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Use collection expression for builder (IDE0304)

  • Article
Property Value
Rule ID IDE0304
Title Use collection expression for builder
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 CreateBuilder() or similar method is called to create a builder type that adds elements and finally constructs a collection type that has the CollectionBuilderAttribute attribute (for example, by calling ImmutableArray<T>.Builder.ToImmutable()). Instead, a collection expression ([...]) could be used to initialize the collection.

Note

This rule requires more recent versions of the immutable APIs (for example, System.Collections.Immutable), which opt into the collection-expression pattern.

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.
when_types_loosely_match
(.NET 9 and later versions)*		 Prefer to use collection expressions even when types match loosely. 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.
var builder = ImmutableArray.CreateBuilder<int>();
builder.Add(1);
builder.AddRange(new int[] { 5, 6, 7 });
ImmutableArray<int> i = builder.ToImmutable();

// Fixed code.
ImmutableArray<int> i = [1, .. new int[] { 5, 6, 7 }];

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 IDE0304
// The code that's violating the rule is on this line.
#pragma warning restore IDE0304

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0304.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.

See also