Logging errors in ASP.NET MVC

I would consider simplifying your web application by plugging in Elmah.

You add the Elmah assembly to your project and then configure your web.config. It will then log exceptions created at controller or page level. It can be configured to log to various different places (like SQL Server, Email etc). It also provides a web frontend, so that you can browse through the log of exceptions.

Its the first thing I add to any asp.net mvc app I create.

I still use log4net, but I tend to use it for logging debug/info, and leave all exceptions to Elmah.

You can also find more information in the question How do you log errors (Exceptions) in your ASP.NET apps?.

MVC3
Create Attribute that inherits from HandleErrorInfoAttribute and includes your choice of logging

public class ErrorLoggerAttribute : HandleErrorAttribute 
{
    public override void OnException(ExceptionContext filterContext)
    {
        LogError(filterContext);
        base.OnException(filterContext);
    }

    public void LogError(ExceptionContext filterContext)
    {
       // You could use any logging approach here

        StringBuilder builder = new StringBuilder();
        builder
            .AppendLine("----------")
            .AppendLine(DateTime.Now.ToString())
            .AppendFormat("Source:\t{0}", filterContext.Exception.Source)
            .AppendLine()
            .AppendFormat("Target:\t{0}", filterContext.Exception.TargetSite)
            .AppendLine()
            .AppendFormat("Type:\t{0}", filterContext.Exception.GetType().Name)
            .AppendLine()
            .AppendFormat("Message:\t{0}", filterContext.Exception.Message)
            .AppendLine()
            .AppendFormat("Stack:\t{0}", filterContext.Exception.StackTrace)
            .AppendLine();

        string filePath = filterContext.HttpContext.Server.MapPath("~/App_Data/Error.log");

        using(StreamWriter writer = File.AppendText(filePath))
        {
            writer.Write(builder.ToString());
            writer.Flush();
        }
    }

Place attribute in Global.asax RegisterGlobalFilters

    public static void RegisterGlobalFilters(GlobalFilterCollection filters)
    {
       // filters.Add(new HandleErrorAttribute());
        filters.Add(new ErrorLoggerAttribute());
    }

You can hook into the OnError event in the Global.asax.

Something like this:

/// <summary>
/// Handles the Error event of the Application control.
/// </summary>
/// <param name="sender">The source of the event.</param>
/// <param name="e">The <see cref="System.EventArgs"/> instance containing the event data.</param>
protected void Application_Error(object sender, EventArgs e)
{
    if (Server != null)
    {
        Exception ex = Server.GetLastError();

        if (Response.StatusCode != 404 )
        {
            Logging.Error("Caught in Global.asax", ex);
        }

    }


}