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title description ms.date f1_keywords helpviewer_keywords dev_langs
IDE0303: Use collection expression for Create
Learn about code analysis rule IDE0303: Use collection expression for Create
12/12/2023
IDE0303
IDE0303
CSharp

Use collection expression for Create() (IDE0303)

Property Value
Rule ID IDE0303
Title Use collection expression for Create
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 Create() method or a similar method that's designated as the collection construction method (using the xref:System.Runtime.CompilerServices.CollectionBuilderAttribute attribute) is used to initialize a collection and offers to replace it with a collection expression ([...]).

Create() methods are common for the immutable collections, for example, ImmutableArray.Create(1, 2, 3).

Note

This rule requires more recent versions of the immutable APIs (for example, xref:System.Collections.Immutable), which opt in to 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, for example, ImmutableArray<int> i = ImmutableArray.Create(1, 2, 3);.
when_types_loosely_match
(.NET 9 and later versions)*		 Prefer to use collection expressions even when types match loosely, for example, IEnumerable<int> i = ImmutableArray.Create(1, 2, 3);. The targeted type must match the type on the right-hand side or be one of the following types: xref:System.Collections.Generic.IEnumerable%601, xref:System.Collections.Generic.ICollection%601, xref:System.Collections.Generic.IList%601, xref:System.Collections.Generic.IReadOnlyCollection%601, xref:System.Collections.Generic.IReadOnlyList%601.
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.
ImmutableArray<int> i = ImmutableArray.Create(1, 2, 3);
IEnumerable<int> j = ImmutableArray.Create(1, 2, 3);

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

The following code snippet shows an example with a custom type that's annotated with xref:System.Runtime.CompilerServices.CollectionBuilderAttribute.

public class Program
{
    public static void Main()
    {
        // IDE0303 violation.
        MyCollection<int> c = MyCollection.Create(1, 2, 3);

        // IDE0303 fixed code.
        MyCollection<int> c = [1, 2, 3];
    }
}

static partial class MyCollection
{
    public static MyCollection<T> Create<T>(System.ReadOnlySpan<T> values) => default;
    public static MyCollection<T> Create<T>(T t1, T t2, T t3) => default;
}

[CollectionBuilder(typeof(MyCollection), "Create")]
class MyCollection<T> : IEnumerable<T>
{
    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 IDE0303
// The code that's violating the rule is on this line.
#pragma warning restore IDE0303

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0303.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