System.IO.IOException:The process cannot access the file 'XX.xml' because it is being used by another process. 解决方案

有时候用StreamReader去读取xml文件时，会出现System.IO.IOException:The process cannot access the file 'XX.xml' because it is being used by another process. 错误，这是因为

string str = ""; 
StreamReader reader = new StreamReader(  "E:\test.xml", Encoding.Default );
str = reader.ReadToEnd();

reader在读取完毕后，没有没有关闭reader和释放资源，xml文件在被占用着，所以报错。

在后面增加代码

 reader.Close();
 reader.Dispose();