How do I redirect users to the login page when a session has timed out in Blazor?

To redirect users to your login page when their session has expired, create a RedirectLogin Razor component and implement the AuthenticationState to check whether sessions are expired or valid.
Follow these steps to redirect to a login page when sessions time out in Blazor:

Create a RedirectLogin component to redirect users to a login page when their session has expired.

[RedirectLogin.razor]

@inject NavigationManager UriHelper
  
 @code {
     [CascadingParameter]
     public Task<AuthenticationState> StateAuthenticate { get; set; }
  
     protected override async Task OnInitializedAsync()
     {
         var authenticationState = await StateAuthenticate;
         if (authenticationState?.User?.Identity is null || !authenticationState.User.Identity.IsAuthenticated)
         {
             var returnUri = UriHelper.ToBaseRelativePath(UriHelper.Uri);
             if (string.IsNullOrWhiteSpace(returnUri))
             {
                 UriHelper.NavigateTo("YourLoginPath", true);
             }
             else
             {
                 UriHelper.NavigateTo($"YourLoginPath?returnUrl={returnUri}", true);
             }
         }
     }
 }

Now, initialize the RedirectLogin component in App.razor to identify whether a session is expired or valid.

[App.razor]

<Router AppAssembly="@typeof(Program).Assembly">
	<Found Context="routeData">
		<AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)">
			<NotAuthorized>
				<RedirectLogin />
			</NotAuthorized>
			// . . .
		</AuthorizeRouteView>
	</Found>
	// . . .
</Router> 

View Sample in GitHub