public class book : System.Web.UI.Page
{
    private string sql;
    public string csql
    {
        set
        {
          sql = value;
        }
    }
    private DataTable obj;
    public DataTable cobj
    {
        get
        {
            return obj;
        }
    }
    public void datatree(string ParentID, string lin)
    {
        string sqltxt = "select id,name,cname='" + lin + "'+name,fid FROM " + sql + " WHERE  fid=" + ParentID + " order by showPage asc";
        DataSet dt = new _DB().GetSqlDataSet(sqltxt);
        if (lin == "")
        {
            obj = dt.Tables[0].Clone();
        }
        if (dt.Tables[0].Rows.Count > 0)
        {

            lin += "__";
            for (int i = 0; i < dt.Tables[0].Rows.Count; i++)
            {
                obj.ImportRow(dt.Tables[0].Rows[i]);
                datatree(dt.Tables[0].Rows[i]["id"].ToString(), lin);                  
            }
        }
        else
        {
            return;
        }
    }

}

        if (dt.Tables[0].Rows.Count > 0)
        {
            if(obj==null)
                obj = new DataTable();
            lin += "__";
            for (int i = 0; i < dt.Tables[0].Rows.Count; i++)
            {
                obj.ImportRow(dt.Tables[0].Rows[i]);
                datatree(dt.Tables[0].Rows[i]["id"].ToString(), lin);                  
            }
        }

------解决方案--------------------
没有满足
       if (lin == "")
       {
           obj = dt.Tables[0].Clone();
       }
条件时候
obj为null
在后台一调用就出错，提示：当前上下文中不存在名称“obj”

------解决方案--------------------
if (lin == "")
       {
           obj = dt.Tables[0].Clone();
       }
else if(obj==null) obj = new DataTable();