How do I handle async operations in Startup.Configure?

The example code in the blog you linked to was only using sync-over-async to populate a database with example data; that call wouldn't exist in a production app.

First, I'd say that if you truly need Configure to be asynchronous, then you should raise an issue with the ASP.NET team so it's on their radar. It would not be too difficult for them to add support for a ConfigureAsync at this point (that is, before release).

Second, you've got a couple of approaches to the problem. You could use task.Wait (or better yet, task.GetAwaiter().GetResult(), which avoids the AggregateException wrapper if an error does occur). Or, you could cache the task rather than the result of the task (which works if IMemoryCache is more of a dictionary than some weird serialize-into-binary-array-in-memory thing - I'm looking at you, previous versions of ASP.NET).

If this is considered an acceptable practice, what - if any - error handling do I need?

Using GetAwaiter().GetResult() would cause the exception (if any) to propagate out of Configure. I'm not sure how ASP.NET would respond would be if configuring the application failed, though.

I haven't provided any mechanism to cancel the async operation.

I'm not sure how you can "cancel" the setup of an application, so I wouldn't worry about that part of it.

Dotnet Core 3.x offers better support for this.

First, you could create a class for your caching process. Have it implement IHostedService like below. There are just two functions to implement:

    private readonly IServiceProvider _serviceProvider;
    public SetupCacheService(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public async Task StartAsync(CancellationToken cancellationToken)
    {
        // Perform your caching logic here. 
        // In the below example I omit the caching details for clarity and 
        // instead show how to get a service using the service provider scope. 
        using (var scope = _serviceProvider.CreateScope())
        {
            // Example of getting a service you registered in the startup
            var sampleService = scope.ServiceProvider.GetRequiredService<IYourService>();

            // Perform the caching or database or whatever async work you need to do. 
            var results = sampleService.DoStuff();
            var cacheEntryOptions = new MemoryCacheEntryOptions(){ // cache options };

            // finish caching setup..
        }
    }

    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;

Now, in Starup.cs

    public virtual void ConfigureServices(IServiceCollection services)
    {
        // Normal service registration stuff. 
        // this is just an example. There are 1000x ways to do this. 
        services.AddTransient(IYourService, ConcreteService);

       // Here you register the async work from above, which will 
       // then be executed before the app starts running
       services.AddHostedService<SetupCacheService>();
    }

And that's it. Note that my solution to this relied heavily on Andrew Lock's article. I'm very grateful for him taking the time to write that up.

From the link I posted by Andrew Lock,

The services will be executed at startup in the same order they are added to the DI container, i.e. services added later in ConfigureServices will be executed later on startup.

Hopefully this helps anyone looking for Dotnet core 3.x+ approach.

You can do some asynchronous work, but the method is synchronous and you cannot change that. This means you need synchronously wait for async calls to be completed.

You don't want to return from a Startup method if the startup is not finished yet, right? Your solution seems to be all right.

As for exception handling: If there's a piece of work that your application can't run properly without, you should let the Startup method fail (see Fail-fast). If it isn't something critical I would enclose the relevant part in a try catch block and just log the problem for later inspection.

The answers in here do not always work correctly if your async code makes further async calls, especially if those are callbacks, then you may find the code deadlocks.

This has happens on numerous occasions for me and have used the Nito.AsyncEx with great effect.

using Nito.AsyncEx;

AsyncContext.Run(async () => { await myThing.DoAsyncTask(); });