How can i download a file on a button’s onClick event in C#?

If you are using MVC then you can try the following code it is working for me :

#region Download File ==>
        public ActionResult downloadfile(string Filename, string MIMEType)
        {
            try
            {
                string file_name = "/Files/EvidenceUploads/" + Filename;
                string contentType = "";
                //Get the physical path to the file.
                string FilePath = Server.MapPath(file_name);

                string fileExt = Path.GetExtension(file_name);

                contentType = MIMEType;

                //Set the appropriate ContentType.
                Response.ContentType = contentType;
                Response.AppendHeader("content-disposition", "attachment; filename=" + (new FileInfo(file_name)).Name);

                //Write the file directly to the HTTP content output stream.
                Response.WriteFile(FilePath);
                Response.End();
                return View(FilePath);
            }
            catch
            {
                Response.End();
                return View();
                //To Do
            }

        }
        #endregion

If you are not using MVC please refer following code:

#region Download File ==>
        public void downloadfile(string Filename, string MIMEType)
        {
            try
            {
                string file_name = "/Files/EvidenceUploads/" + Filename;
                string contentType = "";
                //Get the physical path to the file.
                string FilePath = Server.MapPath(file_name);

                string fileExt = Path.GetExtension(file_name);

                contentType = MIMEType;

                //Set the appropriate ContentType.
                Response.ContentType = contentType;
                Response.AppendHeader("content-disposition", "attachment; filename=" + (new FileInfo(file_name)).Name);

                //Write the file directly to the HTTP content output stream.
                Response.WriteFile(FilePath);
                Response.End();

            }
            catch
            {
                Response.End();

                //To Do
            }

        }
        #endregion

Hope its help to you.