Use collection expression for new (IDE0306)

Article
2025-04-22

Property	Value
Rule ID	IDE0306
Title	Use collection expression for new
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 expression with a spread element (..) could be used to initialize a list instead of new. For example, this rule offers to simplify code like new List<...>(x) into the collection expression form ([.. x]).

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 List<int>() { 1, 2 };`.
	`when_types_loosely_match`*	Prefer to use collection expressions even when types match loosely, for example, `IEnumerable<int> list = new List<int>() { 1, 2 };`. 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	`when_types_loosely_match`*

*When this option is used, the code fix might change the semantics of your code.

Example

// Code with violation.
List<int> l1 = new List<int>(Enumerable.Range(1, 10));
List<int> m1 = new List<int>(new[] { 1, 2, 3 });

// Fixed code.
List<int> l1 = [.. Enumerable.Range(1, 10)];
List<int> m1 = [.. new[] { 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 IDE0306
// The code that's violating the rule is on this line.
#pragma warning restore IDE0306

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0306.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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