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

 Elmah.ErrorLog.GetDefault(null).Log(new Error(ex))

using System;
using System.Text;
using Elmah;

namespace Common.WebToolkit
{
    public static class ElmahLogEx
    {
        public static void LogException(this string ex)
        {
            if (string.IsNullOrWhiteSpace(ex))
                return;
            LogException(new Exception(ex));
        }

        public static void LogException(this Exception ex)
        {
            if (ex == null) return;
            try
            {
                ErrorSignal.FromCurrentContext().Raise(ex);
            }
            catch
            {
                ErrorLog.GetDefault(null).Log(new Error(ex));
            }
        }
    }
}

using System;
using System.Web.Mvc;

namespace MvcApplication13.Controllers
{
    public class HomeController : Controller
    {
        [HandleError]
        public ActionResult Index()
        {
            throw new InvalidOperationException();
            return View();
        }
    }
}

<system.web>
   <customErrors mode="On" />

GlobalFilters.Filters.Add(new HandleErrorAttribute());

[HandleError(View = "CustomErrorView")]

[HandleError(ExceptionType = typeof(NullReferenceException), View = "ErrorHandling")]

@model System.Web.Mvc.HandleErrorInfo
            
@{
    ViewBag.Title = "DbError";
}
 
<h2>An Error Has Occurred</h2>
 
@if (Model != null)
{
    <p>@Model.Exception.GetType().Name<br />
    thrown in @Model.ControllerName @Model.ActionName</p>
}

<customErrors mode="On" defaultRedirect="error">
      <error statusCode="404" redirect="error/notfound" />
      <error statusCode="403" redirect="error/forbidden" />
</customErrors>

using System.Web.Mvc;

namespace MvcApplication13.Controllers
{
    public class ErrorController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }

        public ActionResult NotFound()
        {
            return View();
        }

        public ActionResult Forbidden()
        {
            return View();
        }
    }
}

<configuration>
  <configSections>
    <sectionGroup name="elmah">
      <section name="security" requirePermission="false" type="Elmah.SecuritySectionHandler, Elmah"/>
      <section name="errorLog" requirePermission="false" type="Elmah.ErrorLogSectionHandler, Elmah"/>
      <section name="errorMail" requirePermission="false" type="Elmah.ErrorMailSectionHandler, Elmah"/>
      <section name="errorFilter" requirePermission="false" type="Elmah.ErrorFilterSectionHandler, Elmah"/>
      <section name="errorTweet" requirePermission="false" type="Elmah.ErrorTweetSectionHandler, Elmah"/>
    </sectionGroup>
  </configSections>

<elmah>
    <security allowRemoteAccess="1"/>
    <errorLog type="Elmah.XmlFileErrorLog, Elmah" logPath="~/App_Data/ErrorsLog"/>
  </elmah>

<httpModules>
      <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah"/>      
</httpModules>
<httpHandlers>
      <add verb="POST,GET,HEAD" path="elmah.axd" type="Elmah.ErrorLogPageFactory, Elmah"/>      
</httpHandlers>

<system.webServer>
    <validation validateIntegratedModeConfiguration="false"/>
    <modules runAllManagedModulesForAllRequests="true">
      <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah"/>
    </modules>
    <handlers>
      <add name="Elmah" verb="POST,GET,HEAD" path="elmah.axd" type="Elmah.ErrorLogPageFactory, Elmah"/>
    </handlers>
</system.webServer>

public static void RegisterRoutes(RouteCollection routes)
{
    //routes.IgnoreRoute("elmah.axd");
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

filters.Add(new HandleErrorAttribute());

using System.Web.Mvc;
using Elmah;

namespace MvcApplication13.CustomFilters
{
    public class ElmahHandledErrorLoggerFilter : IExceptionFilter
    {
        public void OnException(ExceptionContext context)
        {
            if (context.ExceptionHandled)
                ErrorSignal.FromCurrentContext().Raise(context.Exception);
            // all other exceptions will be caught by ELMAH anyway
        }
    }
}

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

//add a reference to "Elmah.dll"
using System;
using System.ServiceModel.Channels;
using System.ServiceModel.Dispatcher;
using System.Web;

namespace ElmahWcf
{
   public class HttpErrorHandler : IErrorHandler
   {
       #region IErrorHandler Members
       public bool HandleError(Exception error)
       {
           return false;
       }

       public void ProvideFault(Exception error, MessageVersion version, ref Message fault)
       {
           if (error == null)
               return;

           if (HttpContext.Current == null) //In case we run outside of IIS
               return;

           Elmah.ErrorSignal.FromCurrentContext().Raise(error);
       }
       #endregion
   }
}

