Why does `numbers.addAll(2)` throw a compilation error in Dart, and how do you correctly add multiple items to a List?

Overview

numbers.addAll(2)

addAll()

Iterable<T>

T

The Error

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final numbers = <int>[1, 2, 3];

// Compilation error
numbers.addAll(2);
// Error: The argument type 'int' can't be assigned to the parameter type 'Iterable<int>'

// The signature is:
// void addAll(Iterable<E> iterable)
// It expects a List, Set, Iterable -- not a single element!

Correct Ways to Add Multiple Items

Method 1: addAll() with a List (Iterable)

dartCopy
final numbers = <int>[1, 2, 3];

// Pass a list (Iterable)
numbers.addAll([4, 5, 6]);
print(numbers); // [1, 2, 3, 4, 5, 6]

numbers.addAll({7, 8});        // Set is also Iterable
numbers.addAll({9: true}.keys); // Keys iterable

Method 2: add() for a Single Element

dartCopy
final numbers = <int>[1, 2, 3];

// add() for ONE element
numbers.add(4);
print(numbers); // [1, 2, 3, 4]

Method 3: Spread Operator

dartCopy
final original = [1, 2, 3];
final extras = [4, 5, 6];

// Create new list with spread
final combined = [...original, ...extras];
print(combined); // [1, 2, 3, 4, 5, 6]

Method 4: insert() / insertAll()

dartCopy
final list = [1, 2, 3];

list.insert(1, 99);            // Insert 99 at index 1
list.insertAll(0, [0, -1]);    // Insert multiple at index 0
print(list); // [0, -1, 1, 99, 2, 3]

Full Method Reference

add(element)

E

addAll(iterable)

Iterable<E>

insert(index, element)

insertAll(index, iterable)

[...list1, ...list2]

Summary

dartCopy
final list = <int>[1, 2, 3];

// Wrong -- single int, not iterable
// list.addAll(4);

// Correct options:
list.add(4);           // Add single element
list.addAll([4, 5]);   // Add multiple via list
final merged = [...list, 4, 5]; // New list via spread

Root cause:

textCopy

addAll()

is designed to add all elements from another iterable -- not a single element. Use

textCopy

add()

for one element,

textCopy

addAll([items])

for multiple.