Route Parameter Constraints In Asp.Net Razor Pages

In this article we will understand Route Parameter Constraints In Asp.Net Razor Pages by Sagar Jaybhay

Route Parameter Constraints

Route Constraints is a mechanism where it filters out or restricts unwanted route parameters to reaching out to PageModel methods.

To add route constraints we need to add constraints in route template data. For example, if we want our id only accepts numeric value then for this is below code.

@page "/employee/view/{id:int}"

Now we want to apply constraints with optional parameters then we need to use below syntax.

@page "/employee/view/{id:int?}"

Below is a list of the table this kind of constraints we can apply in the route template.

Constraint Description Example
alpha Matches uppercase or lowercase Latin alphabet characters (a-z, A-Z) {title:alpha}
bool 1 Matches a Boolean value. {isActive:bool}
int 1 Matches a 32-bit integer value. {id:int}
datetime 1 Matches a DateTime value. {startdate:datetime}
decimal 1 Matches a decimal value. {cost:decimal}
double 1 Matches a 64-bit floating-point value. {latitude:double}
float 1 Matches a 32-bit floating-point value. {x:float}
long 1 Matches a 64-bit integer value. {x:long}
guid 1 Matches a GUID value. {id:guid}
length Matches a string with the specified length or within a specified range of lengths. {key:length(8)} {postcode:length(6,8)}
min Matches an integer with a minimum value. {age:min(18)}
max Matches an integer with a maximum value. {height:max(10)}
minlength Matches a string with a minimum length. {title:minlength(2)}
maxlength Matches a string with a maximum length. {postcode:maxlength(8)}
range Matches an integer within a range of values. {month:range(1,12)}
regex Matches a regular expression. {postcode:regex(^[A-Z]{2}\d\s?\d[A-Z]{2}$)}

We can apply more than one constraint at a time for this use below syntax.

For example, we want to Id whose minimum value is 1 and max value 100 can accept

@page "/employee/view/{id:min(1):max(100)}"

How to create Custom Constraints in Asp.Net Razor pages?

There is a five-step method to create Custom Constraints in razor pages.

  • First, we required the inbuilt interface for creating a constraint and below is Interface for that.
public interface IRouteConstraint : IParameterPolicy
        bool Match(HttpContext httpContext, IRouter route, string routeKey, RouteValueDictionary values, RouteDirection routeDirection);
  • The second is to create a class and implement this interface in our newly created class and your required logic in match method which returns true or false.
public bool Match(HttpContext httpContext, IRouter route, string routeKey, RouteValueDictionary values, RouteDirection routeDirection)
                var regex = new Regex("^[a-zA-Z0-9]{4}$");
                if (regex.IsMatch(values[routeKey].ToString()))
                    return true;
            return false;
  • The now third step to add a custom constraint to our route template for this use below code.
@page "/employee/view/{id:custom}"
  • When we run our application we get an error which is shown below.

An unhandled exception occurred while processing the request.

InvalidOperationException: The constraint reference ‘custom’ could not be resolved to a type. Register the constraint type with ‘Microsoft.AspNetCore.Routing.RouteOptions.ConstraintMap’.

Microsoft.AspNetCore.Routing.DefaultParameterPolicyFactory.Create(RoutePatternParameterPart parameter, string inlineText)

  • To overcome this error we need to add this constraint in configureservice method using ConstraintMap method
public void ConfigureServices(IServiceCollection services)
            services.AddSingleton<IEmployeeRepos, DBRepository>();
            services.Configure<RouteOptions>(option =>
                option.LowercaseUrls = true;
                option.LowercaseQueryStrings = true;
                option.AppendTrailingSlash = true;
               option.ConstraintMap.Add("custom", typeof(CustomConstraints.custom));

By doing this our application work perfectly fine but when we pass more than 4 character id it will give below output.

Using custom constraints it will not throw an error but it shows 404 not found.

Now we need to understand each parameter in the Match method.

bool Match(HttpContext httpContext, IRouter route, string routeKey, RouteValueDictionary values, RouteDirection routeDirection);
Name Explnation
httpContext An object that encapsulates information about the HTTP request.
route he router that this constraint belongs to.
routeKey The name of the parameter that is being checked.
values A dictionary that contains the parameters for the URL.
routeDirection An object that indicates whether the constraint check is being performed when an incoming request is being handled or when a URL is being generated.


Sagar Jaybhay, from Maharashtra, India, is currently a Senior Software Developer. He has continuously grown in the roles that he has held in the more than seven years he has been with this company. Sagar Jaybhay is an excellent team member and prides himself on his work contributions to his team and company as a whole.

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