C# List - Group By - Without Linq

Question

I have an object:

IObject
{
    string Account,
    decimal Amount
}

How do I group by Account and Sum the Amount, returning a List without

Answer 1

Use a dictionary to hold the results. Locating an item in a dictionary is close to an O(1) operation, so it's a lot faster than searching for items in a list.

Dictionary sum = new Dictionary();

foreach (IObject obj in objects) {
   if (sum.ContainsKey(obj.Account)) {
      sum[obj.Account].Amount += obj.Amount;
   } else {
      sum.Add(obj.Account, obj.Amount);
   }
}