在服务器上，C盘一般是不允许通过WEB创建文档的，除非你在权限设置中打开，改为其他盘即可。
public static void LOG(string msg = "")
{
try
{
string path = "c:\\log.txt";
// string path = @"c:\log.txt";
if (System.IO.File.Exists(path) == false)
using (System.IO.FileStream writer = System.IO.File.Create(path))
{
writer.Close();
}
using (System.IO.StreamWriter writer = System.IO.File.AppendText(path))
{
writer.WriteLine(System.DateTime.Now.ToString("yyyy-MM-dd HH：mm：ss：ffff") + "->" + string.Concat(msg, Environment.NewLine));
writer.Close();
}
string path = @"d:\log.txt";
System.IO.StreamWriter writer;
if (System.IO.File.Exists(path) == false)
writer = System.IO.File.CreateText(path);
else
writer = System.IO.File.AppendText(path);
writer.WriteLine(System.DateTime.Now.ToString("yyyy-MM-dd HH：mm：ss：ffff") + "->" + string.Concat(msg, Environment.NewLine));
writer.Close();
}
catch (System.Exception ex)
{
int i = 00;
}
}
