Mutable collections are those whose state can be changed. For instance, ``Array`` and ``List<T>`` are mutable, but ``System.Collections.ObjectModel.ReadOnlyCollection<T>`` and ``System.Collections.Immutable.ImmutableList<T>`` are not. Mutable collection class members should not be returned to a caller or accepted and stored directly. Doing so leaves you vulnerable to unexpected changes in your class state.

Instead use and store a copy of the mutable collection, or return an immutable collection wrapper, e.g. ``System.Collections.ObjectModel.ReadOnlyCollection<T>``.

Note that you can't just return your mutable collection through the ``IEnumerable<T>`` interface because the caller of your method/property could cast it down to the mutable type and then change it.

This rule checks that mutable collections are not stored or returned directly.

== Noncompliant Code Example

----
class A 
{
    private List<string> names = new List<string>();

    public ICollection<string> Names => names; // Noncompliant

    public IEnumerable<string> GetNames()  // Noncompliant
    {
        return names;
    }

    public void SetNames(List<string> strings) 
    {
        this.names = strings;  // Noncompliant
    }
}
----

== Compliant Solution

----
class A 
{
    private List<string> names = new List<string>();
    private ReadOnlyCollection<string> readOnlyNames = new ReadOnlyCollection<string>(names);

    public ICollection<string> Names => readOnlyNames; // Return a collection wrapper

    public IEnumerable<string> GetNames()
    {
        names.ToList(); // Make a copy
    }

    public void SetNames(List<string> strings) 
    {
        this.names.Clear();
        this.names.AddRange(strings); // Make a copy
    }
}
----

include::../see.adoc[]