Quickstarters
How to Build a Backend for Xamarin?

30min
Introduction
In this tutorial, you will learn how to build a backend for your Xamarin mobile apps using Back4App’s powerful features. 
We will rely on the Back4App REST API, GraphQL, and other web API options instead of using a dedicated SDK. This approach keeps your mobile client lightweight and flexible, enabling you to connect to your data through simple HTTP calls.
We will cover creating database models, applying security, handling authentication, and performing file operations. 
By following these steps, you’ll see how to build a backend for xamarin quickly and securely. 
You’ll also explore scheduling automated tasks and integrating webhooks to extend your Xamarin app’s functionality, so you can focus on coding your UI instead of juggling server configurations.
Once you’ve completed this guide, you will have a reusable template for building mobile apps that rely on Back4App for their backend. 
You will also understand how to apply Access Control Lists (ACLs), write Cloud Code (if needed), and incorporate advanced workflows like Live Queries or Cloud Jobs in your final solution.
Prerequisites
To get the most out of this tutorial, make sure you have:
A Back4App account
Sign up for free here.﻿
A new Back4App project
Getting Started with Back4app﻿
Xamarin development environment (Visual Studio or Visual Studio for Mac)
Xamarin Installation Docs﻿
Basic knowledge of C# and Xamarin (including how to make web API requests from a public class in C#).
With these prerequisites ready, you’ll be set to follow along and connect your Xamarin project to Back4App.
Step 1 – Creating a New Project on Back4App and Connecting
Create a Back4App project in your Back4App dashboard. This is the foundation for your backend.
Name your project (for example, “Xamarin-Backend-Tutorial”).
Locate your App Keys by going to the app’s “Security & Keys” section. You’ll see the REST, GraphQL, or other keys you might use when making requests from your mobile client.
Configure your Xamarin project to make HTTP requests. Instead of a Parse SDK, you’ll use either HttpClient, or any network library you prefer, to call your Back4App app’s REST or GraphQL endpoints.
For example, you can store your Application ID and REST API Key in a secure place or in a constants file:
C#
public class Back4AppConstants
{
    public const string AppId = "YOUR_APPLICATION_ID";
    public const string RestApiKey = "YOUR_REST_API_KEY";
    public const string ServerUrl = "https://parseapi.back4app.com";
}
public class Back4AppConstants
{
    public const string AppId = "YOUR_APPLICATION_ID";
    public const string RestApiKey = "YOUR_REST_API_KEY";
    public const string ServerUrl = "https://parseapi.back4app.com";
}
﻿
When calling the web api, always include these credentials in your request headers. This ensures your requests are routed to the correct app with the required authorization.
Step 2 – Setting Up the Database
1. Creating a Data Model
Use the Back4App dashboard to define your classes or let them be created dynamically on first request. For instance, if you want a Todo class, you can create it in the Database section or on-the-fly via REST API calls:
Bash
curl -X POST \
  -H "X-Parse-Application-Id: YOUR_APPLICATION_ID" \
  -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"title": "Buy groceries", "isCompleted": false}' \
  https://parseapi.back4app.com/classes/Todo
curl -X POST \
  -H "X-Parse-Application-Id: YOUR_APPLICATION_ID" \
  -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"title": "Buy groceries", "isCompleted": false}' \
  https://parseapi.back4app.com/classes/Todo
﻿
2. Creating a Data Model Using the AI Agent
Back4App provides an AI Agent that can generate complete class structures:
Open the AI Agent in your Back4App project.
Describe your model (e.g., “Please create a new class named ‘Todo’ with title and isCompleted fields.”).
Confirm to generate the schema automatically.
﻿
3. Reading and Writing Data (REST)
Within your Xamarin project, you can write a public class (for example, RestClient) that handles all requests to Back4App:
C#
using System.Net.Http;
using System.Text;
using Newtonsoft.Json;

public class RestClient
{
    private readonly HttpClient _client;

    public RestClient()
    {
        _client = new HttpClient();
        _client.DefaultRequestHeaders.Add("X-Parse-Application-Id", Back4AppConstants.AppId);
        _client.DefaultRequestHeaders.Add("X-Parse-REST-API-Key", Back4AppConstants.RestApiKey);
    }

