App.razor

@inject NavigationManager NavigationManager
@inject AuthenticationStateProvider AuthenticationStateProvider

<CascadingAuthenticationState>
    <Router AppAssembly="@typeof(Program).Assembly">
        <Found Context="routeData">
            <AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)">
                <NotAuthorized>
                    @{
                        HandleNotAuthorized();
                    }
                </NotAuthorized>
            </AuthorizeRouteView>
        </Found>
        <NotFound>
            <LayoutView Layout="@typeof(MainLayout)">
                <p>Seite nicht gefunden.</p>
            </LayoutView>
        </NotFound>
    </Router>
</CascadingAuthenticationState>

@code {

    private async void HandleNotAuthorized()
    {
        var authState = await AuthenticationStateProvider.GetAuthenticationStateAsync();
        if (authState.User.Identity.IsAuthenticated)
        {
            NavigationManager.NavigateTo("/403");
        }
        else
        {
            var returnUrl = NavigationManager.ToBaseRelativePath(NavigationManager.Uri);
            NavigationManager.NavigateTo($"/401?returnUrl={Uri.EscapeDataString(returnUrl)}", true);
        }
    }


}