Routing, model binding, validation and status codes

app.MapGet("/trains/{number}", (string number) => $"Train {number}");
app.MapGet("/orders/{id:int}", (int id) => $"Order {id}");
app.MapGet("/bookings/{date:datetime}", (DateTime date) => date.ToShortDateString());
app.MapGet("/files/{*path}", (string path) => path);   // catch-all: matches a/b/c.txt

var api = app.MapGroup("/api/v1/products");            // shared prefix
api.MapGet("/", () => "all products");
api.MapGet("/{id:int}", (int id) => $"product {id}");

app.MapGet("/search", (string q, int page = 1, int size = 20) =>
    $"Searching '{q}', page {page}, size {size}");
// GET /search?q=kurta&page=2  ->  Searching 'kurta', page 2, size 20

app.MapPost("/orders", (CreateOrder order) => Results.Ok(order));
// POST /orders with JSON {"productId":7,"quantity":2}

public record CreateOrder(int ProductId, int Quantity);

using System.ComponentModel.DataAnnotations;

public class CreateProduct
{
    [Required, StringLength(100, MinimumLength = 2)]
    public string Name { get; set; } = "";

    [Range(1, 1_000_000)]
    public decimal Price { get; set; }

    [EmailAddress]
    public string? SupplierEmail { get; set; }
}

app.MapPost("/products", (CreateProduct p) =>
{
    var errors = new Dictionary<string, string[]>();
    if (string.IsNullOrWhiteSpace(p.Name)) errors["name"] = new[] { "Name is required" };
    if (p.Price <= 0) errors["price"] = new[] { "Price must be positive" };
    if (errors.Count > 0) return Results.ValidationProblem(errors);

    // ... save ...
    return Results.Created($"/products/1", p);
});