How Do I Write In A Common File From Separate Projects?

This is not an appropriate or reliable way for writing logs. Instead, you can use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog%28v=vs.110%29.aspx[^].

Another option is using Apache log4net: http://logging.apache.org/log4net[^].

—SA

Zon-cpp asked:

by EventLog, can i specify a special path beside the exe file?

Who told you that the "path" is "exe file". This is not how it works.

You can customize EventLog to change the sink of data, to make it a file or anything else, but I would recommend to use default behavior. To understand how, please see my past answer: MsBuild OutPut to the TextBox on the fly in Windows Application[^].

Even if you use default sink, system event log, to be viewed by the standard Event Log viewer, you can register (and later unregister) your custom event source, to have your events stored in a separate application-specific node of the event tree. Please see my past answer: How to create event log under a folder[^].

—SA

As I said in the comment, you can create a separate application to do the logging with the below code and then use the assembly of that application on all other projects to do the logging for you.

public void WriteLog(string logMessage, string logErrorPath)
        {
            StreamWriter log;
            string filePath = logErrorPath + "\\AppName" + DateTime.Now.ToString("dd_MM_yy") + ".log";

            if (!File.Exists(filePath))
            {
                log = new StreamWriter(filePath);
            }
            else
            {
                log = File.AppendText(filePath);
            }

            // Write to the file:
            log.WriteLine(DateTime.Now.ToString() + ": " + logMessage);          
            // Close the stream:
            log.Close();
        }