Uredi

Deli z drugimi prek

Implement a custom logging provider in .NET

There are many logging providers available for common logging needs. But you might need to implement a custom ILoggerProvider when one of the available providers doesn't suit your application needs. In this article, you learn how to implement a custom logging provider that can be used to colorize logs in the console.

Tip

The custom logging provider example source code is available in the docs GitHub repo.

Sample custom logger configuration

The sample logger creates different color console entries per log level and event ID using the following configuration type:

using Microsoft.Extensions.Logging;

public sealed class ColorConsoleLoggerConfiguration
{
    public int EventId { get; set; }

    public Dictionary<LogLevel, ConsoleColor> LogLevelToColorMap { get; set; } = new()
    {
        [LogLevel.Information] = ConsoleColor.Green
    };
}

The preceding code sets the default color for the Information level to Green. The EventId is implicitly 0.

Create the custom logger

The following code snippet shows the ILogger implementation:

using Microsoft.Extensions.Logging;

public sealed class ColorConsoleLogger(
    string name,
    Func<ColorConsoleLoggerConfiguration> getCurrentConfig) : ILogger
{
    public IDisposable? BeginScope<TState>(TState state)
        where TState : notnull => default!;

    public bool IsEnabled(LogLevel logLevel) =>
        getCurrentConfig().LogLevelToColorMap.ContainsKey(logLevel);

    public void Log<TState>(
        LogLevel logLevel,
        EventId eventId,
        TState state,
        Exception? exception,
        Func<TState, Exception?, string> formatter)
    {
        if (!IsEnabled(logLevel))
        {
            return;
        }

        ColorConsoleLoggerConfiguration config = getCurrentConfig();
        if (config.EventId == 0 || config.EventId == eventId.Id)
        {
            ConsoleColor originalColor = Console.ForegroundColor;

            Console.ForegroundColor = config.LogLevelToColorMap[logLevel];
            Console.WriteLine($"[{eventId.Id,2}: {logLevel,-12}]");
            
            Console.ForegroundColor = originalColor;
            Console.Write($"     {name} - ");

            Console.ForegroundColor = config.LogLevelToColorMap[logLevel];
            Console.Write($"{formatter(state, exception)}");
            
            Console.ForegroundColor = originalColor;
            Console.WriteLine();
        }
    }
}

Each logger instance is instantiated by passing a category name, which is typically the type where the logger is created. The IsEnabled method checks getCurrentConfig().LogLevelToColorMap.ContainsKey(logLevel) to see if the requested log level is enabled (that is, in the configuration's dictionary of log levels).

It's a good practice to call ILogger.IsEnabled within ILogger.Log implementations since Log can be called by any consumer, and there are no guarantees that it was previously checked. The IsEnabled method should be very fast in most implementations.

if (!IsEnabled(logLevel))
{
    return;
}

The logger is instantiated with the name and a Func<ColorConsoleLoggerConfiguration> that returns the current configuration.

Important

The ILogger.Log implementation checks if the config.EventId value is set. When config.EventId is not set or when it matches the exact logEntry.EventId, the logger logs in color.

Custom logger provider

The ILoggerProvider object is responsible for creating logger instances. It's not necessary to create a logger instance per category, but it makes sense for some loggers, like NLog or log4net. This strategy allows you to choose different logging output targets per category, as in the following example:

using System.Collections.Concurrent;
using System.Runtime.Versioning;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

[UnsupportedOSPlatform("browser")]
[ProviderAlias("ColorConsole")]
public sealed class ColorConsoleLoggerProvider : ILoggerProvider
{
    private readonly IDisposable? _onChangeToken;
    private ColorConsoleLoggerConfiguration _currentConfig;
    private readonly ConcurrentDictionary<string, ColorConsoleLogger> _loggers =
        new(StringComparer.OrdinalIgnoreCase);

    public ColorConsoleLoggerProvider(
        IOptionsMonitor<ColorConsoleLoggerConfiguration> config)
    {
        _currentConfig = config.CurrentValue;
        _onChangeToken = config.OnChange(updatedConfig => _currentConfig = updatedConfig);
    }

    public ILogger CreateLogger(string categoryName) =>
        _loggers.GetOrAdd(categoryName, name => new ColorConsoleLogger(name, GetCurrentConfig));

    private ColorConsoleLoggerConfiguration GetCurrentConfig() => _currentConfig;

    public void Dispose()
    {
        _loggers.Clear();
        _onChangeToken?.Dispose();
    }
}

