how to check if object already exists in a list
Simply use Contains method:
bool alreadyExist = list.Contains(item);
Note that it works based on the equality function Equals. Check the example of the link above if you need to implement Equals function.
It depends on the needs of the specific situation. For example, the dictionary approach would be quite good assuming:
- The list is relatively stable (not a lot of inserts/deletions, which dictionaries are not optimized for)
- The list is quite large (otherwise the overhead of the dictionary is pointless).
If the above are not true for your situation, just use the method Any():
Item wonderIfItsPresent = ...
bool containsItem = myList.Any(item => item.UniqueProperty == wonderIfItsPresent.UniqueProperty);
This will enumerate through the list until it finds a match, or until it reaches the end.