Delegate in C#.net

I have created a basic logging framework for my application that writes log messages to a local text file using an Asynchronous delegate.

My application has many threads running however, and as a result, the order of the log entries in the text file is not chronological.

So I have two competing requirements: on one hand, my WriteToLog() method has to return almost immediately so that it doesn't block in-flight code, while at the same time the log file has to be in chronological order.

Any ideas?  Some of the problems I've encountered are:

1a.  Trying to use a Queue (with a lock{ } around it) to maintain the order seems to result in deadlocks

1b.  Even if I use a Queue, at some point the Queue must be written to disk, so it seems like I'd have to frequently have WriteToLog spin off a "WriteToDisk" like method.

2a.  I've considered instantiating a new thread which polls the Queue from (1a) and writes to disk in a loop, say, every 5 seconds.  I'm leaning more towards this method.

3.  I've looked into log4net and some other stuff but it just seems like overkill, and I'd really like to have my own framework for this seemingly simple task.

Thanks!

Got a user control that has a Listbox on it and I want to be able to pass the selected value back to the main page. 

User Control code:

using System;
using System.Web.UI;

public delegate void SendData(string data);

public partial class _controls_Filter_Staff : UserControl{
    public event SendData mSendData;

    protected void Page_Load(object sender, EventArgs e){
    }

    protected void selStaffList_OnSelectedIndexChanged(object sender, EventArgs e){
        if (Convert.ToInt32(selStaffList.SelectedValue) >= 0 && mSendData != null){
            mSendData(selStaffList.SelectedValue);
        }
    }
}

Main page that has the user control:

    protected void Page_Load(object sender, EventArgs e){

        ucFilterStaff.mSendData += GetSelectedStaff; /* ERROR HERE Line 11 */

    }


    private void GetSelectedStaff(string value){
        TextBox txtUserList = (TextBox) fvMain.FindControl("txtUserList");
        if (txtUserList == null) return;

        txtUserList.Text += value + ";";
    }

This is the error I get back and I've tried everything I know, but cannot sort it.  Probably something stupid but I just cannot see it.

System.Web.HttpUnhandledException: Exce