UnionEnumerator.cs

using System.Collections;
using System.Threading.Tasks;
 
namespace Waher.Persistence
{
    public class UnionEnumerator<T> : IAsyncEnumerator<T>
        where T : class
    {
        private readonly IAsyncEnumerator<T>[] enumerators;
        private readonly int nrEnumerators;
        private int position = 0;
 
        public UnionEnumerator(params IAsyncEnumerator<T>[] Enumerators)
        {
            this.enumerators = Enumerators;
            this.nrEnumerators = this.enumerators.Length;
        }
 
        public T Current => this.enumerators[this.position].Current;
 
        object IEnumerator.Current => this.enumerators[this.position].Current;
 
        public void Dispose()
        {
            int i;
 
            for (i = 0; i < this.nrEnumerators; i++)
                this.enumerators[i].Dispose();
        }
 
        public bool MoveNext()
        {
            while (this.position < this.nrEnumerators)
            {
                if (this.enumerators[this.position].MoveNext())
                    return true;
 
                this.position++;
            }
 
            return false;
        }
 
        public async Task<bool> MoveNextAsync()
        {
            while (this.position < this.nrEnumerators)
            {
                if (await this.enumerators[this.position].MoveNextAsync())
                    return true;
 
                this.position++;
            }
 
            return false;
        }
 
        public void Reset()
        {
            int i;
 
            for (i = 0; i <= this.position; i++)
                this.enumerators[i].Reset();
 
            this.position = 0;
        }
    }
}