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Warning CA1831: Use AsSpan instead of Range-based indexers for string

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.NET code analyzer rule CA1831 is enabled, by default, starting in .NET 5. It produces a build warning for any code where a Range-based indexer is used on a string, but no copy was intended.

Change description

Starting in .NET 5, the .NET SDK includes .NET source code analyzers. Several of these rules are enabled, by default, including CA1831. If your project contains code that violates this rule and is configured to treat warnings as errors, this change could break your build.

Rule CA1831 finds instances where a Range-based indexer is used on a string, but no copy was intended. If the Range-based indexer is used directly on a string to produce an implicit cast, then an unnecessary copy of the requested portion of the string is created. For example:

ReadOnlySpan<char> slice = str[1..3];

CA1831 suggests using the Range-based indexer on a span of the string, instead. For example:

ReadOnlySpan<char> slice = str.AsSpan()[1..3];

Version introduced

5.0

  • To correct your code and avoid unnecessary allocations, call AsSpan(String) or AsMemory(String) before using the Range-based indexer. For example:

    ReadOnlySpan<char> slice = str.AsSpan()[1..3];

  • If you don't want to change your code, you can disable the rule by setting its severity to suggestion or none. For more information, see Configure code analysis rules.

  • To disable code analysis completely, set EnableNETAnalyzers to false in your project file. For more information, see EnableNETAnalyzers.

Affected APIs