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LogInformation

As explained in ASP.NET Core in Action, 3e

LogInformation is an ILogger extension method that records a message at the Information severity level.

LogInformation is an ILogger extension method for recording an Information-level message. The logger sends the message to every configured logging provider. section 26.2

Use LogInformation with these practical rules:

  • Inject ILogger<T> into a class through dependency injection, then call LogInformation with a message template and its values. section 26.2
  • Prefer named placeholders such as {RecipeCount} over string interpolation; logging can retain the named value for structured analysis. section 26.2 section 26.6
  • Expect the final presentation to vary by provider; console, debug, EventLog, and EventSource providers are available in ASP.NET Core. section 26.2 section 26.6

Logging the number of loaded recipes

An IndexModel injects ILogger<IndexModel>, loads recipes in OnGet, and records the result with _log.LogInformation("Loaded {RecipeCount} recipes", Recipes.Count). The provider can display the substituted message while retaining RecipeCount as structured data. section 26.2

Listing 26.2 Injecting ILogger into a class and writing a log message
public class IndexModel : PageModel
{
    private readonly RecipeService _service;
    private readonly ILogger<IndexModel> _log;                  #1
 
    public ICollection<RecipeSummaryViewModel> Recipes { get; set; }
 
    public IndexModel(
        RecipeService service,
        ILogger<IndexModel> log)                                #1
    {
        _service = service;
        _log = log;                                             #1
    }
 
    public void OnGet()
    {
        Recipes = _service.GetRecipes();
        _log.LogInformation(                                    #2
            "Loaded {RecipeCount} recipes", Recipes.Count);     #2
    }
}
#1 Injects the generic ILogger&lt;T&gt; using DI, which implements ILogger
#2 Writes an Information-level log. The RecipeCount variable is substituted in the message.

View the complete Razor Page example showing dependency injection, the LogInformation call, and placeholder substitution. section 26.2

How ASP.NET Core in Action, 3e covers it

How deep it goes
Explains injection, severity, providers, positional placeholders, and structured values, while deferring detailed provider configuration.
Examples
Demonstrates an IndexModel logging the number of recipes loaded with a named RecipeCount placeholder.
What it assumes
Assumes familiarity with C# classes and dependency injection while introducing ILogger injection and log levels.
What you can do after
Apply it via ILogger<T> to emit Information-level messages with provider-friendly structured values.
How it connects
Connects ILogger, ILogger<T>, LogLevel, dependency injection, logging providers, and structured logging.
Cover of ASP.NET Core in Action, 3e
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of ASP.NET Core in Action, 3e. The section links point to the parts of the book it draws on.