse to the location where you want to save the function app (e.g. AzFuncUni.Http).

   📝 Tip - Create a folder with a descriptive name since that will be used as the name for the project.

3. Select the language you'll be using to code the function, in this lesson that is using C#.

4. Select the .NET 6 (isolated) as the runtime.

   If you don't see .NET 6, choose:

   • Change Azure Functions version
   • Select Azure Functions v4
   • Select .NET 6 (isolated)

   📝 Tip - More information about the isolated process can be found in the official Azure documentation.

5. Select HTTPTrigger as the template.

6. Give the function a name (e.g. HelloWorldHttpTrigger).

7. Enter a namespace for the function (e.g. AzFuncUni.Http).

   📝 Tip - Namespaces are used to organize pieces of code into a hierarchy. Make sure you don't use the exact same name as the function/class. Namespaces and classes should be named uniquely to prevent compiler and readability issues.

8. Select Function for the AccessRights.

   🔎 Observation - Now a new Azure Functions project is being generated. Once it's done, look at the files in the project. You will see the following:

   File	Description
   AzFuncUni.Http.csproj	The C# project file which specifies the .NET version, Azure Functions version and package references.
   Program.cs	The C# class containing the startup code for the host instance.
   HelloWorldHttpTrigger.cs	The C# class containing the HTTPTrigger function method.
   host.json	Contains global configuration options for all the functions in a function app.
   local.settings.json	Contains app settings and connectionstrings for local development.

   ❔ Question - Review the generated HTTPTrigger function. What is it doing?

9. Build the project (CTRL+SHIFT+B).

10. Run the Function App by pressing F5.

    🔎 Observation - Eventually you should see a local HTTP endpoint in the output.

11. Now call the function by making a GET request to the above endpoint using a REST client:

    GET http://localhost:7071/api/HelloWorldHttpTrigger

    ❔ Question - What is the result of the function? Is it what you expected?

12. Now call the function by making a POST request to the above endpoint using a REST client:

    POST http://localhost:7071/api/HelloWorldHttpTrigger

    ❔ Question - Do you know why the function works with both GET and POST HTTP methods? What would the response be if you try a PUT or DELETE HTTP method?

2. Changing the template for GET requests

Let's change the template to find out what parameters can be changed. Depending on the trigger, arguments can be added/removed and parameter types can be changed.

Start with only allowing GET requests and reading a value from the query string called name.

Steps

1. Remove the "post" string from the HttpTrigger attribute. Now the function can only be triggered by a GET request.

2. The req parameter is of type HttpRequestData. Although this type has various properties and methods it does not contain anything to read query string values from the url. To read query string values, we can use the HttpUtility.ParseQueryString() method. The HttpUtility class lives in the System.Web namespace, so add this to the using directives at the top of the file:

   using System.Web;

3. To read the name query string value, use the ParseQueryString() method directly underneath the logger.LogInformation(...) line:

   var queryStringCollection = HttpUtility.ParseQueryString(req.Url.Query);
   var name = queryStringCollection["name"];

4. Now that a name value is extracted from the query string it can be used in the response.

   Change

   response.WriteString("Welcome to Azure Functions!");

   To

   response.WriteStringAsync($"Hello, {name}");

   The function should look like this now:

   [Function("HelloWorldHttpTrigger")]
   public HttpResponseData Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequestData req)
   {
       _logger.LogInformation("C# HTTP trigger function processed a request.");
       var queryStringCollection = HttpUtility.ParseQueryString(req.Url.Query);
       var name = queryStringCollection["name"];
       var response = req.CreateResponse(HttpStatusCode.OK);
       response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
       response.WriteStringAsync($"Hello, {name}");
       return response;
   }

5. Run the function, once with a name value in the query string, and once without a name value.

   GET http://localhost:7071/api/HelloWorldHttpTrigger?name=AzureFunctionsUniversity

   ❔ Question - Is the outcome of both runs as expected?

3. Adding a BadRequest response

In the previous exercise, when the name query string value is missing, the function still returned an OK response (HTTP status code 200). When something is missing from a request it is better to return a BadRequest response (HTTP status code 400). This informs the client that the request was not valid and a corrective action needs to be taken.

Let's change the function to return a BadRequest response when the name query string is empty.

Steps

1. Before changing the response, let's update the name of the function in the [Function] attribute. Currently it is a string literal, but some people don't like to use string literals and prefer strings based on .NET types. Instead of the string literal "HelloWorldHttpTrigger", the .NET class name can be used like this: nameof(HelloWorldHttpTrigger). Since the name in the [Function()] attribute is the same as the class name you can use this approach.

