Use collection expression for fluent (IDE0305)

Article
01/25/2024

Property	Value
Rule ID	IDE0305
Title	Use collection expression for fluent
Category	Style
Subcategory	Language rules (expression-level preferences)
Applicable languages	C# 12+
Options	`dotnet_style_prefer_collection_expression`

Overview

This rule flags places where a collection is built in a fluent manner, that is, where methods like Add(), AddRange(), AsSpan(), ToList(), and ToArray() are chained. Instead, a collection expression could be used to initialize the collection. For example, x.AddRange(y).Add(z).AsSpan() is converted to [x, ..y, z].

Options

Options specify the behavior that you want the rule to enforce. For information about configuring options, see Option format.

dotnet_style_prefer_collection_expression

Property	Value	Description
Option name	dotnet_style_prefer_collection_expression
Option values	`true` \| `when_types_exactly_match`	Prefer to use collection expressions only when types match exactly, for example, `List<int> list = new[] { 1, 2, 3 }.ToList();`.
	`when_types_loosely_match` (.NET 9 and later versions)^*	Prefer to use collection expressions even when types match loosely, for example, `IEnumerable<int> list = new[] { 1, 2, 3 }.ToList();`. The targeted type must match the type on the right-hand side or be one of the following types: IEnumerable<T>, ICollection<T>, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T>.
	`false` \| `never`	Disables the rule.
Default option value	`true` in .NET 8 `when_types_loosely_match` in .NET 9 and later versions

^*The code fix when this option is used might change the semantics of your code.

Example

// Code with violation.
List<int> i = new[] { 1, 2, 3 }.ToList();
IEnumerable<int> j = new[] { 1, 2, 3 }.ToList();

// Fixed code.
List<int> i = [1, 2, 3];
IEnumerable<int> j = [1, 2, 3];

Suppress a warning

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

#pragma warning disable IDE0305
// The code that's violating the rule is on this line.
#pragma warning restore IDE0305

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0305.severity = none

To disable all of the code-style rules, set the severity for the category Style to none in the configuration file.

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

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

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