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