How to wrirte an exception in a text file?

I am not sure how you write directly ex (you can try). I think you will have to format or use ex.Message or any other property that you want to save.

To write into a file a simple code as below should do.

TextWriter writer = new StreamWriter(Server.MapPath("~/"LogFileName.txt"));            
writer.WriteLine(ex.Message);

Since this code is going to run on the server, make sure you have set appropriate access to the folder where you are creating this file.

However, if it logging purposes, I would strongly recommend to use a toolset like Log4Net or any other you are comfortable with. This is because with ASP.Net and multiuser scenario, thing can get ugly very easily.

Thanks
Milind

For ASP.NET, you can use ELMAH to log it, and/or email it. There is even a nice UI that will let you look at it via the browser. You can see all of the entire YSOD (yellow screen of death) complete with the stack trace, full request details, and a dump of the server variables. If you have trace on, it will even capture the trace. The great thing about ELMAH is that it's 'configuration only' to add it to your project -- no code will need to be written -- unless you wish to log exceptions that you're catching instead of just the ones you're not. To catch, log, and handle exceptions, you have to make a single call to ELMAH's API with the exception in question as a parameter. You still get the full YSOD for caught exceptions when using this method, even if none is displayed to the user. I swear by it.

You can use log4Net .

FileStream fs = new FileStream(@"c:\ErrorLog.txt", FileMode.CreateNew, FileAccess.ReadWrite);
StreamWriter sw = new StreamWriter(fs);
sw.WriteLine(file + ex);
sw.Close();
fs.Close();

try
{
 //do something 
}
catch(Exception ex) 
{
 MessageBox.Show(ex.ToString());//to see what is the matter not necessary
 System.IO.FileStream writefile;
 byte[] bytedata = null;
 bytedata = Encoding.ASCII.GetBytes(ex.ToString());
 writefile = new FileStream(Application.StartupPath+"Eror.txt",FileMode.Append);
 writefile.Write(bytedata, 0, bytedata.Length);
 writefile.Close();
 }