CA2249: Consider using String.Contains instead of String.IndexOf

Article
12/16/2023

Property	Value
Rule ID	CA2249
Title	Consider using String.Contains instead of String.IndexOf
Category	Usage
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 9	As suggestion

Cause

This rule locates calls to IndexOf where the result is used to check for the presence or absence of a substring, and suggests using Contains instead, to improve readability.

Rule description

When IndexOf is used to check if the result is equal to -1 or greater or equal than 0, the call can be safely substituted with Contains without an impact on performance.

Depending on the IndexOf overload being used, the suggested fix could get a comparisonType argument added:

Overload	Suggested fix
`String.IndexOf(char)`	`String.Contains(char)`
`String.IndexOf(string)`	`String.Contains(string, StringComparison.CurrentCulture)`
`String.IndexOf(char, StringComparison.Ordinal)`	`String.Contains(char)`
`String.IndexOf(string, StringComparison.Ordinal)`	`String.Contains(string)`
`String.IndexOf(char, NON StringComparison.Ordinal)`*	`String.Contains(char, NON StringComparison.Ordinal)`*
`String.IndexOf(string, NON StringComparison.Ordinal)`*	`String.Contains(string, NON StringComparison.Ordinal)`*

* Any StringComparison enum value other than StringComparison.Ordinal:

How to fix violations

The violation can either be fixed manually, or, in some cases, using Quick Actions to fix code in Visual Studio.

Examples

The following two code snippets show all possible violations of the rule in C# and how to fix them:

using System;

class MyClass
{
    void MyMethod()
    {
        string str = "My text";
        bool found;

        // No comparisonType in char overload, so no comparisonType added in resulting fix
        found = str.IndexOf('x') == -1;
        found = str.IndexOf('x') >= 0;

        // No comparisonType in string overload, adds StringComparison.CurrentCulture to resulting fix
        found = str.IndexOf("text") == -1;
        found = str.IndexOf("text") >= 0;

        // comparisonType equal to StringComparison.Ordinal, removes the argument
        found = str.IndexOf('x', StringComparison.Ordinal) == -1;
        found = str.IndexOf('x', StringComparison.Ordinal) >= 0;

        found = str.IndexOf("text", StringComparison.Ordinal) == -1;
        found = str.IndexOf("text", StringComparison.Ordinal) >= 0;

        // comparisonType different than StringComparison.Ordinal, preserves the argument
        found = str.IndexOf('x', StringComparison.OrdinalIgnoreCase) == -1;
        found = str.IndexOf('x', StringComparison.CurrentCulture) >= 0;

        found = str.IndexOf("text", StringComparison.InvariantCultureIgnoreCase) == -1;
        found = str.IndexOf("text", StringComparison.InvariantCulture) >= 0;

        // Suggestion message provided, but no automatic fix offered, must be fixed manually
        int index = str.IndexOf("text");
        if (index == -1)
        {
            Console.WriteLine("'text' Not found.");
        }
    }
}

using System;

class MyClass
{
    void MyMethod()
    {
        string str = "My text";
        bool found;

        // No comparisonType in char overload, so no comparisonType added in resulting fix
        found = !str.Contains('x');
        found = str.Contains('x');

        // No comparisonType in string overload, adds StringComparison.CurrentCulture to resulting fix
        found = !string.Contains("text", StringComparison.CurrentCulture);
        found = string.Contains("text", StringComparison.CurrentCulture);

        // comparisonType equal to StringComparison.Ordinal, removes the argument
        found = !string.Contains('x');
        found = string.Contains('x');

        found = !string.Contains("text");
        found = string.Contains("text");

        // comparisonType different than StringComparison.Ordinal, preserves the argument
        ;found = !string.Contains('x', StringComparison.OrdinalIgnoreCase)
        found = string.Contains('x', StringComparison.CurrentCulture);

        found = !string.Contains("text", StringComparison.InvariantCultureIgnoreCase);
        found = string.Contains("text", StringComparison.InvariantCulture);

        // This case had to be manually fixed
        if (!str.Contains("text"))
        {
            Console.WriteLine("'text' Not found.");
        }
    }
}

Tip

A code fix is available for this rule in Visual Studio. To use it, position the cursor on the violation and press Ctrl+. (period). Choose Consider using 'string.Contains' instead of 'string.IndexOf' from the list of options that's presented.

Code fix for CA2249 - Consider using 'string.Contains' instead of 'string.IndexOf'

When to suppress warnings

It's safe to suppress a violation of this rule if improving code readability is not a concern.

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA2249
// The code that's violating the rule is on this line.
#pragma warning restore CA2249

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

[*.{cs,vb}]
dotnet_diagnostic.CA2249.severity = none

For more information, see How to suppress code analysis warnings.

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