    public async Task CreateTodoAsync(string title, bool isCompleted)
    {
        var newTodo = new { title = title, isCompleted = isCompleted };
        var content = new StringContent(JsonConvert.SerializeObject(newTodo), Encoding.UTF8, "application/json");
        var response = await _client.PostAsync($"{Back4AppConstants.ServerUrl}/classes/Todo", content);
        // Handle the response
    }

    public async Task<List<Todo>> GetTodosAsync()
    {
        var response = await _client.GetAsync($"{Back4AppConstants.ServerUrl}/classes/Todo");
        var jsonString = await response.Content.ReadAsStringAsync();
        // Parse JSON into a C# model (Todo)
        // Return the result
        // ...
        return new List<Todo>(); // Placeholder
    }
}

public class Todo
{
    public string objectId { get; set; }
    public string title { get; set; }
    public bool isCompleted { get; set; }
}
using System.Net.Http;
using System.Text;
using Newtonsoft.Json;

public class RestClient
{
    private readonly HttpClient _client;

    public RestClient()
    {
        _client = new HttpClient();
        _client.DefaultRequestHeaders.Add("X-Parse-Application-Id", Back4AppConstants.AppId);
        _client.DefaultRequestHeaders.Add("X-Parse-REST-API-Key", Back4AppConstants.RestApiKey);
    }

    public async Task CreateTodoAsync(string title, bool isCompleted)
    {
        var newTodo = new { title = title, isCompleted = isCompleted };
        var content = new StringContent(JsonConvert.SerializeObject(newTodo), Encoding.UTF8, "application/json");
        var response = await _client.PostAsync($"{Back4AppConstants.ServerUrl}/classes/Todo", content);
        // Handle the response
    }

    public async Task<List<Todo>> GetTodosAsync()
    {
        var response = await _client.GetAsync($"{Back4AppConstants.ServerUrl}/classes/Todo");
        var jsonString = await response.Content.ReadAsStringAsync();
        // Parse JSON into a C# model (Todo)
        // Return the result
        // ...
        return new List<Todo>(); // Placeholder
    }
}

public class Todo
{
    public string objectId { get; set; }
    public string title { get; set; }
    public bool isCompleted { get; set; }
}
﻿
4. Reading and Writing Data (GraphQL)
For GraphQL queries, you can send requests to the Back4App GraphQL endpoint:
Bash
curl -X POST \
  -H "Content-Type: application/json" \
  -H "X-Parse-Application-Id: YOUR_APPLICATION_ID" \
  -H "X-Parse-Master-Key: YOUR_MASTER_KEY" \
  --data '{"query":"mutation { createTodo(input:{fields:{title:\"Wash the car\" isCompleted:false}}){ todo{ objectId title }}}"}' \
  https://parseapi.back4app.com/graphql
curl -X POST \
  -H "Content-Type: application/json" \
  -H "X-Parse-Application-Id: YOUR_APPLICATION_ID" \
  -H "X-Parse-Master-Key: YOUR_MASTER_KEY" \
  --data '{"query":"mutation { createTodo(input:{fields:{title:\"Wash the car\" isCompleted:false}}){ todo{ objectId title }}}"}' \
  https://parseapi.back4app.com/graphql
﻿
Similarly, from Xamarin, you could POST a JSON body with your GraphQL string to https://parseapi.back4app.com/graphql.
5. Working with Live Queries (Optional)
Although you are using web API calls rather than the Parse SDK, you can still enable Live Queries if you want real-time data updates. You would use specialized connections (WebSockets) to subscribe to class changes. Enable Live Queries from your app’s Server Settings in Back4App, then use a compatible client library in Xamarin, if available. Alternatively, you can build your own WebSocket solution or rely on polls if real-time support is optional for your app.
Step 3 – Applying Security with ACLs and CLPs
1. Overview of ACLs and CLPs
Back4App security includes Class-Level Permissions (CLPs) and Access Control Lists (ACLs). CLPs define which users or roles can read/write to an entire class. ACLs add per-object security. Combine them to make sure only authorized users can manipulate your data.
2. Setting Up Class-Level Permissions
Open the Database section on Back4App.
Select your class (like Todo).
Go to Class Level Permissions to set read/write rules. You might only allow authenticated users to read or write. This ensures that your mobile client must log in before reading data.
Step 4 – Writing Cloud Code Functions
1. Why Cloud Code
Cloud Code allows you to run custom server-side logic. You can create business rules, validations, or triggers that run when data changes. This reduces the risk of tampering since logic runs outside the mobile client.
2. Example Function and Triggers
Below is a simplified sample of a Cloud Function in the main.js file:
JS
Parse.Cloud.define('helloXamarin', async (request) => {
  return 'Hello from Xamarin Cloud Code!';
});

