S6677 — Message template placeholders should be unique

Why is this an issue?

Named placeholders in message templates should be unique. The meaning of the named placeholders is to store the value of the provided argument under that name, enabling easier log querying. Since the named placeholder is used multiple times, it cannot store the different values uniquely with each name hence not serving its original purpose. There can be different behaviours when using the same named placeholder multiple times:

• Microsoft.Extensions.Logging saves the different values under the same name
• Serilog stores only the latest assigned value
• Nlog makes the name unique by suffixing it with _index

The rule covers the following logging frameworks:

• Microsoft.Extensions.Logging
• Serilog
• Nlog

How to fix it

Assign unique names to each template placeholder.

Code examples

Noncompliant code example

public void Checkout(ILogger logger, User user, Order order)
{
    logger.LogDebug("User {Id} purchased order {Id}", user.Id, order.Id);
}

Compliant solution

public void Checkout(ILogger logger, User user, Order order)
{
    logger.LogDebug("User {UserId} purchased order {OrderId}", user.Id, order.Id);
}

Resources

Documentation

• Message Templates - Message template specification
• Microsoft Learn - Log message template formatting
• NLog - How to use structured logging
• Serilog - Structured Data
• Serilog - Serilog005: Unique Property Name Verifier