AsyncLazy will cache the exceptions

[gravity00]

AspNetCoreDiagnosticScenarios/Scenarios/Services/RemoteConnection.cs

Lines 55 to 90 in 5be73c4

	public class LazyRemoteConnection : IRemoteConnection
	{
	private readonly AsyncLazy<RemoteConnection> _connectionTask;
	public LazyRemoteConnection(RemoteConnectionFactory remoteConnectionFactory)
	{
	_connectionTask = new AsyncLazy<RemoteConnection>(() => remoteConnectionFactory.ConnectAsync());
	}
	public async Task PublishAsync(string channel, string message)
	{
	var connection = await _connectionTask.Value;
	await connection.PublishAsync(channel, message);
	}
	public async Task DisposeAsync()
	{
	// Don't connect just to dispose
	if (!_connectionTask.IsValueCreated)
	{
	return;
	}
	var connection = await _connectionTask.Value;
	await connection.DisposeAsync();
	}
	private class AsyncLazy<T> : Lazy<Task<T>>
	{
	public AsyncLazy(Func<Task<T>> valueFactory) : base(valueFactory)
	{
	}
	}
	}

I believe it should be at least noted that if an exception occurs inside the factory function, the exception will be cached by the Task and since everything is a singleton, the application won't be able to recover from the failed state without some restart (unless I'm missing something).

Obviously this may or may not be a problem, depends on the developer needs, but usually this isn't the intended behavior. Most commonly is like not adding an item to a cache if the initialization fails.

[davidfowl]

That doesn’t matter in this case but it does matter for the caching case so I’ll update that.

Thanks for the feedback

[wlidha81]

Same comment as gravity00, when an exception occurs at the fetch if the value to be cached, exception is cached instead. it might be also needed to handle the case when valueFactory returns (Null OR Default) value that we don't want to cache, in that case cache entry should be disposed rather than being saved as Task that returns null.

[bartelink]

I have a TaskCell that implements this wrapping over LazyTask: https://github.com/jet/equinox/blob/master/src/Equinox.Core/TaskCell.fs
Where LazyTask is: https://github.com/jet/equinox/blob/master/src/Equinox/LazyTask.fs

(it also manages expiration of the cached value - see the tests: https://github.com/jet/equinox/blob/master/tests/Equinox.Core.Tests/TaskCellTests.fs https://github.com/jet/equinox/blob/master/tests/Equinox.Core.Tests/LazyTaskTests.fs)
Can probably be ported to C# (it emanated from https://stackoverflow.com/a/33942013/11635)