FastEndpoints for C# (.NET): Complete Guide with Examples

FastEndpoints is a modern, high-performance library for building web APIs on the .NET platform (ASP.NET Core) that implements the REPR pattern (Request-Endpoint-Response). Unlike the traditional MVC (Model-View-Controller) approach or Minimal APIs, FastEndpoints offers a cleaner, more modular, and testable architecture where each endpoint is a separate class responsible for handling one specific request.

Why do you need this library? In large projects with dozens or hundreds of endpoints, traditional controllers quickly turn into "god objects" with bloated logic. FastEndpoints solves this by forcing developers to explicitly separate responsibilities: one class — one endpoint. This simplifies maintenance, testing, and code reuse. Additionally, the library is built on native System.Text.Json and optimized for maximum performance, making it one of the fastest API libraries in the .NET ecosystem.

FastEndpoints is ideal for microservice architectures, Domain-Driven Design (DDD) projects, and Clean Architecture. It provides built-in validation (via FluentValidation), Swagger/OpenAPI support, error handling, caching, endpoint versioning, and much more, saving developers from writing boilerplate code.

Installation

FastEndpoints is installed via NuGet. You'll need .NET SDK 6.0 or higher. Run one of the following commands in your terminal:

dotnet add package FastEndpoints

For Swagger integration (optional but recommended):

dotnet add package FastEndpoints.Swagger

For FluentValidation support (recommended):

dotnet add package FluentValidation

Quick Start — Minimal Working Example

Create a new ASP.NET Core Web API project (empty or with controllers — it doesn't matter, we'll use FastEndpoints anyway).

Step 1: Configure Program.cs

using FastEndpoints;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddFastEndpoints(); // Register FastEndpoints services

var app = builder.Build();
app.UseFastEndpoints(); // Add middleware
app.Run();

Step 2: Create an Endpoint

Create a folder called Endpoints and inside it a file named HelloEndpoint.cs:

using FastEndpoints;

namespace MyApi.Endpoints;

public class HelloEndpoint : Endpoint<EmptyRequest, string>
{
    public override void Configure()
    {
        Get("/api/hello"); // HTTP GET method on /api/hello
        AllowAnonymous();  // Accessible without authentication
    }

    public override async Task HandleAsync(EmptyRequest req, CancellationToken ct)
    {
        await SendAsync("Hello, World!", cancellation: ct);
    }
}

Step 3: Run It

Launch the application (dotnet run) and open your browser at http://localhost:5000/api/hello. You'll see the response: Hello, World!

Core Classes and Methods

Below are the key classes and methods of the FastEndpoints library with usage examples.

1. The Endpoint<TRequest, TResponse> Class

Signature: public abstract class Endpoint<TRequest, TResponse> : BaseEndpoint where TRequest : notnull, new()

What it does: The base class for creating an endpoint. It accepts a request type (TRequest) and a response type (TResponse).

Example:

public class CreateUserEndpoint : Endpoint<CreateUserRequest, Us