App Settings in Client-Side Blazor

I’ve been spoiled by Asp.Net and expect app settings to just work, but there is no easily configured app settings story for client-side Blazor yet. What I’m looking for is the ability to set some settings on a per-environment (dev/test/beta/prod) through Azure App Service Application Settings. The goal is one build artifact that moves from environment to environment with zero changes.

If you host the Blazor app as static files you’d need to change the app settings file for each different environment. So, I’ve gone with Asp.Net Core Web Hosted. I’ve named the app AppSettingsExample, so in the solution we have AppSettingsExample.Client (the WASM app), AppSettingsExample.Server (the host app), and AppSettingExample.Shared (code that is shared between the Client and Server)

Loading Client App Settings

We’ll store and access the client app settings through the built-in configuration in AppSettingsExample.Server. To do that we’ll add an appsettings.json with the following values.

{
  "ClientAppSettings": {
    "BaseApiUrl": "http://somesite.com/api"
  }
}

We’ll also create a class in the AppSettingsExample.Shared project to hold those configurations.

public class ClientAppSettings
{
    public string BaseApiUrl { get; set; }
}

Then in the AppSettingsExample.Server’s Startup we’ll get a reference to the application’s configuration and store it in a local variable.

private readonly IConfiguration _configuration;

public Startup(IConfiguration configuration)
{
    _configuration = configuration;
}

This allows us to use that config to load the settings from appsettings.json and add it to the dependency injection config as a singleton.

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton(_configuration.GetSection("ClientAppSettings").Get<ClientAppSettings>());

Exposing the Settings to the Client

There isn’t an easy way to just pass the settings into the client-side Blazor app, so we’ll need the app to request them from the server. We’ll create a ClientAppSettingsController to AppSettingsExample.Server to serve up these settings.

[Route("api/[controller]")]
[ApiController]
public class ClientAppSettingsController : ControllerBase
{
    private readonly ClientAppSettings _clientAppSettings;

    public ClientAppSettingsController(ClientAppSettings clientAppSettings)
    {
        _clientAppSettings = clientAppSettings;
    }

    [HttpGet]
    public ClientAppSettings GetClientAppSettings()
    {
        return _clientAppSettings;
    }
}

Getting the Settings from the Client

This is where I had the most trouble. I need to completely load these settings before the application can move on. If I did it asynchronously, it would start running the Initialization and ParameterSet methods on the current page before the settings load completed. If I tried to force the asynchronous web request to complete synchronously by calling .Wait(), the application locked up.

To get around this we can create a component that loads the settings and once loaded displays its child content. Then we can wrap our content in this component to make sure it doesn’t start initializing or setting parameters until the settings are loaded. First, we create AppSettingsLoader.razor

@using AppSettingExample.Shared
@inject HttpClient http

@if (IsLoaded)
{
    @ChildContent
}

@code 
{
    [Parameter]
    public RenderFragment ChildContent { get; set; }

    public bool IsLoaded { get; set; }

    protected override async Task OnInitializedAsync()
    {
        await base.OnInitializedAsync();
        if (!IsLoaded)
        {
            var appSettings = await http.GetJsonAsync<ClientAppSettings>("api/ClientAppSettings");
            AppSettings.BaseApiUrl = appSettings.BaseApiUrl;
            IsLoaded = true;
        }
    }
}

Because we can’t (or I couldn’t) load the ClientAppSettings instance into the Dependency Injection to make it available throughout the application, I’m just putting the values in a static class.

Now, in the MainLayout.razor we can wrap the @body with the AppSettingsLoader

<AppSettingsLoader>
@Body
</AppSettingsLoader>

And finally, from the Index.razor page we can reference the AppSettings.BaseApiUrl. To prove it out, I’ll just display it on the page.

@page "/"
<h1>Hello, world!</h1>

@AppSettings.BaseApiUrl

And now we can set any setting we like in the ClientAppSettings sections of the AppSettings.json and it will be treated just like a normal app setting, including being able to set the setting through Configuration section of an Azure App Service.