public override async Task ValidateClientAuthentication(ValidateClientAuthenticationContext context)
        {
            // Note: client authentication is not mandatory for non-confidential client applications like mobile apps
            // (except when using the client credentials grant type) but this authorization server uses a safer policy
            // that makes client authentication mandatory and returns an error if client_id or client_secret is missing.
            // You may consider relaxing it to support the resource owner password credentials grant type
            // with JavaScript or desktop applications, where client credentials cannot be safely stored.
            // In this case, call context.Skipped() to inform the server middleware the client is not trusted.
            if (string.IsNullOrEmpty(context.ClientId) || string.IsNullOrEmpty(context.ClientSecret))
            {
                context.Rejected(
                    error: "invalid_request",
                    description: "Missing credentials: ensure that your credentials were correctly " +
                                 "flowed in the request body or in the authorization header");

                return;
            }

            var database = context.HttpContext.RequestServices.GetRequiredService<ApplicationContext>();

            // Retrieve the application details corresponding to the requested client_id.
            var application = await (from entity in database.Applications
                                     where entity.ApplicationID == context.ClientId
                                     select entity).SingleOrDefaultAsync(context.HttpContext.RequestAborted);

            if (application == null)
            {
                context.Rejected(
                    error: "invalid_client",
                    description: "Application not found in the database: ensure that your client_id is correct");

                return;
            }

            if (!string.Equals(context.ClientSecret, application.Secret, StringComparison.Ordinal))
            {
                context.Rejected(
                    error: "invalid_client",
                    description: "Invalid credentials: ensure that you specified a correct client_secret");

                return;
            }

            context.Validated();
        }
        public override Task ValidateClientAuthentication(ValidateClientAuthenticationContext context) {
            if (context.ClientId == "AspNetContribSample") {
                // Note: the context is marked as skipped instead of validated because the client
                // is not trusted (JavaScript applications cannot keep their credentials secret).
                context.Skipped();
            }

            else {
                // If the client_id doesn't correspond to the
                // intended identifier, reject the request.
                context.Rejected();
            }

            return Task.FromResult(0);
        }