数据库连接字符串保存到配置文件

public static void Save2Config(string cstr)
{
XmlDocument doc = new XmlDocument();
string xmldox = "rpg.exe.config";
doc.Load(xmldox);
XmlNodeList nodes = doc.GetElementsByTagName("add");
for (int i = 0; i ＜ nodes.Count; i++)
{
XmlAttribute att = nodes[i].Attributes["key"];
if (att.Value == "SqlConectionString")
{
att = nodes[i].Attributes["value"];
att.Value = cstr;
break;
}
}
doc.Save(xmldox);
}