reactive/Ix.NET/Source/System.Interactive.Async/Using.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static IAsyncEnumerable<TSource> Using<TSource, TResource>(Func<TResource> resourceFactory, Func<TResource, IAsyncEnumerable<TSource>> enumerableFactory) where TResource : IDisposable
        {
            if (resourceFactory == null)
                throw new ArgumentNullException(nameof(resourceFactory));
            if (enumerableFactory == null)
                throw new ArgumentNullException(nameof(enumerableFactory));

            return Create(() =>
                          {
                              var resource = resourceFactory();
                              var e = default(IAsyncEnumerator<TSource>);

                              try
                              {
                                  e = enumerableFactory(resource)
                                      .GetEnumerator();
                              }
                              catch (Exception)
                              {
                                  resource.Dispose();
                                  throw;
                              }

                              var cts = new CancellationTokenDisposable();
                              var d = Disposable.Create(cts, resource, e);

                              var current = default(TSource);

                              return Create(
                                  async ct =>
                                  {
                                      bool res;
                                      try
                                      {
                                          res = await e.MoveNext(cts.Token)
                                                       .ConfigureAwait(false);
                                      }
                                      catch (Exception)
                                      {
                                          d.Dispose();
                                          throw;
                                      }
                                      if (res)
                                      {
                                          current = e.Current;
                                          return true;
                                      }
                                      d.Dispose();
                                      return false;
                                  },
                                  () => current,
                                  d.Dispose,
                                  null
                              );
                          });
        }
    }
}