Migrating Mvc Logging

ASP.NET MVC Logging and Diagnostics Migration

Overview

Migrate logging, error handling, and diagnostics from ASP.NET Framework patterns to ASP.NET Core equivalents. Framework apps typically used System.Diagnostics.Trace, third-party libraries (log4net, NLog, ELMAH, Serilog) with static access, customErrors in web.config, and Application_Error in Global.asax. Core provides a built-in ILogger<T> abstraction with dependency injection, exception handling middleware, and a health check framework.

Related skills: migrating-mvc-filters covers HandleErrorAttribute → UseExceptionHandler conversion. migrating-mvc-http-pipeline covers Application_Error and logging module pipeline migration. This skill focuses on logging framework migration, error page configuration, and health checks.

Workflow

Track progress across these steps:

Migration Progress:
- [ ] Step 1: Inventory logging and diagnostics usage
- [ ] Step 2: Configure built-in logging providers
- [ ] Step 3: Migrate logging framework calls
- [ ] Step 4: Migrate error pages and exception handling
- [ ] Step 5: Add health checks
- [ ] Step 6: Remove legacy logging references

Step 1: Inventory Logging and Diagnostics Usage

Scan the project for all logging and diagnostics patterns:

• System.Diagnostics.Trace / TraceSource calls
• log4net references (ILog, LogManager.GetLogger, log4net.config)
• NLog references (Logger, LogManager.GetCurrentClassLogger, NLog.config)
• ELMAH references (ErrorSignal, ErrorLog, ELMAH HTTP modules in web.config)
• Serilog references (Log.Information, Log.Error, static Log class usage)
• customErrors section in web.config
• Custom error pages (.aspx or .html error pages)
• Application_Error in Global.asax.cs
• HttpContext.Current used for logging context

Record each pattern found. This inventory drives which sections below apply.

Step 2: Configure Built-in Logging Providers

ASP.NET Core includes logging providers out of the box. Register them in Program.cs:

var builder = WebApplication.CreateBuilder(args);

// Built-in providers are added by default (Console, Debug, EventSource, EventLog on Windows).
// To customize:
builder.Logging.ClearProviders();
builder.Logging.AddConsole();
builder.Logging.AddDebug();

Configure log levels in appsettings.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning",
      "MyApp.DataAccess": "Debug"
    }
  }
}

This replaces <system.diagnostics> trace listener configuration in web.config and XML-based logging configuration files.

Step 3: Migrate Logging Framework Calls

Apply the appropriate migration pattern based on the framework found in Step 1.

System.Diagnostics.Trace → ILogger<T>

Replace static Trace calls with constructor-injected ILogger<T>. The Trace API has no log levels or structured logging — ILogger<T> provides both.

Before:

using System.Diagnostics;

public class OrderService
{
    public void ProcessOrder(int orderId)
    {
        Trace.TraceInformation("Processing order {0}", orderId);
        try
        {
            // processing logic
        }
        catch (Exception ex)
        {
            Trace.TraceError("Order {0} failed: {1}", orderId, ex.Message);
            throw;
        }
    }
}

After:

public class OrderService
{
    private readonly ILogger<OrderService> _logger;

    public OrderService(ILogger<OrderService> logger)
    {
        _logger = logger;
    }

    public void ProcessOrder(int orderId)
    {
        _logger.LogInformation("Processing order {OrderId}", orderId);
        try
        {
            // processing logic
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Order {OrderId} failed", orderId);
            throw;
        }
    }
}

Map Trace methods to ILogger methods:

Use structured logging placeholders ({OrderId}) instead of format strings ({0}) because structured placeholders are captured as named properties by logging providers.

log4net → ILoggerProvider Adapter

log4net works in ASP.NET Core through the Microsoft.Extensions.Logging.Log4Net.AspNetCore adapter package. This preserves existing log4net configuration while routing through ILogger<T>.

Add the NuGet package and register in Program.cs:

builder.Logging.AddLog4Net("log4net.config");

Then replace static LogManager.GetLogger calls with ILogger<T> injection:

Before:

private static readonly ILog _log = LogManager.GetLogger(typeof(OrderService));

public void ProcessOrder(int orderId)
{
    _log.Info($"Processing order {orderId}");
}

After:

private readonly ILogger<OrderService> _logger;

public OrderService(ILogger<OrderService> logger)
{
    _logger = logger;
}

public void ProcessOrder(int orderId)
{
    _logger.LogInformation("Processing order {OrderId}", orderId);
}

Move log4net.config to the project root and set it to copy to output directory. Existing appender configuration (file appenders, rolling file appenders) continues to work through the adapter.

