Inefficient immutable collection modifications

This article describes performance insights when modifying immutable collections such as ImmutableList<T>.

Cause

Modifying an immutable collection (such as Add, Remove, or Insert) results in slow performance.

Performance insight description

Each modification to an immutable collection creates a new copy of the collection, resulting in excessive memory allocations and CPU usage. This is especially inefficient when modifications are performed frequently, in loops, or in performance-critical code paths.

If possible, use a mutable collection (such as List<T>) during construction or in hot paths, and convert to an immutable collection only when necessary. Mutable collections allow for efficient in-place modifications, while immutable collections are best used for thread safety and scenarios where modifications are infrequent.

How to investigate a warning

In the CPU Usage tool, click View source code to find where immutable collections are being modified. If possible, refactor to use a mutable collection in performance-sensitive code, and convert to an immutable collection only when needed.

See also

Immutable collections documentation