//add a ref to "System.ServiceModel" assembly
using System;
using System.Collections.ObjectModel;
using System.ServiceModel;
using System.ServiceModel.Channels;
using System.ServiceModel.Description;
using System.ServiceModel.Dispatcher;

namespace ElmahWcf
{
   public class ServiceErrorBehaviorAttribute : Attribute, IServiceBehavior
   {
       Type errorHandlerType;
       public ServiceErrorBehaviorAttribute(Type errorHandlerType)
       {
           this.errorHandlerType = errorHandlerType;
       }

       #region IServiceBehavior Members

       public void AddBindingParameters(
           ServiceDescription serviceDescription,
           ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints,
           BindingParameterCollection bindingParameters)
       { }

       public void ApplyDispatchBehavior(
           ServiceDescription serviceDescription,
           ServiceHostBase serviceHostBase)
       {
           IErrorHandler errorHandler;
           errorHandler = (IErrorHandler)Activator.CreateInstance(errorHandlerType);
           foreach (ChannelDispatcherBase cdb in serviceHostBase.ChannelDispatchers)
           {
               ChannelDispatcher cd = cdb as ChannelDispatcher;
               cd.ErrorHandlers.Add(errorHandler);
           }
       }

       public void Validate(
           ServiceDescription serviceDescription,
           ServiceHostBase serviceHostBase)
       { }
       #endregion
   }
}

    [ServiceErrorBehavior(typeof(HttpErrorHandler))] //Integrating with ELMAH
   [EnableClientAccess()]
   public partial class MyDomainService : LinqToEntitiesDomainService<myEntities>

<?xml version="1.0" encoding="utf-8"?>
<configuration>
 <configSections>
   <sectionGroup name="elmah">
     <section name="security" requirePermission="false" type="Elmah.SecuritySectionHandler, Elmah"/>
     <section name="errorLog" requirePermission="false" type="Elmah.ErrorLogSectionHandler, Elmah" />
     <section name="errorMail" requirePermission="false" type="Elmah.ErrorMailSectionHandler, Elmah" />
     <section name="errorFilter" requirePermission="false" type="Elmah.ErrorFilterSectionHandler, Elmah"/>
     <section name="errorTweet" requirePermission="false" type="Elmah.ErrorTweetSectionHandler, Elmah"/>
   </sectionGroup>
 </configSections>

 <elmah>
   <security allowRemoteAccess="1" />
   <errorLog type="Elmah.XmlFileErrorLog, Elmah" logPath="~/Errors" />
   <!-- <errorMail
       from="errors@site.net"
       to="nasiri@site.net"
       subject="prj-error"
       async="true"
       smtpPort="25"
       smtpServer="mail.site.net"
       noYsod="true" /> -->
 </elmah>

 <system.webServer>
   <modules runAllManagedModulesForAllRequests="true">
     <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah"/>
     <add name="DomainServiceModule"
          preCondition="managedHandler"
          type="System.ServiceModel.DomainServices.Hosting.DomainServiceHttpModule, System.ServiceModel.DomainServices.Hosting, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
   </modules>
   <validation validateIntegratedModeConfiguration="false" />
   <handlers>
     <add name="Elmah" verb="POST,GET,HEAD" path="myelmah.axd" type="Elmah.ErrorLogPageFactory, Elmah" />
   </handlers>
 </system.webServer>
 <system.web>
   <globalization
       requestEncoding="utf-8"
       responseEncoding="utf-8"
  />
   <authentication mode="Forms">
     <!--one month ticket-->
     <forms name=".403AuthV"
            cookieless="UseCookies"
            slidingExpiration="true"
            protection="All"
            path="/"
            timeout="43200" />
   </authentication>
   <httpHandlers>
     <add verb="POST,GET,HEAD" path="myelmah.axd" type="Elmah.ErrorLogPageFactory, Elmah" />
   </httpHandlers>
   <httpModules>
     <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah"/>
     <add name="DomainServiceModule"
          type="System.ServiceModel.DomainServices.Hosting.DomainServiceHttpModule, System.ServiceModel.DomainServices.Hosting, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
   </httpModules>
   <compilation debug="true" targetFramework="4.0">
     <assemblies>
       <add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
     </assemblies>
   </compilation>
 </system.web>
 <connectionStrings>
 </connectionStrings>
 <system.serviceModel>
   <serviceHostingEnvironment
     aspNetCompatibilityEnabled="true"
     multipleSiteBindingsEnabled="true" />
 </system.serviceModel>
</configuration>

throw new Exception("This is an ELMAH test");

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]

                catch (Exception ex)
                {
                    StringBuilder errorMsg = new StringBuilder();
                    for (Exception current = ex; current != null; current = current.InnerException)
                    {
                        if (errorMsg.Length > 0)
                            errorMsg.Append("\n");

                        errorMsg.Append(current.Message.Replace("See the inner exception for details.", string.Empty));
                    }
                    // log
                    errorMsg.ToString();
                }