How to download txt file from the server system to client?

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C#

ASP.NET

<%@ WebHandler Language="C#" Class="dlde" %>

using System;
using System.Web;

public class dlde : IHttpHandler {

    public void ProcessRequest (HttpContext context) {
        var fileName = @"D:\Error.txt";
        var r = context.Response;
        r.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
        r.ContentType = "text/plain";
        r.WriteFile(context.Server.MapPath(fileName));
    }

    public bool IsReusable {
        get {
            return false;
        }
    }

}

I want to download the .txt file from the server system from the specified directory...i tried with this code,but i am getting error,invalid virtual path..How to get the file ,from the server system ..please help me...