public async Task SingleOrDefaultShouldReturnElement()
{
var sequence = new[] { 10 }.ToAsync();
Assert.AreEqual(10, await sequence.SingleOrDefault());
var sequence2 = new int[] { }.ToAsync();
Assert.AreEqual(0,await sequence2.SingleOrDefault());
var sequence3 = new int[] { 10, 11 }.ToAsync();
Assert.AreEqual(0,await sequence3.SingleOrDefault());
}
}