Ivan Sander de Jong Ivan Sander de Jong - 3 months ago 6
C# Question

Web api with mvc 6 get element based on string

I am creating a web api using mvc 6. now i am trying to get a element from my db. the key in this table is a string (email address). I do not have access to this database so i cant change the key of this table.

Now when creating a demo webapi i was able to create a controller to extract items based on a key which was an int. But when trying to get a element by a string the program crashes.

[Route("api/[controller]")]
public class TodoController : Controller
{

[HttpGet("{id:string}", Name = "GetByIdRoute")]
public IActionResult GetById (string id)
{
var item = _items.FirstOrDefault(x => x.Id == id);
if (item == null)
{
return HttpNotFound();
}

return new ObjectResult(item);
}
}


when trying to access this path (example.com/api/Todo/key) key being the string i get an exception in my startup.cs

the exception in the browser reads:


System.InvalidOperationException
The constraint entry 'id' - 'string' on the route 'api/Todo/{id:string}' could not be resolved by the constraint resolver of type 'DefaultInlineConstraintResolver'.


the part of the startup.cs where the code breaks is:

// Add MVC to the request pipeline.
app.UseMvc(routes =>
{
routes.MapRoute(
name: "default",
template: "{controller}/{action}/{id?}",
defaults: new { controller = "Home", action = "Index" });

});


i cant seem to figure out why i ain't allowed to get a item by a key which is a string.
is this even possible and if so what am i doing wrong?

Answer

Just remove :string. You're not really constraining the value of the id anyway - it's already a string in the URL.

This fairly old blog post lists the available constraints - and you can see there's no :string constraint, because you don't need there to be.

The constraints are used to give "more specific" constraints a priority - e.g. "If that part of the URL is the string representation of a DateTime, use this route" - but as everything is a string (in a URL), there's nothing that a constraint of :string would make it more specific than, if you see what I mean.