RestSharp API Testing in C#: Complete Guide

Last updated: June 27, 2026 · 8 min read

RestSharp is the most popular HTTP client library for .NET API testing. This guide walks you from setup to advanced patterns in 15 minutes. Pair it with our API Testing Tutorial, Postman API Testing, and SpecFlow C# Automation guide.

RestSharp is a simple REST and HTTP API client for .NET. It wraps HttpClient with a fluent API that handles JSON serialization, authentication, and error handling out of the box. The library is maintained on GitHub and documented on restsharp.dev.

dotnet add package RestSharp
dotnet add package Newtonsoft.Json

using RestSharp;

var client = new RestClient("https://api.example.com");
var request = new RestRequest("/users/1", Method.Get);
var response = await client.ExecuteAsync(request);

Console.WriteLine(response.StatusCode); // 200
Console.WriteLine(response.Content);    // {"id":1,"name":"Alice"}

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
}

var response = await client.ExecuteAsync<User>(request);
var user = response.Data;
Console.WriteLine(user.Name); // "Alice"

var request = new RestRequest("/users", Method.Post)
    .AddJsonBody(new
    {
        name = "Alice",
        email = "alice@example.com"
    });

var response = await client.ExecuteAsync(request);
Assert.AreEqual(201, (int)response.StatusCode);

// PUT
var updateRequest = new RestRequest("/users/1", Method.Put)
    .AddJsonBody(new { name = "Alice Updated" });

await client.ExecuteAsync(updateRequest);

// DELETE
var deleteRequest = new RestRequest("/users/1", Method.Delete);
await client.ExecuteAsync(deleteRequest);

Bearer Token

var client = new RestClient("https://api.example.com");
client.AddDefaultHeader("Authorization", $"Bearer {token}");

var request = new RestRequest("/admin/users", Method.Get);
var response = await client.ExecuteAsync(request);

Basic Auth

client.Authenticator = new HttpBasicAuthenticator("admin", "Sup3rSecret!");

Query parameters

var request = new RestRequest("/users", Method.Get)
    .AddQueryParameter("page", "1")
    .AddQueryParameter("limit", "10");

Path parameters

var request = new RestRequest("/users/{id}", Method.Get)
    .AddUrlSegment("id", "1");

Use NUnit, xUnit, or MSTest:

using NUnit.Framework;

Assert.That((int)response.StatusCode, Is.EqualTo(200));
Assert.That(response.IsSuccessful, Is.True);

For more on API testing strategy, see our API Testing Tutorial.

Install JsonPath:

dotnet add package JsonPath.Net

using JsonPath;

var json = JObject.Parse(response.Content);
var name = json.SelectToken("$.name");
Assert.That(name?.ToString(), Is.EqualTo("Alice"));

For production code, use HttpClient (faster, more control). For API testing, RestSharp wins on developer experience.

- uses: actions/setup-dotnet@v4
  with: { dotnet-version: '8.0.x' }
- run: dotnet test --logger "trx;LogFileName=results.trx"
- uses: actions/upload-artifact@v4
  if: ${{ !cancelled() }}
  with: { name: test-results, path: TestResults/ }

For more CI patterns, see our GitHub Actions Selenium CI guide.

Reusable client setup

public class ApiClient
{
    private readonly RestClient _client;

    public ApiClient(string baseUrl) { _client = new RestClient(baseUrl); }

    public async Task<RestResponse<T>> GetAsync<T>(string path)
        => await _client.ExecuteAsync<T>(new RestRequest(path, Method.Get));

    public async Task<RestResponse<T>> PostAsync<T>(string path, object body)
    {
        var request = new RestRequest(path, Method.Post).AddJsonBody(body);
        return await _client.ExecuteAsync<T>(request);
    }

    public async Task<RestResponse> DeleteAsync(string path)
        => await _client.ExecuteAsync(new RestRequest(path, Method.Delete));
}

Request interceptor

client.AddRequestInterceptor(req => {
    Console.WriteLine($"→ {req.Method} {req.Resource}");
    return req;
});

File upload

var request = new RestRequest("/upload", Method.Post)
    .AddFile("file", "/path/to/file.png", "image/png");

var response = await client.ExecuteAsync(request);

Async/Await best practice

// ❌ Bad — blocks the thread
var response = client.Execute(request);

// ✅ Good — async, scalable
var response = await client.ExecuteAsync(request);

Handling rate limits

public async Task<RestResponse> ExecuteWithRetry(RestRequest request, int maxRetries = 3)
{
    for (int i = 0; i < maxRetries; i++)
    {
        var response = await client.ExecuteAsync(request);
        if (response.StatusCode != HttpStatusCode.TooManyRequests)
            return response;

        var retryAfter = response.Headers
            .FirstOrDefault(h => h.Name == "Retry-After")?.Value ?? "1";
        await Task.Delay(int.Parse(retryAfter) * 1000);
    }
    throw new Exception("Rate limit exceeded");
}

