C# ASP.NET Core Cheat Sheet

Covers building ASP.NET Core Web APIs with controllers and minimal APIs, the middleware pipeline, routing, and model binding.

3 PagesIntermediateMar 25, 2026

API Controller

A typical RESTful controller with attribute routing.

csharp

[ApiController][Route("api/[controller]")]public class ProductsController : ControllerBase{    private readonly IProductService _service;    public ProductsController(IProductService service) => _service = service;    [HttpGet]    public async Task<ActionResult<List<Product>>> GetAll()        => Ok(await _service.GetAllAsync());    [HttpGet("{id:int}")]    public async Task<ActionResult<Product>> GetById(int id)    {        var product = await _service.GetByIdAsync(id);        return product is null ? NotFound() : Ok(product);    }    [HttpPost]    public async Task<ActionResult<Product>> Create(Product product)    {        await _service.AddAsync(product);        return CreatedAtAction(nameof(GetById), new { id = product.Id }, product);    }}

Middleware Pipeline

Built-in and custom request/response middleware.

csharp

var app = builder.Build();app.UseHttpsRedirection();app.UseRouting();app.UseAuthentication();app.UseAuthorization();app.UseCors("AllowAll");app.MapControllers();app.Run();// Custom middlewareapp.Use(async (context, next) =>{    Console.WriteLine($"Request: {context.Request.Path}");    await next(context);   // Call the next middleware in the pipeline});

Minimal API Endpoints

Defining routes without a full controller class.

csharp

var app = builder.Build();app.MapGet("/products/{id}", async (int id, IProductService svc) =>{    var p = await svc.GetByIdAsync(id);    return p is not null ? Results.Ok(p) : Results.NotFound();});app.MapPost("/products", async (Product p, IProductService svc) =>{    await svc.AddAsync(p);    return Results.Created($"/products/{p.Id}", p);});app.Run();

Core Building Blocks

Concepts that show up in every ASP.NET Core app.

[ApiController]- Enables automatic model validation and binding-source inference for API controllers
Routing- Attribute routing ([Route], [HttpGet]) or conventional routing via MapControllerRoute
Model binding- Automatically maps route, query, and body values to action method parameters
Middleware order- Executes top-to-bottom for requests and bottom-to-top for responses (a pipeline)
Filters- [Authorize], action filters, and exception filters run around action execution
IActionResult- Ok(), NotFound(), BadRequest(), CreatedAtAction() return the correct HTTP status codes
Options pattern- services.Configure<T>(config.GetSection("Section")) binds config to a strongly-typed class

Pro Tip

Register middleware order carefully — UseAuthentication() must come before UseAuthorization(), and both must come before MapControllers(), or auth checks silently won't apply.

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