I created a custom timer job and implemented it through a feature event in sharepoint.
when i tried to activate it, its showing this particular error:
Object reference not set to an instance of an object.   at TimerJobExample.MyFeatureReceiver.FeatureActivated(SPFeatureReceiverProperties properties) in C:\Documents and Settings\admin-shuklag.INFRADEV\My Documents\Visual Studio 2008\Projects\TimerJobExample\TimerJobExample\MyFeatureReceiver.cs:line 22 
   at Microsoft.SharePoint.SPFeature.DoActivationCallout(Boolean fActivate, Boolean fForce) 
   at Microsoft.SharePoint.SPFeature.Activate(SPSite siteParent, SPWeb webParent, SPFeaturePropertyCollection props, Boolean fForce) 
   at Microsoft.SharePoint.SPFeatureCollection.AddInternal(Guid featureId, SPFeaturePropertyCollection properties, Boolean force, Boolean fMarkOnly) 
   at Microsoft.SharePoint.SPFeatureCollection.Add(Guid featureId) 
   at Microsoft.SharePoint.WebControls.FeatureActivator.BtnActivateFeature_Click(Object objSender, EventArgs evtargs) 
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e) 
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) 
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) 
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) 
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) 
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) 
The error line is highlighted below
public override void FeatureActivated(SPFeatureReceiverProperties properties)
    {
        SPSite site = properties.Feature.Parent as SPSite;
        // make sure the job isn't already registered
//error line
      **foreach (SPJobDefinition job in site.WebApplication.JobDefinitions)**
        {
            if (job.Name == List_JOB_NAME)
                job.Delete();
        }
        SampleTimer listLoggerJob = new SampleTimer(List_JOB_NAME, site.WebApplication);
        SPMinuteSchedule schedule = new SPMinuteSchedule();
        schedule.BeginSecond = 0;
        schedule.EndSecond = 59;
        schedule.Interval = 5;
        listLoggerJob.Schedule = schedule;
        listLoggerJob.Update();
    }
what could be the problem?
 
     
    