Routing - Mapping URLs to Code

Level: Beginner

ℹ️ Where This Fits

Routing connects URLs to code. Learn this before controllers, actions, model binding, and Web API endpoints.

ℹ️ What You'll Learn

• Routing purpose: Match URL pattern + HTTP method to C# handler code
• MapGet("/students", ...): Handle GET requests to /students (list all)
• MapPost("/students", ...): Handle POST requests to /students (create new)
• MapPut("/students/{id}", ...): Handle PUT requests to /students/101 (update one)
• MapDelete("/students/{id}", ...): Handle DELETE requests to /students/101 (delete one)
• Route parameters: {id} captures value from URL (GET /students/101 → id=101)
• Route constraints: {id:int} only matches integer (/students/abc won't match)
• Query string vs route: /students/101 is route (identifies resource), /students?class=10A is query (filter)
• REST conventions: Use nouns (/students), not verbs (/getStudents). HTTP method implies action
• School Management routes: GET /students, POST /students, GET /classes/:classId/students, POST /attendance, GET /reports/fees
• Multiple parameters: Route can capture className and studentId from URL path segments
• Optional parameters: Route parameters can be optional using route constraints for pagination
• Debugging routing: Catch-all routes show unmatched URL patterns for troubleshooting

What is Routing?

Routing is how ASP.NET Core decides which C# code should run for a URL.

Example:

GET /students/101

Routing matches this to:

app.MapGet("/students/{id}", (int id) => ...);

Then ASP.NET Core passes:

id = 101

Basic Routes

💻 Try It — Console App

💡 Add these routes in Program.cs.⌥ GitHub

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "SchoolPortal Home");

app.MapGet("/students", () => "All students");

app.MapGet("/teachers", () => "All teachers");

app.Run();

URL	Handler
/	Home route
/students	Students route
/teachers	Teachers route

Route Parameters

Route parameters capture values from the URL.

app.MapGet("/students/{id}", (int id) =>
{
    return $"Student id is {id}";
});

Request:

/students/101

Result:

id = 101

Multiple Parameters

app.MapGet("/classes/{className}/students/{studentId}", 
    (string className, int studentId) =>
{
    return $"Class: {className}, Student: {studentId}";
});

Request:

/classes/10A/students/101

Result:

className = 10A
studentId = 101

HTTP Methods and Routes

The same URL can behave differently based on HTTP method.

Method	Route	Meaning
GET	/students	Read students
POST	/students	Create student
GET	/students/101	Read one student
PUT	/students/101	Update student
DELETE	/students/101	Delete student

app.MapGet("/students", () => "Get all students");

app.MapPost("/students", () => "Create student");

app.MapGet("/students/{id}", (int id) => $"Get student {id}");

app.MapPut("/students/{id}", (int id) => $"Update student {id}");

app.MapDelete("/students/{id}", (int id) => $"Delete student {id}");

Route Constraints

Constraints limit what values a route accepts.

app.MapGet("/students/{id:int}", (int id) =>
{
    return $"Student {id}";
});

This matches:

/students/101

This does not match:

/students/abc

Common constraints:

Constraint	Example	Meaning
int	{id:int}	Must be integer
guid	{id:guid}	Must be GUID
min	{id:min(1)}	Minimum value
alpha	{name:alpha}	Letters only

Query String vs Route Parameter

Route parameter:

/students/101

Use when the value identifies a specific resource.

Query string:

/students?className=10A&status=active

Use for filters, sorting, searching, and paging.

Example:

app.MapGet("/students", (string? className, string? status) =>
{
    return $"Class: {className}, Status: {status}";
});

School Management Route Examples

Feature	Route
Get all students	GET /students
Get one student	GET /students/{id}
Add student	POST /students
Update student	PUT /students/{id}
Get class students	GET /classes/{classId}/students
Mark attendance	POST /attendance
Get monthly report	GET /reports/monthly-fees?month=5&year=2026

Good Route Design

Prefer nouns:

/students
/teachers
/attendance
/reports

Avoid action-heavy names:

/getStudents
/createStudent
/deleteStudentById

HTTP method already tells the action.

Minimal API Routing vs Controller Routing

Minimal API:

app.MapGet("/students/{id}", (int id) => $"Student {id}");

Controller route:

[ApiController]
[Route("api/students")]
public class StudentsController : ControllerBase
{
    [HttpGet("{id}")]
    public IActionResult GetStudent(int id)
    {
        return Ok($"Student {id}");
    }
}

Both use routing. Controllers organize routes better in larger projects.

Common Beginner Mistakes

Mistake	Better Approach
/getStudent/101	GET /students/101
/student/create	POST /students
Using route values for filters	Use query string for filters
No route constraints	Use {id:int} where needed
Very long route names	Keep routes readable and resource-based
Mixing singular/plural randomly	Prefer plural resources like /students

What to Remember

Routing = URL + HTTP method -> C# handler
Route parameter = value inside path
Query string = filters/search/paging
Constraint = rule for route value

🎯 Interview Favourite

Q: How does routing work in ASP.NET Core?

Good Answer: "Routing maps an incoming HTTP request to the correct endpoint or controller action. ASP.NET Core checks the HTTP method and URL path against route patterns like /students/{id}. Values in the URL can be captured as route parameters and passed to the handler. Routes can also use constraints such as {id:int}. In Web API design, routes usually represent resources, while HTTP methods represent actions."

Practice Before Next Article

Create routes for:

1. Get all teachers.
2. Get teacher by id.
3. Get students in a class.
4. Mark attendance.
5. Get fee report by month and year using query string.

🤖Use AI to Learn Faster

Use ChatGPT, Claude, or Copilot to go deeper on ASP.NET Core routing. Try these prompts:

• "Create REST-style routes for a School Management System"
• "Explain route parameters vs query strings"
• "What are route constraints?"
• "Review these routes and make them REST friendly"

💡 Tip: After reading this article, paste your own code into AI and ask "What could go wrong here and why?" — fastest way to find edge cases and deepen understanding.

Next Article

-> Dependency Injection (DI)