   Change

   [Function("HelloWorldHttpTrigger")]

   To

   [Function(nameof(HelloWorldHttpTrigger))]

   📝 Tip .NET uses strong typing, this means that types and their values are checked at compile time (while you write your code). Languages that use weakly typing do type checking at runtime (when your program is running). Strong typing prevents you from making certain mistakes during programming, such as making typos, or using completely incorrect types or operations on types.

2. Now let's apply the proper HTTP status code in the response. Inside the function, after extracting the name value from the query string, add an if statement that checks the name field. If name is null, then return a HttpStatusCode.BadRequest. If the name is not null return a HttpStatusCode.OK.

   The function should look like this now:

   [Function(nameof(HelloWorldHttpTrigger))]
   public HttpResponseData Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequestData req)
   {
       _logger.LogInformation("C# HTTP trigger function processed a request.");
       var queryStringCollection = HttpUtility.ParseQueryString(req.Url.Query);
       var name = queryStringCollection["name"];
   
       var response = req.CreateResponse();
       response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
       if (string.IsNullOrEmpty(name))
       {
           response.StatusCode = HttpStatusCode.BadRequest;
           response.WriteStringAsync($"Please provide a value for the name query string parameter.");
       }
       else
       {
           response.StatusCode = HttpStatusCode.OK;
           response.WriteStringAsync($"Hello, {name}");
       }
   
       return response;
   }

3. Run the function, once with a value in the name query string parameter, once without a value, and once without the name query parameter.

   GET http://localhost:7071/api/HelloWorldHttpTrigger?name=AzureFunctionsUniversity

   ❔ Question - Are all situations handled properly now?

4. Handling POST requests with string data

Let's change the function to also allow POST requests and test it by posting a request with a name in the body (as plain text).

Steps

1. Update the HttpTrigger attribute of the function to include the POST HTTP verb;

   [HttpTrigger(AuthorizationLevel.Function, "get", "post")]

2. Now let's make the function a bit smarter and have it check the query string parameter only for GET requests, and check the request body only for POST requests. This can be done by adding and if statement that checks the Method property of the request:

   Replace

       var queryStringCollection = HttpUtility.ParseQueryString(req.Url.Query);
       var name = queryStringCollection["name"];

   With

   string name = default;
   if (req.Method.Equals("get", StringComparison.OrdinalIgnoreCase))
   {
       var queryStringCollection = HttpUtility.ParseQueryString(req.Url.Query);
       name = queryStringCollection["name"];
   }
   else
   {
       name = await req.ReadAsStringAsync();
   }

3. Notice that the ReadAsStringAsync() method is an asynchronous method. The function needs to wait until that method is completed so the result is stored in the name field. That's why the await keyword is there before the method call. The await keyword can only be used in asynchronous methods and the function isn't. Let's make it asynchronous by adding the async keyword to the function:

   Change

   public HttpResponseData Run(...)

   To

   public async Task<HttpResponseData> Run(...)

   🔎 Observation - async methods always return a Task or Task. In this case our response type is HttpResponseData, so the output type is Task<HttpResponseData>. The Task type is available in the System.Threading.Tasks namespace so add that to the using directives.

4. There was already an async method used in the function: response.WriteStringAsync(). Let's add an await to those two method calls as well:

   Change

   response.WriteStringAsync(...)

   To

   await response.WriteStringAsync(...)

5. Now run the function and do a POST request by submitting a string value in the body. If you're using the VSCode REST client you can use this in a .http file and execute it:

   POST http://localhost:7071/api/HelloWorldHttpTrigger
   Content-Type: text/plain
   
   AzureFunctionsUniversity

   ❔ Question - Is the outcome of the POST as expected?

   ❔ Question - What is the response when you use an empty body?

5. Handling POST requests with JSON data

Let's add a new function that only handles POST requests with a specific JSON structure in the request body.

Steps

1. Create a copy of the HelloWorldHttpTrigger.cs file and rename the file, the class name and the [Function() attribute value to HelloWorldHttpTrigger5.

   📝 Tip - Function names need to be unique within a Function App.

2. Add a new file named Person.cs and add the following code:

   namespace AzFuncUni.Http
   {
       record Person
       {
           public string Name { get; init; }
       }
   }

   This record will be used in the function to read a JSON structure from the request body.

   📝 Tip - This is a .NET record type, used for data classes which are supposed to be immutable. For more information see the official Azure docs.

3. Remove the "get" verb from the HttpTrigger attribute since this function will only be triggered by POST requests. Make sure the "post" verb is still included in the attribute:

4. Since this function will only handle POST requests with JSON body the if statement can be replaced with retrieving a Person object from the request body:

   Replace

   string name = default;
   if (req.Method.Equals("get", StringComparison.OrdinalIgnoreCase))
   {
       var queryStringCollection = HttpUtility.ParseQueryString(req.UrlQuery);
       name = queryStringCollection["name"];
   }
   else
   {
       name = await req.ReadAsStringAsync();
   }

   With

   var person = await req.ReadFromJsonAsync<Person>();

5. A Person object is now available in the person variable. Let's use the person.Name property in the two places where the name variable was used.

