ASP.NET Core - Route Constraints

Routing constraints let you restrict how the parameters in the route template are matched. It helps to filter out the input parameter and action method it can accept.

For example, if the URL Parameter is restricted to have int value, the route engine will match the controller action having integer value in the parameter or restrict it.

How Route Constraints are applied:

Using constraint parameter in the MapControllerRoute at the application startup where the endpoints are defined; i.e. Inline Constraint
Route attribute at the controller or action method

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

When the route engine matches the incoming url, it invokes the routing constraint to the values in the url which match the pattern. In this case which is seperated by : in the above example restricts the value should be int or null.

Here's a quick example on how the routing constraints are defined and used.

Inline Constraint

Inline constraint are added after the URL parameter and sperated by : (colon) with the primitive type and also defined if the parameter can be nullable. Below code see – {id:int?}

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

The int constraints checks if the parameter value sent is integer and allows you to execute the action method by passing the parameter value to the matching method.

The other way to define the same is to pass the default values and constraint parameter in the MapControllerRoute

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

Constraints in route attribute

You can also define the constraints in route attribute as follows,

[Route("Home/Index/{id:int?}")]
public IActionResult Index(int? id)
{
return View();
}

Here is the list of constraints from the Microsoft doc site, try it yourself,

Constraint	Description	Example
alpha	Matches uppercase or lowercase Latin alphabet characters (a-z, A-Z)	{x:alpha}
bool	Matches a Boolean value.	{x:bool}
datetime	Matches a DateTime value.	{x:datetime}
decimal	Matches a decimal value.	{x:decimal}
double	Matches a 64-bit floating-point value.	{x:double}
float	Matches a 32-bit floating-point value.	{x:float}
guid	Matches a GUID value.	{x:guid}
int	Matches a 32-bit integer value.	{x:int}
length	Matches a string with the specified length or within a specified range of lengths.	{x:length(6)} {x:length(1,20)}
long	Matches a 64-bit integer value.	{x:long}
max	Matches an integer with a maximum value.	{x:max(10)}
maxlength	Matches a string with a maximum length.	{x:maxlength(10)}
min	Matches an integer with a minimum value.	{x:min(10)}
minlength	Matches a string with a minimum length.	{x:minlength(10)}
range	Matches an integer within a range of values.	{x:range(10,50)}
regex	Matches a regular expression.	{x:regex(^\d{3}-\d{3}-\d{4}$)}