How to create a Net Core Middleware to validate a route parameter

Using Net Core 7 I have Razor Pages with a culture route parameter:

@page "/{culture:validculture}/about"

I created a custom route constraint to check if culture has a valid value.

When the culture value is invalid I am redirected to a 404 error page.

public class CultureRouteConstraint : IRouteConstraint {

    private readonly RequestLocalizationOptions _options;

  public CultureRouteConstraint(IOptionsSnapshot<RequestLocalizationOptions> options) {

        _options = options.Value;

  } 

  public Boolean Match(HttpContext? httpContext, IRouter? route, String routeKey, RouteValueDictionary values, RouteDirection routeDirection) {
    
    String? culture = Convert.ToString(value, CultureInfo.InvariantCulture);

    List<String>? cultures = _options.SupportedCultures?.Select(x => x.TwoLetterISOLanguageName).ToList();

    if (culture != null && cultures != null)
      return cultures.Contains(culture, StringComparer.InvariantCultureIgnoreCase);

  }

}

This works great if I hardcode de valid cultures, e.g:

List<String>? cultures = new() { "en", "pt" };

But if I inject RequestLocalizationOptions I get the error:

RouteCreationException: An error occurred while trying to create an instance of 'CultureRouteConstraint'.

Maybe I need to use a Middleware for this? How can I do this?

Just because IOptionsSnapshot Is registered as Scoped and therefore can't be injected into a Singleton service.

You could try with IOptions (Not Options<>)instead If you don't have to update the options for different request),For Example,I tried as below and it works well in my case:

In program.cs:

builder.Services.Configure<RequestLocalizationOptions>(opts =>
{
    var supportedcultures = new List<CultureInfo>()
    {
        new CultureInfo("en-US"),
        new CultureInfo("zh-CN")
    };
    opts.SupportedCultures = supportedcultures;
    opts.SupportedUICultures= supportedcultures;
});
builder.Services.AddRazorPages();
builder.Services.AddRouting(options =>
    options.ConstraintMap.Add("validculture", typeof(CultureRouteConstraint)));



var app = builder.Build();

The constraint:

public class CultureRouteConstraint : IRouteConstraint
    {



       private readonly RequestLocalizationOptions _options;



       public CultureRouteConstraint(IOptions<RequestLocalizationOptions> options)
        {



           _options = options.Value;



       }



       public Boolean Match(HttpContext? httpContext, IRouter? route, String routeKey, RouteValueDictionary values, RouteDirection routeDirection)
        {



           String? culture = Convert.ToString(values["culture"], CultureInfo.InvariantCulture);



           List<String>? cultures = _options.SupportedCultures?.Select(x => x.TwoLetterISOLanguageName).ToList();



           if (culture != null && cultures != null)
                return cultures.Contains(culture, StringComparer.InvariantCultureIgnoreCase);
            else
                return false;



       }



   }

The Result:

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