关于字符串为空的判断条件

最新推荐文章于 2023-07-25 09:42:52 发布

转载最新推荐文章于 2023-07-25 09:42:52 发布 · 439 阅读

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原文链接：http://www.cnblogs.com/trey/p/4682689.html

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#前端 #ViewUI

第一篇随笔，正好今天解决了一个问题，立马记录下，希望是好习惯的开始。

问题描述：读取一个记录SQL查询语句的XML文档，生成SQL查询字符串。

XML文档如下所示：

<?xml version="1.0" encoding="utf-8"?>
<SubQuery>
  <DBName>SQLServer</DBName>
  <Select>*</Select>
  <From>t_product</From>
  <Where>ID=1</Where>
  <Order>
  </Order>
  <Group>
  </Group>
</SubQuery

希望得到的字符串结果为：select * from t_product where ID=1

先贴下我一开始编写的代码：

public string XmlToSQL(XmlDocument doc)
    {
        string str_select, str_from, str_where, str_order, str_group;
        str_select = null;
        str_from = null;
        str_where = null;
        str_order = null;
        str_group = null;
        XmlNode node_select, node_from, node_where, node_order, node_group;
        XmlNode root = doc.DocumentElement;
        node_select = root.SelectSingleNode("Select");
        str_select = node_select.InnerText;
        node_from = root.SelectSingleNode("From");
        str_from = node_from.InnerText;
        node_where = root.SelectSingleNode("Where");
        str_where = node_where.InnerText;
        node_order = root.SelectSingleNode("Order");
        str_order = node_order.InnerText;
        node_group = root.SelectSingleNode("Group");
        str_group = node_group.InnerText;
        string str = "select " + str_select + " from " + str_from;
        if (str_where.Length !=null )
        {
            str += " " + " where " + str_where;
        }
        if (str_order.Length !=null)
        {
            str += " " + " order by "+str_order;
        }
        if (str_group.Length !=null)
        {
            str += " " + " group by " +str_group;
        }
        return str;
    }

运行结果为：select * from t_product where ID=1 order by group by

不是预期结果。

当时思考str_order和str_group两个字符串的结果应该为null，不清楚哪里出错。

然后编写测试程序，测试发现，这两个字符串结果不为null。

字符串为空，但不为null。这个点，我之前一直都是理解错误了。

然后想到通过测试字符串的长度作为判断条件。

于是修改代码如下：

public string XmlToSQL(XmlDocument doc)
    {
        string str_select, str_from, str_where, str_order, str_group;
        str_select = null;
        str_from = null;
        str_where = null;
        str_order = null;
        str_group = null;
        XmlNode node_select, node_from, node_where, node_order, node_group;
        XmlNode root = doc.DocumentElement;
        node_select = root.SelectSingleNode("Select");
        str_select = node_select.InnerText;
        node_from = root.SelectSingleNode("From");
        str_from = node_from.InnerText;
        node_where = root.SelectSingleNode("Where");
        str_where = node_where.InnerText;
        node_order = root.SelectSingleNode("Order");
        str_order = node_order.InnerText;
        node_group = root.SelectSingleNode("Group");
        str_group = node_group.InnerText;
        string str = "select " + str_select + " from " + str_from;
        if (str_where.Length >0 )
        {
            str += " " + " where " + str_where;
        }
        if (str_order.Length > 0)
        {
            str += " " + " order by "+str_order;
        }
        if (str_group.Length > 0)
        {
            str += " " + " group by " +str_group;
        }
        return str;
    }