求解释，代码很简单，但是我想不明白，望明白的认识指点下

<%@ WebHandler Language="C#" Class="deldata" %>
using System;
using System.Web;
using System.Data;
using System.Data.SqlClient;
using System.Text;
using System.Collections.Generic;
public class deldata : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        Model model = new Model();
        model.FName = context.Request.Form["FName"];
        model.FCategoryID = IsNull(context.Request.Form["Fcate"]) ? (int?)Convert.ToInt32(context.Request.Form["Fcate"]) : null;
        model.FAuthor = context.Request.Form["Fauthor"];
        model.IsDel = IsNull(context.Request.Form["Fcate"]) ? (bool?)Convert.ToBoolean(context.Request.Form["IdDel"]) : null; //context.Request.Form["IdDel"];
        model.FYearPublished = IsNull(context.Request.Form["Fyear"]) ? (int?)Convert.ToInt32(context.Request.Form["Fyear"]) : null;//(int?) Convert.ToInt32(context.Request.Form["Fyear"]);

        //这里不理解，求解释
        int rel = DelExecuteNonQuery(model, context);
        if (rel <= 0)
        {
            context.Response.Write("alert(\"删除失败\");");
        }
        else
        {
            context.Response.Redirect("SelecteData.ashx");
        }
    }
    public bool IsNull(object value)
    {
        if (value.ToString().Trim().Length == 0)
        {
            return false;
        }
        else
        {
            return true;
        }
    }
    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
    public int DelExecuteNonQuery(Model model, HttpContext context)
    {
        List<SqlParameter> sqlParameterList = new List<SqlParameter>();
        List<string> sqlList = new List<string>();
        string sqlStr = "delete from T_Book";
        if (model.FName.ToString().Trim().Length > 0)
        {
            sqlList.Add("FName like @FName");
            sqlParameterList.Add(new SqlParameter("@FName", "%" + model.FName + "%"));
        }
        if (model.FCategoryID != null)
        {
            sqlList.Add("FCategoryID like @FCategoryID");
            sqlParameterList.Add(new SqlParameter("@FCategoryID", "%" + model.FCategoryID + "%"));
        }
        if (model.FYearPublished != null)
        {
            sqlList.Add("FYearPublished like @FYearPublished");
            sqlParameterList.Add(new SqlParameter("@FYearPublished", "%" + model.FYearPublished + "%"));
        }
        if (model.IsDel != null)
        {
            sqlList.Add("IsDel like @IsDel");
            sqlParameterList.Add(new SqlParameter("@IsDel", "%" + model.IsDel + "%"));
        }
        if (model.FAuthor != string.Empty)
        {
            sqlList.Add("FAuthor like @FAuthor");
            sqlParameterList.Add(new SqlParameter("@FAuthor", "%" + model.FAuthor + "%"));
        }
        if (sqlList.Count > 0)
        {
            string sqlTxt = string.Join(" and ", sqlList.ToArray());
            sqlStr = sqlStr + " where " + sqlTxt;
        }
        else
        {
            context.Response.Write("alert(\" 你会把所有数据都删除掉的！！！\");");
            return 0;
        }
        SqlConnection con = new SqlConnection(@"Data Source=.;Initial Catalog=Books;User ID=sa;Password=123");
        con.Open();
        SqlCommand com = new SqlCommand();
        com.Connection = con;
         = sqlStr;
        com.Parameters.AddRange(sqlParameterList.ToArray());
        int rel = com.ExecuteNonQuery();
        return rel;
    }
}

这算个模糊查询删除记录的一个小程序吧，HTML代码就不写了，这个是一般处理程序，问题就是我明明已经把要删除的记录删除了，并且上述做标记处代码中的rel变量是大于0的，但是调试过程中不知道什么原因，（我在if判断出设了断点），走到if判断处，按F11，语句往下执行，但是再按F11 又回到if判断处了，并且rel变量为0，求解释。期待中。。。。。。