How to use Http.GetJsonAsync() in Blazor Server App?

This is how this can work in the server-side Blazor:

Add reference to Microsoft.AspNetCore.Blazor.HttpClient package

 <PackageReference Include="Microsoft.AspNetCore.Blazor.HttpClient" Version="3.0.0-preview9.19465.2" PrivateAssets="all" />

In Startup.cs, add HttpClient to DI container:

public void ConfigureServices(IServiceCollection services)
{
 ...
    if (!services.Any(x => x.ServiceType == typeof(HttpClient)))
    {
        services.AddSingleton<HttpClient>();
    }
  ...
}

In your "service" class, inject HttpClient via constructor:

public PizaaMenuService(HttpClient httpClient)
{
    _httpClient = httpClient;
}

Then you should be able to use the HTTP and deserialization like this:

public async Task<Menu> GetPizzaMenuAsync()
        {
            string sUrl = _ApiUrlBase + "api/pizzamenu";
            return await _httpClient.GetJsonAsync<Menu>(sUrl);
        }

Note: as you are already running the application on the server, in case your API is located on the same server that serves the application, you possibly do not need to call the API via HttpClient; instead, you could directly instantiate your business objects (the same what you now do in your API controller).

The GetJsonAsync() method is an extension method for HttpClient, but it looks like what you are really trying to achieve here is one level of abstractiobn higher - i.e. deserialization to an array of Customers. RestClient.Net can do that in Blazor without the extra step.

Here is a similar call in a Blazor page:

private List<RestCountry> countries;

protected override async Task OnInitializedAsync()
{
    countries = await new Client(new NewtonsoftSerializationAdapter(), baseUri: new Uri("https://restcountries.eu/rest/v2/")).GetAsync<List<RestCountry>>();
}

Code Reference

This code works on both the server-side and client-side rendering and avoids the need to call GetJsonAsync. This article explains how to get the sample working.