CA2227: Collection properties should be read only

Cause

An externally visible, writable property is of a type that implements System.Collections.ICollection. This rule ignores arrays, indexers (properties with the name 'Item'), immutable collections, readonly collections, and permission sets.

Rule description

A writable collection property allows a user to replace the collection with a completely different collection. A read-only or init-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 repopulate 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 or init-only. If the design requires it, add methods to clear and repopulate the collection.

When to suppress warnings

You can suppress the warning if the property is part of a Data Transfer Object (DTO) class.

Otherwise, do not suppress warnings from this rule.

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA2227
// The code that's violating the rule is on this line.
#pragma warning restore CA2227

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.CA2227.severity = none

For more information, see How to suppress code analysis warnings.

Example

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

public class WritableCollection
{
    public ArrayList SomeStrings
    {
        get;

        // This set accessor violates rule CA2227.
        // To fix the code, remove this set accessor or change it to init.
        set;
    }

    public WritableCollection()
    {
        SomeStrings = new ArrayList(new string[] { "one", "two", "three" });
    }
}

class ReplaceWritableCollection
{
    static void Main2227()
    {
        ArrayList newCollection = new ArrayList(new string[] { "a", "new", "collection" });

        WritableCollection collection = new WritableCollection();

        // This line of code demonstrates how the entire collection
        // can be replaced by a property that's not read only.
        collection.SomeStrings = newCollection;

        // If the intent is to replace an entire collection,
        // implement and/or use the Clear() and AddRange() methods instead.
        collection.SomeStrings.Clear();
        collection.SomeStrings.AddRange(newCollection);
    }
}

Public Class WritableCollection

    ' This property violates rule CA2227.
    ' To fix the code, add the ReadOnly modifier to the property:
    ' ReadOnly Property SomeStrings As ArrayList
    Property SomeStrings As ArrayList

    Sub New()
        SomeStrings = New ArrayList(New String() {"one", "two", "three"})
    End Sub

End Class

Class ViolatingVersusPreferred

    Shared Sub Main2227()
        Dim newCollection As New ArrayList(New String() {"a", "new", "collection"})

        Dim collection As New WritableCollection()

        ' This line of code demonstrates how the entire collection
        ' can be replaced by a property that's not read only.
        collection.SomeStrings = newCollection

        ' If the intent is to replace an entire collection,
        ' implement and/or use the Clear() and AddRange() methods instead.
        collection.SomeStrings.Clear()
        collection.SomeStrings.AddRange(newCollection)
    End Sub

End Class

Related rules

• CA1819: Properties should not return arrays

See also

• Properties in C#