private static async Task <T> FirstAsync <T>(IAsyncEnumerable <T> source, Boolean throwIfNone)
{
T retVal;
var enumerator = source.GetAsyncEnumerator();
try
{
var success = await enumerator.WaitForNextAsync();
retVal = success ? enumerator.TryGetNext(out success) : default;
if (!success)
{
if (throwIfNone)
{
throw AsyncProviderUtilities.EmptySequenceException();
}
else
{
retVal = default;
}
}
}
finally
{
await enumerator.DisposeAsync();
}
return(retVal);
}
/// <summary>
/// Similarly to <see cref="System.Linq.Enumerable.Aggregate{TSource}(IEnumerable{TSource}, Func{TSource, TSource, TSource})"/> method, this method potentially asynchronously enumerates this <see cref="IAsyncEnumerable{T}"/> and aggregates a single value using the given <paramref name="asyncFunc"/> potentially asynchronous callback.
/// </summary>
/// <typeparam name="T">The type of elements being enumerated.</typeparam>
/// <param name="source">This <see cref="IAsyncEnumerable{T}"/>.</param>
/// <param name="asyncFunc">The potentially asynchronous callback function to perform aggregation. First argument is previous element, second argument is current element, and return value is the new aggregated value.</param>
/// <returns>An aggregated value.</returns>
/// <exception cref="NullReferenceException">If this <see cref="IAsyncEnumerable{T}"/> is <c>null</c>.</exception>
/// <exception cref="ArgumentNullException">If <paramref name="asyncFunc"/> is <c>null</c>.</exception>
/// <exception cref="InvalidOperationException">If this <see cref="IAsyncEnumerable{T}"/> does not contain at least one element.</exception>
public async Task <T> AggregateAsync <T>(IAsyncEnumerable <T> source, Func <T, T, ValueTask <T> > asyncFunc)
{
ArgumentValidator.ValidateNotNullReference(source);
ArgumentValidator.ValidateNotNull(nameof(asyncFunc), asyncFunc);
var state = INITIAL;
T prev = default;
await source.EnumerateAsync(async item =>
{
if (state == INITIAL)
{
prev = item;
Interlocked.Exchange(ref state, FIRST_SEEN);
}
else
{
prev = await asyncFunc(prev, item);
}
});
return(state == FIRST_SEEN ? prev : throw AsyncProviderUtilities.EmptySequenceException());
}
public Task <T> AggregateAsync <T>(IAsyncEnumerable <T> source, Func <T, T, ValueTask <T> > asyncFunc) => throw AsyncProviderUtilities.EmptySequenceException();