CA2227: Collection properties should be read only

TypeName CollectionPropertiesShouldBeReadOnly
CheckId CA2227
Category Microsoft.Usage
Breaking Change Breaking


An externally visible writable property is a type that implements System.Collections.ICollection. Arrays, indexers (properties with the name 'Item'), and permission sets are ignored by the rule.

Rule Description

A writable collection property allows a user to replace the collection with a completely different collection. A read-only property stops the collection from being replaced but still allows the individual members to be set. If replacing the collection is a goal, the preferred design pattern is to include a method to remove all the elements from the collection and a method to re-populate the collection. See the Clear and AddRange methods of the System.Collections.ArrayList class for an example of this pattern.

Both binary and XML serialization support read-only properties that are collections. The System.Xml.Serialization.XmlSerializer class has specific requirements for types that implement ICollection and System.Collections.IEnumerable in order to be serializable.

How to Fix Violations

To fix a violation of this rule, make the property read-only and, if the design requires it, add methods to clear and re-populate the collection.

When to Suppress Warnings

Do not suppress a warning from this rule.


The following example shows a type with a writable collection property and shows how the collection can be replaced directly. Additionally, the preferred manner of replacing a read-only collection property using Clear and AddRange methods is shown.

using System;
using System.Collections;

namespace UsageLibrary
   public class WritableCollection
      ArrayList strings;

      public ArrayList SomeStrings
         get { return strings; }

         // Violates the rule.
         set { strings = value; }

      public WritableCollection()
         strings = new ArrayList(
            new string[] {"IEnumerable", "ICollection", "IList"} );

   class ReplaceWritableCollection
      static void Main()
         ArrayList newCollection = new ArrayList(
            new string[] {"a", "new", "collection"} );

         // strings is directly replaced with newCollection.
         WritableCollection collection = new WritableCollection();
         collection.SomeStrings = newCollection;

         // newCollection is added to the cleared strings collection.
Imports System
Imports System.Collections

Namespace UsageLibrary

   Public Class WritableCollection
      Dim strings As ArrayList

      Property SomeStrings As ArrayList
            Return strings
         End Get

         ' Violates the rule.
            strings = Value
         End Set
      End Property

      Sub New()
         strings = New ArrayList( _
            New String() {"IEnumerable", "ICollection", "IList"} )
      End Sub

   End Class

   Class ViolatingVersusPreferred

      Shared Sub Main()
         Dim newCollection As New ArrayList( _
            New String() {"a", "new", "collection"} )

         ' strings is directly replaced with newCollection.
         Dim collection As New WritableCollection()
         collection.SomeStrings = newCollection

         ' newCollection is added to the cleared strings collection.
      End Sub

   End Class

End Namespace
using namespace System;
using namespace System::Collections;

namespace UsageLibrary
   public ref class WritableCollection
      // Violates the rule.
      property ArrayList^ SomeStrings;

         SomeStrings = gcnew ArrayList(
            gcnew array<String^> {"IEnumerable", "ICollection", "IList"} );

using namespace UsageLibrary;

void main()
   ArrayList^ newCollection = gcnew ArrayList(
      gcnew array<String^> {"a", "new", "collection"} );

   // strings is directly replaced with newCollection.
   WritableCollection^ collection = gcnew WritableCollection();
   collection->SomeStrings = newCollection;

   // newCollection is added to the cleared strings collection.

CA1819: Properties should not return arrays