Use collection expression for empty (IDE0301)

Статия
01/25/2024

Property	Value
Rule ID	IDE0301
Title	Use collection expression for empty
Category	Style
Subcategory	Language rules (expression-level preferences)
Applicable languages	C# 12+
Options	`dotnet_style_prefer_collection_expression`

Overview

This rule looks for code similar to Array.Empty<T>() (a method call that returns an empty collection) or ImmutableArray<T>.Empty (a property that returns an empty collection) and offers to replace it with a collection expression ([]).

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, `int[] i = Array.Empty<int>();`.
	`when_types_loosely_match` (.NET 9 and later versions)^*	Prefer to use collection expressions even when types match loosely, for example, `IEnumerable<int> i = Array.Empty<int>();`. 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 violations.
int[] i = Array.Empty<int>();
IEnumerable<int> j = Array.Empty<int>();
ReadOnlySpan<int> span = ReadOnlySpan<int>.Empty;

// Fixed code.
int[] i = [];
IEnumerable<int> j = [];
ReadOnlySpan<int> span = [];

The following code snippet shows an example with a custom type.

public class Program
{
    public static void Main()
    {
        // IDE0301 violation.
        MyList<int> x = MyList<int>.Empty;

        // IDE0301 fixed code.
        MyList<int> x = [];
    }
}

class MyList<T> : IEnumerable<T>
{
    public static MyList<T> Empty { get; }

    public IEnumerator<T> GetEnumerator() => default;
    IEnumerator IEnumerable.GetEnumerator() => default;
}

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 IDE0301
// The code that's violating the rule is on this line.
#pragma warning restore IDE0301

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0301.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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