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Use pattern matching to avoid 'is' check followed by a cast (IDE0020 and IDE0038)

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This article describes two related rules, IDE0020 and IDE0038.

Property Value
Rule ID IDE0020
Title Use pattern matching to avoid is check followed by a cast (with variable)
Category Style
Subcategory Language rules (pattern matching preferences)
Applicable languages C#
Options csharp_style_pattern_matching_over_is_with_cast_check
Property Value
Rule ID IDE0038
Title Use pattern matching to avoid is check followed by a cast (without variable)
Category Style
Subcategory Language rules (pattern matching preferences)
Applicable languages C#
Options csharp_style_pattern_matching_over_is_with_cast_check

Overview

This style rule concerns the use of C# pattern matching, for example, o is int i, over an is check followed by a cast, for example, if (o is int) { ... (int)o ... }. Enable either IDE0020 or IDE0038 based on whether or not the cast expression should be saved into a separate local variable:

  • IDE0020: Cast expression is saved into a local variable. For example, if (o is int) { var i = (int)o; } saves the result of (int)o in a local variable.
  • IDE0038: Cast expression is not saved into a local variable. For example, if (o is int) { if ((int)o == 1) { ... } } does not save the result of (int)o into a local variable.

Options

Set the value of the associated option for this rule to specify whether pattern matching or is check followed by a type cast is preferred.

For more information about configuring options, see Option format.

csharp_style_pattern_matching_over_is_with_cast_check

Property Value Description
Option name csharp_style_pattern_matching_over_is_with_cast_check
Option values true Prefer pattern matching instead of is expressions with type casts.
false Disables the rule.
Default option value true 
// csharp_style_pattern_matching_over_is_with_cast_check = true
if (o is int i) {...}

// csharp_style_pattern_matching_over_is_with_cast_check = false
if (o is int) {var i = (int)o; ... }

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 IDE0020 // Or IDE0038
// The code that's violating the rule is on this line.
#pragma warning restore IDE0020 // Or IDE0038

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0020.severity = none
dotnet_diagnostic.IDE0038.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