When To Use ToList() and ToArray()?

It’s not just a matter of taste, the choice between .ToList() and .ToArray() should be intentional, based on how the resulting collection will be consumed.

If you’ve worked with LINQ in C#, you’ve probably written something like this:

var items = source.Where(x => x.IsValid).ToList();
// or
var items = source.Where(x => x.IsValid).ToArray();

Both materialize the query results immediately, but they behave differently.

The Simple Rule

Use .ToList(): If the consumer will modify the collection (add/remove items).
Use .ToArray(): If the consumer will only read or mutate existing values without changing the collection size.

Example

// We plan to add/remove elements later
var list = source.Where(x => x.IsValid).ToList();
list.Add(newItem);   // works
list.RemoveAt(0);    // works

// Array is fixed size, this won't work
var array = source.Where(x => x.IsValid).ToArray();
array.Add(newItem); // compile-time error

Performance Considerations

For small collections, the performance difference is negligible; pick the one that matches your intent.
For collections around 10,000+ items, benchmarks have shown that .ToList() was slightly faster than .ToArray() in older .NET versions.
However, with .NET 9, significant optimizations were made to .ToArray(), making it highly efficient even for large datasets.

Next time you materialize a LINQ query, pause and pick the one that aligns with how the collection will actually be used.