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Collection properties should be read only







Breaking Change


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.

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.

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.

Do not exclude 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.