Build a Postman Clone: Class Library Design for API Calls

Introduction and Setup

Tim begins by explaining the goal of this lesson: creating the business logic and data access layer to make API calls work in the application. He emphasizes that this is an MVP—a functional version that can be expanded later.

Before diving into code, Tim notes that the course is designed to be portfolio-friendly, although he cautions against directly copying the project. Instead, he encourages developers to use it as inspiration to create unique projects that demonstrate skills in C#, API interaction, and UI design.

Creating the API Access Class

Tim walks us through opening the class library and starting with a clean slate. He deletes the default Class1 and creates a new class named APIAccess. This will handle all API interactions.

He explains his approach to method design: starting with public void methods, adding parameters like string url, and then gradually refining them into async tasks that can handle real-world API requests.

public class APIAccess
{
    private readonly HttpClient client = new();

    public async Task<string> CallApiAsync(string url)
    {
        var response = await client.GetAsync(url);
        if (response.IsSuccessStatusCode)
        {
            return await response.Content.ReadAsStringAsync();
        }
        return $"error: {response.StatusCode}";
    }
}

Tim emphasizes creating a single HTTP client instance to avoid reinitializing it with each call, which improves performance.

Handling API Responses

Once the HTTP client is in place, Tim shows how to retrieve the response from the API call. He points out the importance of returning a Taskinstead of async void, except in event handlers.

To demonstrate, Tim uses a sample API from JSON Placeholder, which provides fake data like posts, comments, and todos. He pastes the API URL into the UI form HTML and uses the results.Text field to display the response HTML or JSON.

results.Text = await api.CallApiAsync(apiText.Text);

Tim notes that the raw JSON output is computer-readable but not user-friendly, which leads to the next step: formatting the JSON.

Formatting JSON Output

Tim shows how to make the response JSON more readable using JsonSerializer:

var jsonElement = JsonSerializer.Deserialize<JsonElement>(responseJson);
var prettyJson = JsonSerializer.Serialize(jsonElement, new JsonSerializerOptions { WriteIndented = true });

This allows developers to display pretty JSON in the UI, which is easier to read in JSON text editors or when testing endpoints. Tim also adds an option to toggle between raw and formatted output, giving flexibility depending on whether the data will be displayed in the UI or processed programmatically.

Planning for Future Enhancements

Even though the MVP only supports GET requests, Tim demonstrates how to plan for other HTTP actions like POST, PATCH, PUT, and DELETE. He creates an enum called HTTPAction with a default value of GET, preparing the code to scale without rewriting existing methods.

public enum HTTPAction
{
    GET
}

This forward-thinking design is a great practice for developers wanting to build their own Postman clone that is maintainable and expandable.

URL Validation

Tim introduces a URL validation method to ensure that users only provide valid HTTPS endpoints:

public bool IsValidURL(string url)
{
    if (string.IsNullOrWhiteSpace(url)) return false;
    return Uri.TryCreate(url, UriKind.Absolute, out Uri uriResult) && uriResult.Scheme == Uri.UriSchemeHttps;
}

He explains the importance of never trusting user input and validating it multiple times where necessary. This ensures that the application does not break due to invalid URLs and prevents error messages from disrupting the workflow.

Integrating API Access with the UI

Once validation is in place, Tim shows how to integrate API access with the dashboard UI:

1. Instantiate the APIAccess class.

2. Validate the URL.

3. Display the response JSON in the results text editor.

4. Show meaningful error messages for invalid or failed requests.

if (!api.IsValidURL(apiText.Text))
{
    systemStatus.Text = "Invalid URL";
    results.Text = string.Empty;
    return;
}
results.Text = await api.CallApiAsync(apiText.Text);

Tim highlights the importance of clean UI design, starting each request with an empty results area and updating the system status based on success or failure.

Using Interfaces for Dependency Injection and Unit Testing

Tim introduces IAPIAccess, an interface for APIAccess. This is a best practice for unit testing and preparing the code for dependency injection:

public interface IAPIAccess
{
    Task<string> CallApiAsync(string url);
    bool IsValidURL(string url);
}

By coding against an interface rather than the concrete class, developers can swap implementations for testing or upgrade the API logic without changing the UI or other dependent code. Tim stresses that although this is slightly overkill for the MVP, it is valuable for future application enhancements.

Testing and Running the Application

With all the pieces in place, Tim runs the application on Windows, pastes the JSON Placeholder URL, and successfully displays the formatted JSON response. He shows how invalid URLs are properly rejected, ensuring the app is robust even with user input errors.

This forms a functional Postman clone capable of making GET requests, validating input, and displaying responses in a user-friendly format.

Next Steps: Portfolio and GitHub Integration

Tim closes the lesson by emphasizing the importance of turning this project into a portfolio-ready item. He suggests:

• Creating a GitHub repository for the project.

• Adding a clear README that explains the application.

• Including a downloadable executable for others to test.

• Highlighting the UI and features in screenshots or GIFs.

• Documenting the process, setup, and code structure.

He cautions against simply copying his code and uploading it as your own. Instead, developers should use these lessons to create their own Postman clone or a similar app that reflects personal style and skillset.

By following this approach, developers not only demonstrate coding proficiency but also show the ability to explore, update, and maintain a software project, which is invaluable for potential employers.

Conclusion

Tim Corey’s video provides a comprehensive guide on building a Postman clone from scratch. From setting up a class library to handling API calls, formatting JSON responses, validating input, and preparing the project for future enhancements with interfaces and dependency injection, this lesson covers a full application development process.

By following this approach, developers can create an MVP Postman clone using plain C#, integrate UI elements for displaying response HTML or JSON, and prepare a GitHub project ready for showcasing in a portfolio. This step-by-step methodology not only teaches code but also emphasizes planning, process, and design thinking, which are critical skills for professional software developers.