CA1863: Use 'CompositeFormat'

Article
11/21/2023

Property	Value
Rule ID	CA1863
Title	Use `CompositeFormat`
Category	Performance
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 8	No

Cause

Code calls String.Format(String, Object[]) or StringBuilder.AppendFormat(String, Object[]) with a static format string that hasn't been cached to a CompositeFormat instance.

Rule description

It's expensive to parse a format string at run time. This rule locates places in your code where you can cache and use a CompositeFormat instance as the argument to a formatting operation, rather than passing in the original format string. A CompositeFormat instance parses the composite format string when it's created, which means the "hot path" of string formatting can execute much faster.

How to fix violations

Create an instance of CompositeFormat by calling CompositeFormat.Parse(String) and pass it to String.Format(IFormatProvider, CompositeFormat, Object[]) or StringBuilder.AppendFormat(IFormatProvider, CompositeFormat, Object[]) instead of the original format string.

Example

The following example shows two violations of the rule:

class C
{
    private static readonly string StaticField = "Format one value: {0}";

    static void Main()
    {
        _ = string.Format(StaticField, 42);

        StringBuilder sb = new();
        sb.AppendFormat(StaticField, 42);
    }
}

The following example shows code that fixes both violations:

class C
{
    private static readonly CompositeFormat StaticField = CompositeFormat.Parse("Format one value: {0}");

    static void Main()
    {
        _ = string.Format(null, StaticField, 42);

        StringBuilder sb = new();
        sb.AppendFormat(null, StaticField, 42);
    }
}

When to suppress warnings

It's safe to suppress diagnostics from this rule 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 CA1863
// The code that's violating the rule is on this line.
#pragma warning restore CA1863

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

[*.{cs,vb}]
dotnet_diagnostic.CA1863.severity = none

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

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