[c#] No authenticationScheme was specified, and there was no DefaultChallengeScheme found with default authentification and custom authorization

Your initial statement in the marked solution isn't entirely true. While your new solution may accomplish your original goal, it is still possible to circumvent the original error while preserving your AuthorizationHandler logic--provided you have basic authentication scheme handlers in place, even if they are functionally skeletons.

Speaking broadly, Authentication Handlers and schemes are meant to establish + validate identity, which makes them required for Authorization Handlers/policies to function--as they run on the supposition that an identity has already been established.

ASP.NET Dev Haok summarizes this best best here: "Authentication today isn't aware of authorization at all, it only cares about producing a ClaimsPrincipal per scheme. Authorization has to be aware of authentication somewhat, so AuthenticationSchemes in the policy is a mechanism for you to associate the policy with schemes used to build the effective claims principal for authorization (or it just uses the default httpContext.User for the request, which does rely on DefaultAuthenticateScheme)." https://github.com/aspnet/Security/issues/1469

In my case, the solution I'm working on provided its own implicit concept of identity, so we had no need for authentication schemes/handlers--just header tokens for authorization. So until our identity concepts changes, our header token authorization handlers that enforce the policies can be tied to 1-to-1 scheme skeletons.

Tags on endpoints:

[Authorize(AuthenticationSchemes = "AuthenticatedUserSchemeName", Policy = "AuthorizedUserPolicyName")]

Startup.cs:

        services.AddAuthentication(options =>
        {
            options.DefaultAuthenticateScheme = "AuthenticatedUserSchemeName";
        }).AddScheme<ValidTokenAuthenticationSchemeOptions, ValidTokenAuthenticationHandler>("AuthenticatedUserSchemeName", _ => { });

        services.AddAuthorization(options =>
        {
            options.AddPolicy("AuthorizedUserPolicyName", policy =>
            {
                //policy.RequireClaim(ClaimTypes.Sid,"authToken");
                policy.AddAuthenticationSchemes("AuthenticatedUserSchemeName");
                policy.AddRequirements(new ValidTokenAuthorizationRequirement());
            });
            services.AddSingleton<IAuthorizationHandler, ValidTokenAuthorizationHandler>();

Both the empty authentication handler and authorization handler are called (similar in setup to OP's respective posts) but the authorization handler still enforces our authorization policies.

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