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[]

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== Comments And Links
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include::comments-and-links.adoc[]
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