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Showing content from https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1831 below:

CA1831: Use AsSpan instead of Range-based indexers for string when appropriate (code analysis) - .NET

Property Value Rule ID CA1831 Title Use AsSpan instead of Range-based indexers for string when appropriate Category Performance Fix is breaking or non-breaking Non-breaking Enabled by default in .NET 9 As warning Cause

A range-indexer is used on a string and the value is implicitly assigned to ReadOnlySpan<char>.

Rule description

This rule fires when you use a range-indexer on a string and assign it to a span type. The range indexer on a Span<T> is a non-copying Slice operation, but for the range indexer on a string, the method Substring will be used instead of Slice. This produces a copy of the requested portion of the string. This copy is usually unnecessary when it's implicitly used as a ReadOnlySpan<T> or ReadOnlyMemory<T> value. If a copy isn't intended, use the AsSpan method to avoid the unnecessary copy. If the copy is intended, either assign it to a local variable first or add an explicit cast. The analyzer only reports when an implicit cast is used on the result of the range indexer operation.

Detects

Implicit conversion:

ReadOnlySpan<char> slice = str[a..b];

Does not detect

Explicit conversion:

ReadOnlySpan<char> slice = (ReadOnlySpan<char>)str[a..b];

How to fix violations

To fix a violation of this rule, use AsSpan instead of the Range-based indexer on the string to avoid creating unnecessary data copies.

public void TestMethod(string str)
{
    // The violation occurs
    ReadOnlySpan<char> slice = str[1..3];
    ...
}

public void TestMethod(string str)
{
    // The violation fixed with AsSpan extension method
    ReadOnlySpan<char> slice = str.AsSpan()[1..3];
    ...
}

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 Use AsSpan instead of the Range-based indexer on a string from the list of options that's presented.

You can also add an explicit cast to avoid this warning.

public void TestMethod(string str)
{
    // The violation occurs.
    ReadOnlySpan<char> slice = str[1..3];
    ...
}

public void TestMethod(string str)
{
    // The violation avoided with explicit casting.
    ReadOnlySpan<char> slice = (ReadOnlySpan<char>)str[1..3];
    ...
}

When to suppress warnings

It's safe to suppress a violation of this rule if creating a copy is intended.

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 CA1831
// The code that's violating the rule is on this line.
#pragma warning restore CA1831

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

[*.{cs,vb}]
dotnet_diagnostic.CA1831.severity = none

To disable this entire category of rules, set the severity for the category to none in the configuration file.

[*.{cs,vb}]
dotnet_analyzer_diagnostic.category-Performance.severity = none

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