SingleOrDefault method
Returns the single element in the element, optionally matching a
condition
, or a defaultValue
if no such element exists.
If condition
is omitted, SingleOrDefault will return the single value
in this enumerable. If the enumerable is empty, the value specified by
defaultValue
is returned instead. If the enumerable contains more than
one element, an OperationError is thrown.
In this scenario, the SingleOrDefault function will short-circuit after iterating a maximum of two elements into the enumerable.
If condition
is supplied, SingleOrDefault will iterate over the entire
enumerable, applying the condition
function to each element. At the end
of the iteration, if a single element matches the condition
, that element
is returned. If no elements match the condition
, the value specified by
defaultValue
is returned instead. If more than one element matches the
condition
, an OperationError is thrown.
In this scenario, the SingleOrDefault function will short-circuit after
finding a second element that matches the condition
. In the worst-case
scenario, SingleOrDefault will iterate over the entire enumerable.
Implementation
T SingleOrDefault(T defaultValue, [Condition<T> condition]) {
final _iterator = this.iterator;
if (!_iterator.moveNext()) return defaultValue;
bool valueSet = false;
T value;
if (condition != null) {
do {
if (condition(_iterator.current)) {
if (valueSet)
throw OperationError(
'More than one element matches the condition.');
value = _iterator.current;
valueSet = true;
}
} while (_iterator.moveNext());
if (!valueSet) return defaultValue;
return value;
}
do {
if (valueSet)
throw OperationError('More than one element exists in the enumerable.');
value = _iterator.current;
valueSet = true;
} while (_iterator.moveNext());
if (!valueSet) throw UnexpectedStateError();
return value;
}