C# streamwriter invoke

i want to reach and write a txt file in cross-threads but software gives an exception because multiple threads wants to reach the same file at the same time. How can i invoke streamwriter to avoid exceptions? Here is my code:

void WriteLog(string LogStr)
 {
     StreamWriter sw = new StreamWriter("Log.txt", true);
     sw.WriteLine(LogStr);
     sw.Close();
 }
 

I call WriteLog method in threads.

Thank you.

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