Get & Set Indexer

How can I both get and set a value in an object in my indexer? DataIdxer_Instance[3].Accum += 123000; Error - cannot modify the return value of ...DataIdxer.this[int] Because it is NOT a variable. public class DataIdxer { protected ArrayList Datas = new ArrayList(); public DataObj this[int idx] { get { if (idx > -1 && idx < Datas.Count) { return ((DataObj)Datas[idx]); } else { throw new InvalidOperationException("[Data indexer get item] index out of range"); } } set { if (idx > -1 && idx < Datas.Count) { Datas[idx] = value; } else if (idx == Datas.Count) { Datas.Add(value); } else { throw new InvalidOperationException("[Datas indexer set item outofrange]"); } } } } public struct DataObj { public Int64 Accum; public DataObj(int D) { Accum = D; } }