CA1858: Use StartsWith instead of IndexOf

Article
12/16/2023

Property	Value
Rule ID	CA1858
Title	Use StartsWith instead of IndexOf
Category	Performance
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 9	As suggestion

Cause

String.IndexOf is called and its result is compared against zero.

Rule description

It's more efficient and clearer to call String.StartsWith than to call String.IndexOf and compare the result with zero to determine whether a string starts with a given prefix.

IndexOf searches the entire string, while StartsWith only compares at the beginning of the string.

How to fix violations

Replace the call to String.IndexOf with a call to String.StartsWith.

Example

The following code snippet shows a violation of CA1858:

bool M(string s)
{
    return s.IndexOf("abc") == 0;
}

Function M(s As String) As Boolean
    Return s.IndexOf("abc") = 0
End Function

The following code snippet fixes the violation:

bool M(string s)
{
    return s.StartsWith("abc");
}

Function M(s As String) As Boolean
    Return s.StartsWith("abc")
End Function

When to suppress warnings

It's safe to suppress this warning if performance isn't 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 CA1858
// The code that's violating the rule is on this line.
#pragma warning restore CA1858

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

[*.{cs,vb}]
dotnet_diagnostic.CA1858.severity = none

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

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