Edit

CA2254: Template should be a static expression

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Property Value
Rule ID CA2254
Title Template should be a static expression
Category Usage
Fix is breaking or non-breaking Non-breaking
Enabled by default in .NET 8 As suggestion

Cause

A message template passed to a logger API is not constant. This occurs when the template passed uses either string concatenation or interpolation. Instead, the template should be a constant value that represents the log message in message template format. For example: "User {User} logged in from {Address}". For more information, see Log message template formatting.

Rule description

When performing logging, it's desirable to preserve the structure of the log (including placeholder names) along with the placeholder values. Preserving this information allows for better observability and search in log aggregation and monitoring software.

Preferred:

var firstName = "Lorenz";
var lastName = "Otto";

// This tells the logger that there are FirstName and LastName properties
// on the log message, and correlates them with the argument values.
logger.Warning("Person {FirstName} {LastName} encountered an issue", firstName, lastName);

Not preferred:

// DO NOT DO THIS

var firstName = "Lorenz";
var lastName = "Otto";

// Here, the log template itself is changing, and the association between named placeholders and their values is lost.
logger.Warning("Person " + firstName + " " + lastName + " encountered an issue");

// String interpolation also loses the association between placeholder names and their values.
logger.Warning($"Person {firstName} {lastName} encountered an issue");

The logging message template should not vary between calls.

How to fix violations

Update the message template to be a constant expression. If you're using values directly in the template, refactor the template to use named placeholders instead.

logger.Warning("Person {FirstName} {LastName} encountered an issue", firstName, lastName);

When to suppress errors

It's safe to suppress a warning from this rule if your use case doesn't require structured logging. It's also safe to suppress this rule if your log message template is defined in a resource file.

See also