Structured logging that you can actually query

When an integration fails at 3 a.m., logs are how you find out what happened. Whether you can find it quickly depends on one small habit: how you write the log call.

#Interpolation hides your data

logger.LogInformation($"Order {order.Id} sent to billing in {elapsed} ms");

This produces a perfectly readable line of text. In Application Insights it's also just text. To find every message for one order, you're searching strings.

#Message templates keep it structured

logger.LogInformation("Order {OrderId} sent to billing in {ElapsedMs} ms", order.Id, elapsed);

The output reads the same, but OrderId and ElapsedMs are now stored as separate properties. In Application Insights they land in customDimensions, and you can query them directly:

traces
| where customDimensions.OrderId == "4815"
| order by timestamp asc

or chart them:

traces
| where message startswith "Order"
| summarize avg(todouble(customDimensions.ElapsedMs)) by bin(timestamp, 1h)

The analyzer rule CA2254 warns when a log message isn't a constant template, so you can catch interpolation in code review automatically.

#Add context once with scopes

Instead of repeating the order ID in every log call, wrap the work in a scope:

using (logger.BeginScope(new Dictionary<string, object> { ["OrderId"] = order.Id }))
{
    logger.LogInformation("Validating order");
    logger.LogInformation("Sending to billing");
}

Every log entry inside the scope carries OrderId, including entries written by code that doesn't know about it.

#Use the source generator on hot paths

For logging in frequently called code, LoggerMessage generates the logging method at compile time, avoiding boxing and repeated template parsing:

public static partial class Log
{
    [LoggerMessage(Level = LogLevel.Information, Message = "Order {OrderId} sent to billing in {ElapsedMs} ms")]
    public static partial void OrderSent(ILogger logger, string orderId, long elapsedMs);
}

Log.OrderSent(logger, order.Id, elapsed);

#Check what's actually captured

By default, the Application Insights logger only sends Warning and above. If your Information logs are missing, set the level explicitly:

{
  "Logging": {
    "ApplicationInsights": {
      "LogLevel": { "Default": "Information" }
    }
  }
}

Be deliberate about it, because every log line costs ingestion.

#Takeaway

Write log calls with message templates, never interpolation, so every value becomes a queryable field. Use scopes for context that applies to a whole operation, the LoggerMessage generator on hot paths, and check your log level so the entries you need actually reach Application Insights.