Question

I am trying

   Dictionary<int, int> list = new Dictionary<int, int>();

   for (int i = 0; i < IDList.Count; i++)
   {
     list.Add(Convert.ToInt32(Cmd.Parameters["@ReturnVal"].Value.ToString()), IDList[i]);
   }
   return list;

When I addi a value twice there is no error but the third time it shows the error

an item with the same key has already been added. dictionary

Someone please tell me how to solve this.

Answer 1

Assumption #1:

You name your dictionary “list”, which suggests, you are not using a dictionary for what it was ment for and should possibly be using a List<T> instead.

Dictionary requires that each key (the first parameter to Add()) is unique, however Cmd.Parameters["@ReturnVal"] looks suspiciously like it is the Parameters property of an SqlCommand (or one of its parent classes). If so, it will always return the same value in the code you show.

So, if you really want to create a collection of tuples where the first value always is “@ReturnValue” and the second value is IDList[i], then you should use something like List<Tuple<int, int>> instead.

Example:

  List<Tuple<int, int>> list = new List<Tuple<int, int>>();

  for (int i = 0; i < IDList.Count; i++)
  {
    list.Add(
      Tuple.Create(Convert.ToInt32(Cmd.Parameters["@ReturnVal"].Value), IDList[i])
    );
  }

  return list;

Assumption #2:

Actually, the IDList[i] values (again, as the name suggests) are the unique values, i.e. your keys, and you simply have the Dictionary<TKey,TValue>.Add() parameters the wrong way around.