   The final function should look like this:

   [Function(nameof(HelloWorldHttpTrigger5))]
   public async Task<HttpResponseData> Run([HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequestData req)
   {
       _logger.LogInformation("C# HTTP trigger function processed a request.");
       
       var person = await req.ReadFromJsonAsync<Person>();
       var response = req.CreateResponse();
       response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
       if (string.IsNullOrEmpty(person.Name))
       {
           response.StatusCode = HttpStatusCode.BadRequest;
           await response.WriteStringAsync($"Please provide a value for the name in JSON format in the body.");
       }
       else
       {
           response.StatusCode = HttpStatusCode.OK;
           await response.WriteStringAsync($"Hello, {person.Name}");
       }
   
       return response;
   }

6. Run the Function App.

   🔎 Observation - You should see the new HTTP endpoint in the output of the console.

7. Trigger the new endpoint by making a POST request and submitting a JSON body with a name parameter. If you're using the VSCode REST client you can use this in a .http file and execute it:

   POST http://localhost:7071/api/HelloWorldHttpTrigger5
   Content-Type: application/json
   
   {
       "name": "AzureFunctionsUniversity"
   }

   ❔ Question - Is the outcome as expected?

6. Changing the route for a custom greeting

Instead returning "Hello {name}" all the time, it would be nice if we can supply our own greeting. So we could return "Hi {name}" or "Ola {name}". We can do this by changing the route of the function so it contains the greeting.

Steps

1. Create a copy of the HelloWorldHttpTrigger.cs class that can handle a GET request and rename the file, the class and the [Function()] attribute to HelloWorldHttpTrigger6.cs.

2. Now add the Route parameter in the HttpTrigger binding as follows:

   [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = "HelloWorldHttpTrigger6/{greeting:alpha?}")

   🔎 Observation - The Route uses a route argument named greeting and it has an alpha constraint. This means that greeting may only contain characters from the alphabet (a-zA-Z). The question mark indicates the greeting parameter is optional. More info on route constraints in the official Azure docs.

   📝 Tip - If you want a less restrictive route that allows multiple words you can use {greeting:regex(^[\\w\\s]*$)?} instead.

3. Although a greeting parameter is now part of the route the function is still unaware of such parameter. It needs to be added to the function Run() method as a separate parameter:

   string greeting,

4. Locate the response.WriteStringAsync() for the OK result and add the greeting parameter:

   Change

   await response.WriteStringAsync($"Hello", {name}");

   To

   await response.WriteStringAsync($"{greeting ?? "Hello"}, {name}");

   🔎 Observation - Here a null-coalescing operator is used to set the default value of greeting to "Hello" if it is null.

   The entire function should look like this:

   [Function(nameof(HelloWorldHttpTrigger6))]
   public async Task<HttpResponseData> Run([HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = "HelloWorldHttpTrigger6/{greeting:alpha?}")] 
       HttpRequestData req,
       string greeting)
   {
       _logger.LogInformation("C# HTTP trigger function processed a request.");
   
       string name = default;
       if (req.Method.Equals("get", StringComparison.OrdinalIgnoreCase))
       {
           var queryStringCollection = HttpUtility.ParseQueryString(req.Url.Query);
           name = queryStringCollection["name"];
       }
       else
       {
           name = await req.ReadAsStringAsync();
       }
   
       var response = req.CreateResponse();
       response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
       if (string.IsNullOrEmpty(name))
       {
           response.StatusCode = HttpStatusCode.BadRequest;
           await response.WriteStringAsync($"Please provide a value for the name query string parameter or in the body as plain text.");
       }
       else
       {
           response.StatusCode = HttpStatusCode.OK;
           await response.WriteStringAsync($"{greeting ?? "Hello"}, {name}");
       }
   
       return response;
   }

5. Run the Function App.

   🔎 Observation - You should see the new HTTP endpoint (ending with HelloWorldHttpTrigger6/{greeting:alpha?}) in the output of the console.

6. Trigger the new endpoint by making a GET request to the following endpoint.

   GET http://localhost:7071/api/HelloWorldHttpTrigger6/Hi?name=AzureFunctionsUniversity

   ❔ Question - Is the outcome as expected?

   ❔ Question - What happens when you remove the greeting parameter?

7. Homework

Ready to get hands-on? Checkout the homework assignment for this lesson.

8. More info

• For more info about the HTTP Trigger have a look at the official Azure Functions HTTP Trigger documentation.

• Information about running .NET 6 in isolated mode.

9. Feedback

We love to hear from you! Was this lesson useful to you? Is anything missing? Let us know in a Feedback discussion post here on GitHub.

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