Enumerable/AsyncEnumerable conversions

EnumerableExtensions.cs

using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Threading.Tasks;

internal static class EnumerableExtensions
{
	public static IAsyncEnumerable<T>? ToAsyncEnumerable<T>(IEnumerable<T>? data) =>
		data == null ? null : ToAsyncEnumerableImpl(data);

	async static IAsyncEnumerable<T> ToAsyncEnumerableImpl<T>(IEnumerable<T> data)
	{
		foreach (var dataItem in data)
			yield return dataItem;
	}

	[return: NotNullIfNotNull(nameof(data))]
	public static IEnumerable<T>? ToEnumerable<T>(this IAsyncEnumerable<T>? data) =>
		data == null ? null : ToEnumerableImpl(data);

	static IEnumerable<T> ToEnumerableImpl<T>(IAsyncEnumerable<T> data)
	{
		var enumerator = data.GetAsyncEnumerator();

		try
		{
			while (WaitForValueTask(enumerator.MoveNextAsync()))
				yield return enumerator.Current;
		}
		finally
		{
			WaitForValueTask(enumerator.DisposeAsync());
		}
	}

	static void WaitForValueTask(ValueTask valueTask)
	{
		var valueTaskAwaiter = valueTask.GetAwaiter();
		if (valueTaskAwaiter.IsCompleted)
			return;

		// Let the task complete on a thread pool thread while we block the main thread
		Task.Run(valueTask.AsTask).GetAwaiter().GetResult();
	}

	static T WaitForValueTask<T>(ValueTask<T> valueTask)
	{
		var valueTaskAwaiter = valueTask.GetAwaiter();
		if (valueTaskAwaiter.IsCompleted)
			return valueTaskAwaiter.GetResult();

		// Let the task complete on a thread pool thread while we block the main thread
		return Task.Run(valueTask.AsTask).GetAwaiter().GetResult();
	}
}