Mocking external APIs with WireMock.NET

dotnet add package WireMock.Net

var server = WireMockServer.Start(8080);
server.Given(Request.Create().WithPath("/api/external").UsingGet())
      .RespondWith(Response.Create().WithStatusCode(200)
                           .WithBody("{ \"id\": 1, \"name\": \"Mock\" }"));

var client = new RestClient("http://localhost:8080");

Schema validation

var schemaJson = File.ReadAllText("schemas/user.json");
var schema = JSchema.Parse(schemaJson);

var response = await client.ExecuteAsync(request);
var body = JObject.Parse(response.Content);

var isValid = body.IsValid(schema, out var errors);
Assert.That(isValid, Is.True);

• API Testing Ultimate Guide
• Postman API Testing Tutorial
• SpecFlow C# Automation
• API Testing Interview Questions
• SDET Career Roadmap

Junior engineers reach for RestClient directly in the test body. Senior SDETs isolate the client behind a typed API facade so tests read like requirements, not HTTP plumbing. Below is the pattern most .NET 8 teams now ship, distilled from ~30 SDET take-home reviews we ran in H1 2026.

1. Layered client with a shared ClientFactory

public static class ClientFactory {
  private static readonly Lazy<RestClient> _authed = new(() => {
    var opts = new RestClientOptions(EnvConfig.BaseUrl) {
      Timeout = TimeSpan.FromSeconds(30),
      ThrowOnAnyError = false
    };
    var client = new RestClient(opts);
    client.AddDefaultHeader("Authorization", $"Bearer {TokenCache.Get()}");
    client.AddDefaultHeader("X-Trace-Id", TraceId.New());
    return client;
  });
  public static RestClient Authed => _authed.Value;
}

[Test]
public async Task GetOrder_ReturnsExpectedShape() {
  var response = await OrdersApi.Get(id: 42);
  Assert.That(response.IsSuccessStatusCode, Is.True);
  Assert.That(response.Data!.Currency, Is.EqualTo("USD"));
}

One client per authenticated persona, shared across the suite. Every test that opens with OrdersApi.Get(...) is one line of intent, zero lines of ceremony — and swapping a base URL becomes an env-var flip, not a suite-wide refactor.

2. Contract-first assertions with JSON Schema

Store the expected response schema in Resources/schemas/ and fail the build when the shape drifts. That single habit catches ~65% of backend-to-frontend regressions before any Playwright suite runs.

var schema = JSchema.Parse(File.ReadAllText("Resources/schemas/order.json"));
var body = JObject.Parse(response.Content!);
Assert.That(body.IsValid(schema, out IList<string> errors), Is.True,
  $"Schema drift: {string.Join(\", \", errors)}");

3. Parallel execution without shared-state pain

RestSharp calls are stateless; the danger is your fixtures. Pair xUnit's collection fixtures with a per-test data factory that provisions a unique tenant via an internal /test-setup endpoint.

[Fact]
public async Task Checkout_HappyPath() {
  var buyer = await TestBuyers.FreshAsync();
  var response = await CheckoutApi.PayAsync(buyer.CartId, "card_visa_4242");
  Assert.Equal(HttpStatusCode.Created, response.StatusCode);
  Assert.Equal("PAID", response.Data!.Status);
}

Every test creates and disposes its own tenant, so 20 xUnit workers cannot corrupt each other's state. Interview panels love this pattern — it demonstrates test isolation at the framework layer, not the assertion layer.

4. What senior interviewers ask about RestSharp in 2026

• Why RestSharp over HttpClient? Fluent DSL, built-in JSON handling, and lower ceremony for test code. HttpClient still wins in production services.
• How do you keep tokens fresh? A TokenCache singleton with a TTL just below the token's real expiry and a lazy refresh on 401 — never a login before every test.
• How do you contract-test against a stub? Pair RestSharp with WireMock.NET for consumer-driven contract tests and run the same suite against real environments in a nightly job.
• How do you report? Allure with an Allure.NUnit adapter, published as a GitHub Actions artifact and posted to Slack via the Allure GitHub App.

Original SoftwareTestPilot research relevant to API testing:

• Shift-left benchmarks (2026) — where API contract tests move the defect-cost needle.
• Test pyramid benchmarks (2026) — the API layer has grown from 20% (Cohn) to 26% median in 2026.
• Enterprise automation tool benchmarks (2026) — flake and cost data.
• API testing interview questions with senior-SDET answers.