What is Routing?

Routing in .NET is the mechanism that directs an HTTP request to the appropriate controller action or endpoint. It plays a critical role in determining how URLs are interpreted by the application and how requests are handled. When a request comes in, the routing engine examines the URL and matches it against a defined set of routes. If a match is found, the corresponding action or endpoint is invoked to process the request.

Setting Up Routing in ASP.NET Core
In ASP.NET Core, routing is configured in the Startup.cs file within the Configure method. Here's a basic example:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });
}

In this example, the UseRouting middleware is added to the pipeline to enable routing. The UseEndpoints method is used to define the route pattern. The pattern parameter specifies the route template, where {controller}, {action}, and {id?} are placeholders that correspond to the controller name, action method, and optional ID parameter.

Attribute Routing

In addition to conventional routing, ASP.NET Core supports attribute routing. Attribute routing allows you to define routes directly on controller actions, providing more control and flexibility.

[Route("products")]
public class ProductsController : Controller
{
    [HttpGet("{id}")]
    public IActionResult Details(int id)
    {
        // Action logic here
        return View();
    }

    [HttpPost("create")]
    public IActionResult Create(Product product)
    {
        // Action logic here
        return RedirectToAction("Index");
    }
}

In this example, the Details action method is accessible via products/{id}, and the Create action method is accessible via products/create. This approach is particularly useful when you want to have more descriptive and SEO-friendly URLs.

Advanced Routing Techniques

Route Constraints
Route constraints are used to restrict the routes based on certain conditions. For instance, you can enforce that a route parameter must be an integer:

endpoints.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id:int?}");

Here, the id parameter must be an integer, and the route will only match if the condition is satisfied.

Custom Route Handlers
In some cases, you might need more control over how routes are handled. You can create custom route handlers by implementing the IRouter interface. This allows you to define your routing logic and handle requests in a unique way.

public class CustomRouteHandler : IRouter
{
    public Task RouteAsync(RouteContext context)
    {
        var requestPath = context.HttpContext.Request.Path.Value;

        if (requestPath.Contains("custom-route"))
        {
            // Custom logic here
            context.Handler = async ctx =>
            {
                await ctx.Response.WriteAsync("This is a custom route handler!");
            };
        }

        return Task.CompletedTask;
    }

    public VirtualPathData GetVirtualPath(VirtualPathContext context)
    {
        return null;
    }
}

This custom route handler checks if the request path contains "custom-route" and handles it accordingly.

Routing in .NET is a versatile and powerful feature that allows you to design clean and efficient URL structures for your web applications. Whether you’re using conventional routing, attribute routing, or advanced techniques like route constraints and custom route handlers, understanding how routing works will enable you to build more scalable and maintainable applications.