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Update option values for prefer collection expressions (#45790)
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‎docs/fundamentals/code-analysis/style-rules/ide0028.md

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---
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title: "IDE0028: Use collection initializers or expressions"
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description: "Learn about code analysis rule IDE0028: Use collection initializers or expressions"
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ms.date: 12/12/2023
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ms.date: 04/11/2025
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f1_keywords:
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- IDE0028
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- dotnet_style_collection_initializer
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| | `false` | Don't prefer collection initializers. |
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| **Default option value** | `true` | |
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### dotnet_style_prefer_collection_expression (C# only)
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` | Prefer to use collection expressions. |
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| | `false` | Don't prefer collection expressions. |
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| **Default option value** | `true` | |
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(This option applies only to C#.)
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## Examples
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```csharp
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// IDE0028 violation.
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List<int> list = new List<int>();
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list.Add(1);
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list.Add(2);
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list.Add(3);
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List<int> list = new List<int>() { 1, 2, 3 };
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// Fixed code (with dotnet_style_prefer_collection_expression = true)
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List<int> list = [1, 2, 3];

‎docs/fundamentals/code-analysis/style-rules/ide0300.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly, for example, `int[] i = new int[] { 1, 2, 3 };`. |
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| | `when_types_loosely_match`<br />(.NET 9 and later versions)<sup>\*</sup> | Prefer to use collection expressions even when types match loosely, for example, `IEnumerable<int> i = new int[] { 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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `true` in .NET 8<br />`when_types_loosely_match` in .NET 9 and later versions | |
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<sup>\*</sup>When this option is used, the code fix might change the semantics of your code. For example, if you had `IEnumerable<int> x = new int[] { 1, 2, 3 };`, then in the original code, an array is produced. But in the new code (`IEnumerable<int> x = [1, 2, 3];`), an internal compiler-synthesized type is produced instead. You can observe this difference if you use an `is` check or a cast.
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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For example, if you had `IEnumerable<int> x = new int[] { 1, 2, 3 };`, then in the original code, an array is produced. But in the new code (`IEnumerable<int> x = [1, 2, 3];`), an internal compiler-synthesized type is produced instead. You can observe this difference if you use an `is` check or a cast.
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## Example
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‎docs/fundamentals/code-analysis/style-rules/ide0301.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly, for example, `int[] i = Array.Empty<int>();`. |
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| | `when_types_loosely_match`<br />(.NET 9 and later versions)<sup>\*</sup> | 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: <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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `true` in .NET 8<br />`when_types_loosely_match` in .NET 9 and later versions | |
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<sup>\*</sup>When this option is used, the code fix might change the semantics of your code.
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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## Example
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‎docs/fundamentals/code-analysis/style-rules/ide0302.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `true` | |
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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The `when_types_loosely_match` value for does not apply to this rule `IDE0302`, but is listed here for completeness of the `dotnet_style_prefer_collection_expression` option (which is shared by multiple rules). The default value is effectively `true`.
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## Example
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‎docs/fundamentals/code-analysis/style-rules/ide0303.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly, for example, `ImmutableArray<int> i = ImmutableArray.Create(1, 2, 3);`. |
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| | `when_types_loosely_match`<br />(.NET 9 and later versions)<sup>\*</sup> | 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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `true` in .NET 8<br />`when_types_loosely_match` in .NET 9 and later versions | |
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<sup>\*</sup>When this option is used, the code fix might change the semantics of your code.
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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## Example
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‎docs/fundamentals/code-analysis/style-rules/ide0304.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly. |
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| | `when_types_loosely_match`<br />(.NET 9 and later versions)<sup>\*</sup> | 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: <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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `true` in .NET 8<br />`when_types_loosely_match` in .NET 9 and later versions | |
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<sup>\*</sup>When this option is used, the code fix might change the semantics of your code.
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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## Example
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‎docs/fundamentals/code-analysis/style-rules/ide0305.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly, for example, `List<int> list = new[] { 1, 2, 3 }.ToList();`. |
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| | `when_types_loosely_match`<br />(.NET 9 and later versions)<sup>\*</sup> | 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: <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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `true` in .NET 8<br />`when_types_loosely_match` in .NET 9 and later versions | |
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<sup>\*</sup>When this option is used, the code fix might change the semantics of your code.
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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## Example
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‎docs/fundamentals/code-analysis/style-rules/ide0306.md

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Options specify the behavior that you want the rule to enforce. For information about configuring options, see [Option format](language-rules.md#option-format).
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly, for example, `List<int> m1 = new List<int>(new[] { 1, 2, 3 });`. |
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| | `when_types_loosely_match`<sup>\*</sup> | Prefer to use collection expressions even when types match loosely, for example, `IEnumerable<int> m1 = new List<int>(new[] { 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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `when_types_loosely_match` | |
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<sup>\*</sup>When this option is used, the code fix might change the semantics of your code.
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[!INCLUDE [dotnet-style-prefer-collection-expression](includes/dotnet-style-prefer-collection-expression.md)]
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## Example
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‎docs/fundamentals/code-analysis/style-rules/includes/dotnet-style-prefer-collection-expression.md

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---
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ms.topic: include
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---
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### dotnet_style_prefer_collection_expression
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| Property | Value | Description |
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|--------------------------|-------------------------------------------|---------------------------------------|
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| **Option name** | dotnet_style_prefer_collection_expression | |
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| **Option values** | `true` &#124; `when_types_exactly_match` | Prefer to use collection expressions only when types match exactly, for example, `List<int> list = new List<int>() { 1, 2 };`. |
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| | `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: <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>. |
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| | `false` &#124; `never` | Disables the rule. |
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| **Default option value** | `when_types_loosely_match`\* | |
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\*When this option is used, the code fix might change the semantics of your code.

‎docs/fundamentals/code-analysis/style-rules/index.md

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> | [IDE0025](ide0025.md) | Use expression body for properties | [csharp_style_expression_bodied_properties](ide0025.md#csharp_style_expression_bodied_properties) |
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> | [IDE0026](ide0026.md) | Use expression body for indexers | [csharp_style_expression_bodied_indexers](ide0026.md#csharp_style_expression_bodied_indexers) |
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> | [IDE0027](ide0027.md) | Use expression body for accessors | [csharp_style_expression_bodied_accessors](ide0027.md#csharp_style_expression_bodied_accessors) |
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> | [IDE0028](ide0028.md) | Use collection initializers | [dotnet_style_collection_initializer](ide0028.md#dotnet_style_collection_initializer)<br/>[dotnet_style_prefer_collection_expression (C# only)](ide0028.md#dotnet_style_prefer_collection_expression-c-only) |
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> | [IDE0028](ide0028.md) | Use collection initializers | [dotnet_style_collection_initializer](ide0028.md#dotnet_style_collection_initializer)<br/>[dotnet_style_prefer_collection_expression](ide0028.md#dotnet_style_prefer_collection_expression) |
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> | [IDE0029](ide0029-ide0030-ide0270.md) | Null check can be simplified | [dotnet_style_coalesce_expression](ide0029-ide0030-ide0270.md#dotnet_style_coalesce_expression) |
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> | [IDE0030](ide0029-ide0030-ide0270.md) | Null check can be simplified | [dotnet_style_coalesce_expression](ide0029-ide0030-ide0270.md#dotnet_style_coalesce_expression) |
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> | [IDE0031](ide0031.md) | Use null propagation | [dotnet_style_null_propagation](ide0031.md#dotnet_style_null_propagation) |

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