Collections · interview question

What happens when this code runs?

What does this code output?Medium

var list = new List<int> { 1, 2, 3 };
foreach (var x in list)
{
    if (x == 2) list.Remove(x);
}
Console.WriteLine(list.Count);

It throws InvalidOperationException.It prints 2.It prints 3.It prints 1.

Answer and explanation

Correct answer

A. It throws InvalidOperationException.

Why

List<T> tracks a version number; modifying it during foreach makes the enumerator throw "Collection was modified". Use list.RemoveAll(x => x == 2), iterate a copy, or loop backwards with an index.

References

Microsoft Learn: Collections and data structures

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