Parse.Cloud.beforeSave('Todo', (request) => {
  const todo = request.object;
  if (!todo.get('title')) {
    throw 'Title is required';
  }
});
Parse.Cloud.define('helloXamarin', async (request) => {
  return 'Hello from Xamarin Cloud Code!';
});

Parse.Cloud.beforeSave('Todo', (request) => {
  const todo = request.object;
  if (!todo.get('title')) {
    throw 'Title is required';
  }
});
﻿
3. Deployment
To deploy, you can use the Back4App CLI or the Cloud Code section in your dashboard. Once deployed, you can call your functions from Xamarin via REST:
Bash
curl -X POST \
  -H "X-Parse-Application-Id: YOUR_APP_ID" \
  -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{}' \
  https://parseapi.back4app.com/functions/helloXamarin
curl -X POST \
  -H "X-Parse-Application-Id: YOUR_APP_ID" \
  -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{}' \
  https://parseapi.back4app.com/functions/helloXamarin
﻿
4. NPM Modules
If you need extra libraries, install them via npm in your Cloud Code project folder. After that, require them in your main.js. This approach is useful for calling third-party APIs from the server side.
Step 5 – Configuring Authentication
1. Enable User Authentication
In Back4App, the _User class manages user credentials. Under App Settings, you can enable user authentication, email verification, and password reset settings.
2. User Sign Up / Log In (REST)
From your Xamarin project, you could write C# methods to handle sign-up and login:
C#
public async Task SignUpUserAsync(string username, string password, string email)
{
    var user = new 
    { 
        username = username, 
        password = password,
        email = email 
    };

    var content = new StringContent(JsonConvert.SerializeObject(user), Encoding.UTF8, "application/json");
    var response = await _client.PostAsync($"{Back4AppConstants.ServerUrl}/users", content);
    // Parse response
}

public async Task LogInUserAsync(string username, string password)
{
    var loginEndpoint = $"{Back4AppConstants.ServerUrl}/login?username={username}&password={password}";
    var response = await _client.GetAsync(loginEndpoint);
    // Parse response
}
public async Task SignUpUserAsync(string username, string password, string email)
{
    var user = new 
    { 
        username = username, 
        password = password,
        email = email 
    };

    var content = new StringContent(JsonConvert.SerializeObject(user), Encoding.UTF8, "application/json");
    var response = await _client.PostAsync($"{Back4AppConstants.ServerUrl}/users", content);
    // Parse response
}

public async Task LogInUserAsync(string username, string password)
{
    var loginEndpoint = $"{Back4AppConstants.ServerUrl}/login?username={username}&password={password}";
    var response = await _client.GetAsync(loginEndpoint);
    // Parse response
}
﻿
You can then store session tokens client-side for future requests.
3. Social Login
To integrate social logins (e.g., Google, Facebook), consult the Back4App Social Login docs. Each provider has its own OAuth flow, which you can handle from your mobile client. Then, pass the returned tokens to Back4App.
Step 6 – Handling File Storage
1. Setting up File Storage
Files can be uploaded by sending a POST request with the file data in the body. For instance, to store an image from your Xamarin app, read the image into a byte array, then send:
Bash
curl -X POST \
  -H "X-Parse-Application-Id: YOUR_APP_ID" \
  -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \
  -H "Content-Type: image/png" \
  --data-binary "@local-image.png" \
  https://parseapi.back4app.com/files/myImage.png
curl -X POST \
  -H "X-Parse-Application-Id: YOUR_APP_ID" \
  -H "X-Parse-REST-API-Key: YOUR_REST_API_KEY" \
  -H "Content-Type: image/png" \
  --data-binary "@local-image.png" \
  https://parseapi.back4app.com/files/myImage.png
﻿
2. Example Upload with C#
C#
public async Task<string> UploadFileAsync(byte[] fileData, string fileName)
{
    var fileContent = new ByteArrayContent(fileData);
    fileContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("image/png");

