CA1858: Use StartsWith instead of IndexOf

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

xref:System.String.IndexOf%2A?displayProperty=nameWithType is called and its result is compared against zero.

Rule description

It's more efficient and clearer to call xref:System.String.StartsWith%2A?displayProperty=nameWithType than to call xref:System.String.IndexOf%2A?displayProperty=nameWithType 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 xref:System.String.IndexOf%2A?displayProperty=nameWithType with a call to xref:System.String.StartsWith%2A?displayProperty=nameWithType.

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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