Remove unnecessary expression value (IDE0058)

Article
07/25/2023

Property	Value
Rule ID	IDE0058
Title	Remove unnecessary expression value
Category	Style
Subcategory	Language rules (expression-level preferences)
Applicable languages	C# and Visual Basic
Options	`csharp_style_unused_value_expression_statement_preference`
	`visual_basic_style_unused_value_expression_statement_preference`

Overview

This rule flags unused expression values. For example:

void M()
{
  Compute(); // IDE0058: computed value is never used.
}

int Compute();

You can take one of the following actions to fix this violation:

If the expression has no side effects, remove the entire statement. This improves performance by avoiding unnecessary computation.
If the expression has side effects, replace the left side of the assignment with a discard (C# only) or a local variable that's never used. This improves code clarity by explicitly showing the intent to discard an unused value.
```
_ = Compute();
```

Options

The options for this specify whether to prefer the use of a discard or an unused local variable:

C# - csharp_style_unused_value_expression_statement_preference
Visual Basic - visual_basic_style_unused_value_expression_statement_preference

For information about configuring options, see Option format.

csharp_style_unused_value_expression_statement_preference

Property	Value	Description
Option name	csharp_style_unused_value_expression_statement_preference
Applicable languages	C#
Option values	`discard_variable`	Prefer to assign an unused expression to a discard
	`unused_local_variable`	Prefer to assign an unused expression to a local variable that's never used
Default option value	`discard_variable`

// Original code:
System.Convert.ToInt32("35");

// After code fix for IDE0058:

// csharp_style_unused_value_expression_statement_preference = discard_variable
_ = System.Convert.ToInt32("35");

// csharp_style_unused_value_expression_statement_preference = unused_local_variable
var unused = Convert.ToInt32("35");

visual_basic_style_unused_value_expression_statement_preference

Property	Value	Description
Option name	visual_basic_style_unused_value_expression_statement_preference
Applicable languages	Visual Basic
Option values	`unused_local_variable`	Prefer to assign an unused expression to a local variable that's never used
Default option value	`unused_local_variable`

' visual_basic_style_unused_value_expression_statement_preference = unused_local_variable
Dim unused = Computation()

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 IDE0058
// The code that's violating the rule is on this line.
#pragma warning restore IDE0058

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

[*.{cs,vb}]
dotnet_diagnostic.IDE0058.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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