    var response = await _client.PostAsync($"{Back4AppConstants.ServerUrl}/files/{fileName}", fileContent);
    var jsonResponse = await response.Content.ReadAsStringAsync();
    // Parse the response to get file URL
    return "File URL from response";
}
public async Task<string> UploadFileAsync(byte[] fileData, string fileName)
{
    var fileContent = new ByteArrayContent(fileData);
    fileContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("image/png");

    var response = await _client.PostAsync($"{Back4AppConstants.ServerUrl}/files/{fileName}", fileContent);
    var jsonResponse = await response.Content.ReadAsStringAsync();
    // Parse the response to get file URL
    return "File URL from response";
}
﻿
3. Security Considerations
To secure file uploads, go to your Back4App Server Settings and adjust the File Permissions. For instance, you can only allow authenticated users to upload.
Step 7 – Email Verification and Password Reset
1. Overview
Email verification ensures that users own the email they registered with. Password reset links allow them to regain access if they forget their passwords.
2. Back4App Dashboard Configuration
Enable email verification under Email Settings.
Edit the password reset template and set the “From” address.
You can also customize the email content to match your branding.
3. Implementation
Once enabled, if a user signs up with an email, they will get a verification link. Password reset calls are made to the same base API with the user’s email to trigger a reset email.
Step 8 – Scheduling Tasks with Cloud Jobs
1. What Cloud Jobs Do
Cloud Jobs let you schedule routine tasks, such as cleaning old data or sending daily digest emails, all from the Back4App platform.
2. Example
JS
Parse.Cloud.job('cleanupOldTodos', async (request) => {
  const Todo = Parse.Object.extend('Todo');
  const query = new Parse.Query(Todo);
  const now = new Date();
  const THIRTY_DAYS_AGO = new Date(now - 30 * 24 * 60 * 60 * 1000);
  query.lessThan('createdAt', THIRTY_DAYS_AGO);
  const oldTodos = await query.find({ useMasterKey: true });
  await Parse.Object.destroyAll(oldTodos, { useMasterKey: true });
  return `Deleted ${oldTodos.length} old items.`;
});
Parse.Cloud.job('cleanupOldTodos', async (request) => {
  const Todo = Parse.Object.extend('Todo');
  const query = new Parse.Query(Todo);
  const now = new Date();
  const THIRTY_DAYS_AGO = new Date(now - 30 * 24 * 60 * 60 * 1000);
  query.lessThan('createdAt', THIRTY_DAYS_AGO);
  const oldTodos = await query.find({ useMasterKey: true });
  await Parse.Object.destroyAll(oldTodos, { useMasterKey: true });
  return `Deleted ${oldTodos.length} old items.`;
});
﻿
Deploy the Cloud Code, then schedule the job under App Settings > Server Settings > Background Jobs.
Step 9 – Integrating Webhooks
Define a webhook on Back4App by navigating to More > WebHooks and clicking “Add Webhook.”
Provide the endpoint, such as https://your-service.com/webhook-endpoint.
Choose triggers, like “New object in Todo class.”
﻿
Webhooks can notify third-party services, so your Xamarin app can remain lightweight while external systems handle extra logic or notifications.
Step 10 – Exploring the Back4App Admin Panel
The Back4App Admin App is a user-friendly way to handle CRUD operations without writing queries. Enable it in App Dashboard > More > Admin App. Create an admin user, choose a subdomain, and log in.
﻿
This Admin App is ideal for non-technical team members who need to manage data but shouldn’t have direct database access.
Conclusion
You have just seen how to build a backend for xamarin using Back4App APIs. You created secure classes, added CLPs and ACLs, handled authentication, and explored file uploads, scheduling tasks, and integrating with external services. This approach ensures your mobile apps stay fast and flexible while leveraging the power of a hosted database and cloud environment.
Next Steps
Expand your Xamarin application to handle more complex logic, caching, or advanced roles and permissions.
Dive into advanced features, such as push notifications, or integrate real-time data with Live Queries if you need collaborative updates.
Visit the official Back4App documentation for deeper guides on security, performance, and debugging.
Create real-world solutions by mixing third-party APIs (payment gateways, analytics, social media) with your Back4App backend. This combination can enhance your mobile client’s functionality and keep your code organized.
﻿
Updated 15 Jan 2025
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