>>508の指摘にそって改良

var 元 = new[]
{
new {Index = 0, Value = "あ"},
new {Index = 1, Value = "い"},
new {Index = 3, Value = "う"}
};

var array = Enumerable.Range(0, 4)
.Select(index => 元.FirstOrDefault(moto => moto.Index == index)?.Value)
.ToArray();