IMemoryCache does not save data at application startup

Question

I created a fresh ASP.NET Core Web API project. Here is ConfigureServices in Startup.cs:

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        services.AddMemoryCache();
        var serviceProvider = services.BuildServiceProvider();
        var cache = serviceProvider.GetService<IMemoryCache>();

        cache.Set("key1", "value1");
        //_cahce.Count is 1
    }

As you see I add an item to IMemoryCache. Here is my controller:

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    private readonly IMemoryCache _cache;
    public ValuesController(IMemoryCache cache)
    {
        _cache = cache;

    }
    [HttpGet("{key}")]
    public ActionResult<string> Get(string key)
    {
        //_cahce.Count is 0
        if(!_cache.TryGetValue(key, out var value))
        {
            return NotFound($"The value with the {key} is not found");
        }

        return value + "";
    }
}

When I request https://localhost:5001/api/values/key1, the cache is empty and I receive a not found response.

Answer 1

In short, the cache instance you're setting the value in is not the same as the one that is later being retrieved. You cannot do stuff like while the web host is being built (i.e. in ConfigureServices/Configure. If you need to do something on startup, you need to do it after the web host is built, in Program.cs:

public class Program
{
    public static void Main(string[] args)
    {
        var host = CreateWebHostBuilder(args).Build();

        var cache = host.Services.GetRequiredService<IMemoryCache>();
        cache.Set("key1", "value1");

        host.Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
 }