# CA2227: Collection properties should be read only

Value
Rule ID CA2227
Category Usage
Fix is breaking or non-breaking Breaking

## 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


To disable this entire category of rules, set the severity for the category to none in the configuration file.

[*.{cs,vb}]
dotnet_analyzer_diagnostic.category-Usage.severity = none


## 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,
collection.SomeStrings.Clear();
}
}

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,