.NET Core Web API + Third-Party API Integration – 20 Q&A

 

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Here are 20 essential interview questions (with answers) on .NET Core Web API integration with third-party APIs that will help you land interviews, grow skills, and impress clients.

Q1: How do you call a third-party API in .NET Core Web API?

A: Using HttpClient, injected via DI.
public class MyService
{
    private readonly HttpClient _httpClient;
    public MyService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<string> GetDataAsync()
    {
        var response = await _httpClient.GetAsync("https://api.example.com/data");
        return await response.Content.ReadAsStringAsync();
    }
}

Q2: How do you register HttpClient in Startup.cs or Program.cs?

A: services.AddHttpClient<IMyService, MyService>();

Q3: What is a DelegatingHandler?

A: It's middleware for HttpClient to handle cross-cutting concerns like logging, authentication, retry policies.

Q4: How do you send headers (e.g., Authorization) with a request?

A: _httpClient.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", token);

Q5: How do you handle JSON responses from third-party APIs?

A: Use System.Text.Json or Newtonsoft.Json:
var data = JsonSerializer.Deserialize<MyDto>(responseBody);

Q6: How do you send a POST request with a JSON body?

A: var content = new StringContent(JsonSerializer.Serialize(myModel), Encoding.UTF8, "application/json");
   var response = await _httpClient.PostAsync("url", content);

Q7: How to retry failed requests automatically?

A: Use Polly with HttpClientFactory:
services.AddHttpClient("API").AddPolicyHandler(
    HttpPolicyExtensions.HandleTransientHttpError().RetryAsync(3));

Q8: How do you integrate with Stripe/PayPal in .NET Core?

A: Use official SDKs (e.g., Stripe.net) or REST APIs. Always validate webhooks and secure secrets.

Q9: How do you securely store API keys/secrets?

A: Store in appsettings.json, Environment Variables, or use Azure Key Vault.

Q10: How to consume a SOAP service in .NET Core Web API?

A: Use Connected Services in Visual Studio or tools like dotnet-svcutil to generate client proxies.

Q11: What is a Webhook, and how to consume it?

A: Webhook is an HTTP callback. You create a POST endpoint to receive and process incoming webhook requests.

Q12: How do you handle timeouts in HttpClient?

A: httpClient.Timeout = TimeSpan.FromSeconds(30);

Q13: How do you log external API calls?

A: Use middleware, ILogger, or tools like Serilog, Seq, or Application Insights.

Q14: How do you consume paginated APIs?

A: Keep calling API while nextPageUrl or hasMore flag exists in the response.

Q15: How do you handle rate limiting from a third-party API?

A: Respect Retry-After headers, use Polly to delay or fallback.

Q16: How do you test external API integrations locally?

A: Use Postman, Swagger, or mock services with WireMock.Net or Moq.

Q17: How do you use third-party authentication (Google, Microsoft) in .NET Core Web API?

A: Use OAuth/OpenID Connect libraries like Microsoft.AspNetCore.Authentication.Google with JWT validation.

Q18: How to handle large data downloads from an API?

A: Stream the response using HttpClient.GetStreamAsync() instead of loading entire content in memory.

Q19: How do you use Swagger to document third-party API consumption?

A: Add comments and models that explain outbound calls; mention integration points in endpoint descriptions.

Q20: What to consider when integrating multiple third-party APIs in a single Web API?

A: Timeout and retry settings, Circuit breaker pattern, Logging & monitoring, Configurable base URLs, Consistent DTO models.