CA1002: Do not expose generic lists

#Cause

A type contains an externally visible member that is a System.Collections.Generic.List type, returns a System.Collections.Generic.List type, or whose signature includes a System.Collections.Generic.List parameter.

By default, this rule only looks at externally visible types, but this is configurable.

#Rule description

System.Collections.Generic.List is a generic collection that's designed for performance and not inheritance. System.Collections.Generic.List does not contain virtual members that make it easier to change the behavior of an inherited class. The following generic collections are designed for inheritance and should be exposed instead of System.Collections.Generic.List.

• System.Collections.ObjectModel.Collection

• System.Collections.ObjectModel.ReadOnlyCollection

• System.Collections.ObjectModel.KeyedCollection

• System.Collections.Generic.IList

• System.Collections.Generic.ICollection

#How to fix violations

To fix a violation of this rule, change the System.Collections.Generic.List type to one of the generic collections that's designed for inheritance.

#Example

sample · csharpdocs/fundamentals/code-analysis/quality-rules/snippets/csharp/all-rules/ca1002.cssnippet1

using System.Collections.Generic;

namespace ca1001
{
    //<snippet1>
    // This class violates the rule.
    public class MutableItems
    {
        // CA1002: Change 'List<string>' in 'MutableItems.Items' to
        // use 'Collection<T>', 'ReadOnlyCollection<T>' or 'KeyedCollection<K,V>'.
        public List<string> Items { get; } = [];

        public void Add(string item)
        {
            Items.Add(item);
        }
    }

    // This class satisfies the rule.
    public class ReadOnlyItems
    {
        private readonly List<string> _items = [];

        public IReadOnlyCollection<string> Items => _items.AsReadOnly();

        public void Add(string item)
        {
            _items.Add(item);
        }
    }
    //</snippet1>
}

#When to suppress warnings

Do not suppress a warning from this rule unless the assembly that raises this warning is not meant to be a reusable library. For example, it would be safe to suppress this warning in a performance-tuned application where a performance benefit was gained from the use of generic lists.

#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 CA1002
// The code that's violating the rule is on this line.
#pragma warning restore CA1002

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

[*.{cs,vb}]
dotnet_diagnostic.CA1002.severity = none

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

#Configure code to analyze

Use the following option to configure which parts of your codebase to run this rule on.

• Include specific API surfaces

You can configure this option for just this rule, for all rules it applies to, or for all rules in this category (Design) that it applies to. For more information, see Code quality rule configuration options.

#Include specific API surfaces

You can configure which parts of your codebase to run this rule on, based on their accessibility, by setting the api_surface option. For example, to specify that the rule should run only against the non-public API surface, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.api_surface = private, internal

#Related rules

CA1005: Avoid excessive parameters on generic types

CA1010: Collections should implement generic interface

CA1000: Do not declare static members on generic types

CA1003: Use generic event handler instances

#See also

Generics