reactive/Ix.NET/Source/System.Interactive/System/Linq/Operators/Defer.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.Collections.Generic;

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Creates an enumerable sequence based on an enumerable factory function.
        /// </summary>
        /// <typeparam name="TResult">Result sequence element type.</typeparam>
        /// <param name="enumerableFactory">Enumerable factory function.</param>
        /// <returns>Sequence that will invoke the enumerable factory upon a call to GetEnumerator.</returns>
        public static IEnumerable<TResult> Defer<TResult>(Func<IEnumerable<TResult>> enumerableFactory)
        {
            if (enumerableFactory == null)
                throw new ArgumentNullException(nameof(enumerableFactory));

            return DeferCore(enumerableFactory);
        }

        private static IEnumerable<TSource> DeferCore<TSource>(Func<IEnumerable<TSource>> enumerableFactory)
        {
            foreach (var item in enumerableFactory())
            {
                yield return item;
            }
        }
    }
}