If you do that, your original data will be lost. Is that what you want to do?
If you want to merge the two dictionaries, you need to add the contents from the returned collection to the old collection.
[Update]
----------
In response to your comment, and using some imaginary code for those extra classes:
class CaseService
{
internal bool ValidStageTransition(int p1, int p2, int p3,
out Dictionary<string, string> newList)
{
newList = new Dictionary<string, string>();
newList.Add("aaa", "1");
return true;
}
}
Here's how you can merge in the extra collection.
Dictionary<string, string> _list = new Dictionary<string,string>();
_list.Add("bbb", "a0");
Dictionary<string, string> newList = null;
bool valid = new CaseService().ValidStageTransition(0, 0, 0, out newList);
foreach (var item in newList)
{
_list.Add(item.Key, item.Value);
}