GetOrAddAsync for ConcurrentDictionary<TKey, ValueTask<TValue>> -- https://stackoverflow.com/questions/54117652/concurrentdictionary-getoradd-async

ConcurrentDictionaryExtensions.cs

/// <summary>
/// Returns an existing task from the concurrent dictionary, or adds a new task
/// using the specified asynchronous factory method. Concurrent invocations for
/// the same key are prevented, unless the task is removed before the completion
/// of the delegate. Failed tasks are evicted from the concurrent dictionary.
/// </summary>
public static ValueTask<TValue> GetOrAddAsync<TKey, TValue>(
    this ConcurrentDictionary<TKey, ValueTask<TValue>> source, TKey key,
    Func<TKey, Task<TValue>> valueFactory)
{
    ArgumentNullException.ThrowIfNull(source);
    ArgumentNullException.ThrowIfNull(valueFactory);
    ValueTask<TValue> currentTask;
    if (source.TryGetValue(key, out currentTask)) return currentTask;

    Task<Task<TValue>> newTaskTask = new(() => valueFactory(key));
    ValueTask<TValue> newTask = default;
    newTask = new(newTaskTask.Unwrap().ContinueWith(task =>
    {
        if (task.IsCompletedSuccessfully)
            source.TryUpdate(key, new(task.Result), newTask);
        else
            source.TryRemove(KeyValuePair.Create(key, newTask));
        return task;
    }, default, TaskContinuationOptions.DenyChildAttach | TaskContinuationOptions.ExecuteSynchronously,
        TaskScheduler.Default).Unwrap());

    currentTask = source.GetOrAdd(key, newTask);
    if (currentTask == newTask) newTaskTask.RunSynchronously(TaskScheduler.Default);

    return currentTask;
}