NLog → NLog.Extensions.Logging

NLog integrates with ASP.NET Core through NLog.Web.AspNetCore. This is the recommended approach because it preserves NLog targets and rules.

Add the NuGet package and register in Program.cs:

builder.Logging.ClearProviders();
builder.Host.UseNLog();

Most NLog layout renderers (${aspnet-request}, ${aspnet-session}, ${aspnet-user-identity}) work identically in Core through the NLog.Web.AspNetCore package. Replace static logger access with constructor injection:

Before:

private static readonly Logger _logger = LogManager.GetCurrentClassLogger();

After:

private readonly ILogger<OrderService> _logger;

public OrderService(ILogger<OrderService> logger)
{
    _logger = logger;
}

NLog targets (file, database, email) continue to work. The integration package bridges ILogger<T> calls to NLog's pipeline.

ELMAH → Serilog + Exception Middleware

ELMAH has no ASP.NET Core port. Replace it with a combination of structured logging (Serilog recommended) and exception handling middleware.

Add NuGet packages: Serilog.AspNetCore, and optionally Serilog.Sinks.Seq or Serilog.Sinks.ApplicationInsights for centralized error viewing.

Configure in Program.cs:

builder.Host.UseSerilog((context, config) => config
    .ReadFrom.Configuration(context.Configuration)
    .Enrich.FromLogContext()
    .WriteTo.Console()
    .WriteTo.File("logs/app-.log", rollingInterval: RollingInterval.Day));

Replace ELMAH error signaling with ILogger:

Before:

catch (Exception ex)
{
    Elmah.ErrorSignal.FromCurrentContext().Raise(ex);
}

After:

catch (Exception ex)
{
    _logger.LogError(ex, "Operation failed in {Method}", nameof(ProcessOrder));
}

Remove ELMAH NuGet packages (Elmah, Elmah.Mvc), HTTP modules from web.config, and the elmah configuration section. The ELMAH dashboard (/elmah.axd) is replaced by the Serilog sink dashboard (Seq UI, Application Insights, or log file inspection).

Static Logger Access → Constructor Injection

Any pattern that accesses a logger through a static field or service locator must change to constructor injection. ASP.NET Core's DI container provides ILogger<T> automatically — no registration needed.

Before (service locator pattern):

public class OrderService
{
    private readonly ILog _logger = ServiceLocator.Current.GetInstance<ILog>();
}

After:

public class OrderService
{
    private readonly ILogger<OrderService> _logger;

    public OrderService(ILogger<OrderService> logger)
    {
        _logger = logger;
    }
}

For classes not created by DI (e.g., utility or static helper classes), use ILoggerFactory obtained from DI and passed explicitly, or restructure the class to be DI-friendly.

Step 4: Migrate Error Pages and Exception Handling

customErrors → Exception Handling Middleware

Replace customErrors in web.config with middleware in Program.cs:

Before (web.config):

<system.web>
  <customErrors mode="On" defaultRedirect="~/Error">
    <error statusCode="404" redirect="~/Error/NotFound" />
    <error statusCode="403" redirect="~/Error/Forbidden" />
    <error statusCode="500" redirect="~/Error/ServerError" />
  </customErrors>
</system.web>

After (Program.cs):

if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
}

app.UseStatusCodePagesWithReExecute("/Error/{0}");

The UseExceptionHandler middleware catches unhandled exceptions and re-executes the pipeline to the error path. UseStatusCodePagesWithReExecute handles non-exception status codes (404, 403) by re-executing to a status-specific route.

Create an ErrorController (or add to an existing controller):

public class ErrorController : Controller
{
    [Route("Error")]
    public IActionResult Index()
    {
        return View();
    }

    [Route("Error/{statusCode}")]
    public IActionResult StatusCode(int statusCode)
    {
        return statusCode switch
        {
            404 => View("NotFound"),
            403 => View("Forbidden"),
            _ => View("GenericError", statusCode)
        };
    }
}

Application_Error → Exception Middleware or IExceptionHandler

If Application_Error in Global.asax.cs performed custom logging or error processing beyond displaying an error page, convert it to an IExceptionHandler implementation (.NET 8+):

Before (Global.asax.cs):

protected void Application_Error()
{
    var exception = Server.GetLastError();
    Logger.Error("Unhandled exception", exception);
    Server.ClearError();
    Response.Redirect("~/Error");
}