In the preceding code, CreateLogger(String) creates a single instance of the ColorConsoleLogger per category name and stores it in the ConcurrentDictionary<TKey,TValue>.

The ColorConsoleLoggerProvider class is decorated with two attributes:

[UnsupportedOSPlatform("browser")]
[ProviderAlias("ColorConsole")]
public sealed class ColorConsoleLoggerProvider : ILoggerProvider

The configuration can be specified with any valid configuration provider. Consider the following appsettings.json file:

{
    "Logging": {
        "ColorConsole": {
            "LogLevelToColorMap": {
                "Information": "DarkGreen",
                "Warning": "Cyan",
                "Error": "Red"
            }
        }
    }
}

The appsettings.json file specifies that the color for the Information log level is DarkGreen, which overrides the default value set in the ColorConsoleLoggerConfiguration object.

Usage and registration of the custom logger

By convention, services are registered for dependency injection as part of the startup routine of an application. In this example, the logging service is registered directly from the Program.cs file.

To add the custom logging provider and corresponding logger, add an ILoggerProvider by calling a custom extension method, AddColorConsoleLogger, on the ILoggingBuilder from the IHostApplicationBuilder.Logging property:

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

HostApplicationBuilder builder = Host.CreateApplicationBuilder(args);

builder.Logging.ClearProviders();
builder.Logging.AddColorConsoleLogger(configuration =>
{
    // Replace value of "Cyan" from appsettings.json.
    configuration.LogLevelToColorMap[LogLevel.Warning]
        = ConsoleColor.DarkCyan;
    // Replace value of "Red" from appsettings.json.
    configuration.LogLevelToColorMap[LogLevel.Error]
        = ConsoleColor.DarkRed;
});

using IHost host = builder.Build();

ILogger<Program> logger = host.Services.GetRequiredService<ILogger<Program>>();

logger.LogDebug(1, "Does this line get hit?");    // Not logged
logger.LogInformation(3, "Nothing to see here."); // Logs in ConsoleColor.DarkGreen
logger.LogWarning(5, "Warning... that was odd."); // Logs in ConsoleColor.DarkCyan
logger.LogError(7, "Oops, there was an error.");  // Logs in ConsoleColor.DarkRed
logger.LogTrace(5, "== 120.");                    // Not logged

await host.RunAsync();

By convention, extension methods on ILoggingBuilder are used to register the custom provider:

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection.Extensions;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Configuration;

public static class ColorConsoleLoggerExtensions
{
    public static ILoggingBuilder AddColorConsoleLogger(
        this ILoggingBuilder builder)
    {
        builder.AddConfiguration();

        builder.Services.TryAddEnumerable(
            ServiceDescriptor.Singleton<ILoggerProvider, ColorConsoleLoggerProvider>());

        LoggerProviderOptions.RegisterProviderOptions
            <ColorConsoleLoggerConfiguration, ColorConsoleLoggerProvider>(builder.Services);

        return builder;
    }

    public static ILoggingBuilder AddColorConsoleLogger(
        this ILoggingBuilder builder,
        Action<ColorConsoleLoggerConfiguration> configure)
    {
        builder.AddColorConsoleLogger();
        builder.Services.Configure(configure);

        return builder;
    }
}

The ILoggingBuilder creates one or more ILogger instances. The ILogger instances are used by the framework to log the information.

The instantiation code overrides the color values from the appsettings.json file for LogLevel.Warning and LogLevel.Error.

When you run this simple app, it renders color output to the console window similar to the following image:

Color console logger sample output

See also

  • Last updated on