string filePath = @"C:\Error.txt";

​

Exception ex = ...

​

using( StreamWriter writer = new StreamWriter( filePath, true ) )

{

    writer.WriteLine( "-----------------------------------------------------------------------------" );

    writer.WriteLine( "Date : " + DateTime.Now.ToString() );

    writer.WriteLine();

​

    while( ex != null )

    {

        writer.WriteLine( ex.GetType().FullName );

        writer.WriteLine( "Message : " + ex.Message );

        writer.WriteLine( "StackTrace : " + ex.StackTrace );

​

        ex = ex.InnerException;

    }

}