Check whether a list contain an attribute of an object in dart

Contains() compares the whole objects.

Besides overriding == operator or looping over, you can use list's singleWhere method:

  if ((myItemsList.singleWhere((it) => it.itemId == myitem.itemId,
          orElse: () => null)) != null) {

Edit: As Dharaneshvar experienced and YoApps mentioned in the comments .singleWhere raises StateError when more elements are found.

This is desired when you expect unique elements such as in the case of comparing IDs.

Raised error is the friend here as it shows that there is something wrong with the data.

For other cases .firstWhere() is the right tool:

  if ((myItemsList.firstWhere((it) => it.itemName == myitem.itemName,
          orElse: () => null)) != null) {

// EO Edit

Whole example:

List<MyItem> myItemsList = new List();
​
class MyItem {
  final String itemId;
  final String itemName;
  int itemQuantity;
​
  MyItem({
    this.itemId,
    this.itemName,
    this.itemQuantity,
  });
}
​
void main() {
  MyItem myitem = new MyItem(
    itemId: "id00",
    itemName: "name",
    itemQuantity: 50,
  );
​
  myItemsList.add(myitem);
​
  String idToCheck = "id00";
​
  if ((myItemsList.singleWhere((it) => it.itemId == idToCheck,
          orElse: () => null)) != null) {
    
    print('Already exists!');
  } else {
    print('Added!');
  }
}

As already said before, contains compares two Objects with the == operator. So you currently compare MyItem with String itemId, which will never be the same.

To check whether myItemsList contains myitem.itemId you can use one of the following:

myItemsList.map((item) => item.itemId).contains(myitem.itemId);

or

myItemsList.any((item) => item.itemId == myitem.itemId);

You're using contains slightly wrong.

From: https://api.dartlang.org/stable/2.2.0/dart-core/Iterable/contains.html

bool contains(Object element) {
  for (E e in this) {
    if (e == element) return true;
  }
  return false;
}

You can either override the == operator, see: https://dart-lang.github.io/linter/lints/hash_and_equals.html

@override    
bool operator ==(Object other) => other is Better && other.value == value;

Or you can loop over your list and search the normal way one by one, which seems slightly easier.