After (GlobalExceptionHandler.cs):

public class GlobalExceptionHandler : IExceptionHandler
{
    private readonly ILogger<GlobalExceptionHandler> _logger;

    public GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger)
    {
        _logger = logger;
    }

    public async ValueTask<bool> TryHandleAsync(
        HttpContext httpContext,
        Exception exception,
        CancellationToken cancellationToken)
    {
        _logger.LogError(exception, "Unhandled exception occurred");
        return false; // Let UseExceptionHandler handle the response
    }
}

Register in Program.cs:

builder.Services.AddExceptionHandler<GlobalExceptionHandler>();
app.UseExceptionHandler("/Error");

Developer Exception Page

In development, ASP.NET Core shows a detailed exception page (replacing the Framework "yellow screen of death"). This is enabled by default when ASPNETCORE_ENVIRONMENT is Development. Verify it is not exposed in production:

if (app.Environment.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
}
else
{
    app.UseExceptionHandler("/Error");
}

Step 5: Add Health Checks

ASP.NET Framework has no built-in health check mechanism. ASP.NET Core provides Microsoft.Extensions.Diagnostics.HealthChecks for readiness and liveness probes.

Register health checks in Program.cs:

builder.Services.AddHealthChecks()
    .AddSqlServer(
        builder.Configuration.GetConnectionString("DefaultConnection")!,
        name: "database",
        tags: new[] { "ready" })
    .AddCheck("self", () => HealthCheckResult.Healthy(), tags: new[] { "live" });

Map health check endpoints:

app.MapHealthChecks("/health/ready", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("ready")
});

app.MapHealthChecks("/health/live", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("live")
});

Add the AspNetCore.HealthChecks.SqlServer NuGet package for SQL Server checks. Other common packages:

Health checks are optional — add them only when the project deploys to an environment that supports health probes (Kubernetes, Azure App Service, load balancers). Skip this step if the deployment target does not use them.

Step 6: Remove Legacy Logging References

Remove all legacy logging and error handling artifacts:

• Delete customErrors section from web.config
• Remove ELMAH NuGet packages and configuration sections from web.config
• Remove Application_Error from Global.asax.cs (if Global.asax is still present)
• Remove using System.Diagnostics where only used for Trace calls
• Remove static logger fields (private static readonly ILog, private static readonly Logger)
• Remove HttpContext.Current references used for logging context
• Search for remaining Trace., Debug.WriteLine, LogManager.Get, ErrorSignal.From calls

Common Patterns

Logging in Middleware

When converting IHttpModule logging to middleware, inject ILogger<T> through the constructor:

public class RequestTimingMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger<RequestTimingMiddleware> _logger;

    public RequestTimingMiddleware(RequestDelegate next, ILogger<RequestTimingMiddleware> logger)
    {
        _next = next;
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var sw = Stopwatch.StartNew();
        await _next(context);
        sw.Stop();
        _logger.LogInformation("Request {Method} {Path} completed in {ElapsedMs}ms",
            context.Request.Method, context.Request.Path, sw.ElapsedMilliseconds);
    }
}

Scoped Logging with Correlation IDs

Framework apps that used HttpContext.Current.Items to store correlation IDs should switch to ILogger scopes:

using (_logger.BeginScope(new Dictionary<string, object>
{
    ["CorrelationId"] = context.Request.Headers["X-Correlation-Id"].FirstOrDefault()
        ?? Guid.NewGuid().ToString()
}))
{
    await _next(context);
}

Troubleshooting

• log4net configuration not loading: Ensure log4net.config is set to "Copy to Output Directory" in the project file and the path in AddLog4Net() matches.
• NLog layout renderers missing: Install NLog.Web.AspNetCore (not just NLog.Extensions.Logging) for ASP.NET-specific layout renderers.
• Health check endpoint returns 404: Verify MapHealthChecks is called after UseRouting() in the middleware pipeline.
• IExceptionHandler not invoked: Requires .NET 8+. For earlier versions, use UseExceptionHandler with a lambda or a custom exception middleware.

Success Criteria

• All System.Diagnostics.Trace / TraceSource calls replaced with ILogger<T>
• Third-party logging frameworks (log4net, NLog, Serilog) integrated via ILoggerProvider adapters or replaced
• ELMAH references removed and replaced with structured logging and exception middleware
• Static logger access patterns converted to constructor injection
• customErrors removed from web.config and replaced with UseExceptionHandler / UseStatusCodePagesWithReExecute
• Application_Error logic migrated to IExceptionHandler or exception middleware
• Health check endpoints configured (when applicable to deployment target)
• No references remain to Elmah, Trace.TraceInformation, Trace.TraceError, or HttpContext.Current for logging
• Project builds without errors