Constraints

Inline constraints inside the routing attributes are used by placing a colon with the constraint name :constraint-name  after the route parameter name, where constraint-name is a constraint that you define by creating a class that implements the IRouteConstraint interface, or simply by using one of the built-in constraints specified in the ASP.NET documentation, available at https://docs.microsoft.com/en-us/aspnet/core/fundamentals/routing?highlight=routing#route-constraint-reference.

The following example shows how we can add a search method to our ProductsController that searches by the date that the product was posted online. The URL for this action must constrain the date parameter to datetime formats only; therefore, we will use the datetime constraint like this:

[HttpGet("search/{date:datetime}/{keyword}/")]
public string[] Search(string date, string keyword)
{
    return new[]
    {
        $"Date: {date}, keyword: {keyword}"
    };
}

The routing infrastructure in ASP.NET Core is very sophisticated, and there are plenty more features out of the scope of this book that therefore aren't covered. For more details about the routing capabilities in ASP.NET Core, refer to the documentation at https://docs.microsoft.com/en-us/aspnet/core/fundamentals/routing.