StreamWriter出现下面错误“正由另一进程使用，因此该进程无法访问该文件”

出现下面错误“正由另一进程使用，因此该进程无法访问该文件”
代码如下

if (!Directory.Exists(Server.MapPath("~//Logs")))
{
Directory.CreateDirectory(Server.MapPath("~//Logs"));
}

StreamWriter sw = new StreamWriter(Server.MapPath(@"~/Logs/" + System.DateTime.Now.Year + "-" + System.DateTime.Now.Month + "-" + System.DateTime.Now.Day + ".txt"), true);

sw.WriteLine(System.DateTime.Now.ToString().Trim() + ":" + str);
sw.Flush();
sw.Close();
sw.Dispose();

解决方案：

FileStream _file = new FileStream(@"c:/a.txt", FileMode.Create, FileAccess.ReadWrite);
using (StreamWriter writer1 = new StreamWriter(_file))
{
writer.WriteLine(看你要写什么);
writer1.Flush();
writer1.Close();

_file.Close();
}