CA1855: Use Span<T>.Clear() instead of Span<T>.Fill()

Property	Value
Rule ID	CA1855
Title	Use Span<T>.Clear() instead of Span<T>.Fill()
Category	Performance
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 9	As suggestion

Cause

Span<T>.Fill(T) is called to fill the elements of a span with a default value.

Rule description

It's more efficient to call Span<T>.Clear() than to call Span<T>.Fill(T) to fill the elements of the span with a default value.

How to fix violations

Replace the call to Span<T>.Fill(T) with a call to Span<T>.Clear().

Example

The following code snippet shows a violation of CA1855:

void M(Span<byte> span)
{
    span.Fill(0);
}

The following code snippet fixes the violation:

void M(Span<byte> span)
{
    span.Clear();
}

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 CA1855
// The code that's violating the rule is on this line.
#pragma warning restore CA1855

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

[*.{cs,vb}]
dotnet_diagnostic.CA1855.severity = none

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