== Why is this an issue?

A writable collection property can be replaced by a completely different collection. Making it ``++readonly++`` prevents that while still allowing individual members to be set. If you want to allow the replacement of the whole collection the recommended pattern is to implement a method to remove all the elements (e.g. ``++System.Collections.List<T>.Clear++``) and a method to populate the collection (e.g. ``++System.Collections.List<T>.AddRange++``).


This rule raises an issue when an externally visible writable property is of a type that implements ``++System.Collections.ICollection++`` or ``++System.Collections.Generic.ICollection<T>++``.


=== Noncompliant code example

[source,text]
----
using System;
using System.Collections;

namespace MyLibrary
{
  public class Foo
  {
    List<string> strings;

    public List<string> SomeStrings
    {
      get { return strings; }
      set { strings = value; } // Noncompliant
    }
  }
}
----


=== Compliant solution

[source,text]
----
using System;
using System.Collections;

namespace MyLibrary
{
  public class Foo
  {
    List<string> strings;

    public List<string> SomeStrings
    {
      get { return strings; }
    }
  }
}
----


=== Exceptions

This rule does not raise issues for

* ``++string++``, ``++Array++`` and ``++PermissionSet,++``
* properties marked as ``++DataMemberAttribute++``
* classes marked as ``++Serializable++``
* properties overriding a base class member
* properties implementing interface