Why should you set Initial Capacity when creating List<T> and Dictionary<TKey, TValue>?

Short answer

It prevents repeated internal array allocations and copying data on the Heap each time the collection grows beyond its current size.

Explanation

List<T> starts with a small internal array (for example, four elements). Adding a fifth element creates a new array of double the size (eight), copies all elements, and leaves the old array for the GC. Adding 10,000 elements this way repeats the costly process 12 times. new List<T>(10000) allocates the required array once, avoiding extra copies and maximizing speed.

Code example

// Preallocate the capacity
var list = new List<Customer>(expectedCount);
var dict = new Dictionary<int, Customer>(expectedCount);

Common mistakes

• Creating lists and dictionaries without initial capacity when reading a known-size dataset from a database.

Follow-up questions

• Why are string objects Immutable, and when should you use StringBuilder or String.Create?

Sources

• Microsoft Learn — List<T>.Capacity Property