SyncLock and very many threads VB .Net

Question

My case is I have the following method that uses SyncLock to ensure the writing of file by one thread at a time.

Private Shared lockThis As New Object 

Public Sub Process()
  SyncLock lockThis
    File.AppendAllText("c:\jamo\foo.txt","foo")
  End SyncLock 
End Sub 

I'm using many threads running at time:

Public Sub CreateThreads()
  Dim trd as Thread
  Dim X as Integer = 10
  For i as integer = 1 to X
    trd = New Thread(AddressOf Process)
    trd.Start()
  Next Sub
End Sub

My problem is when X is big (like 500), one o more threads write to file at same time. Why is happening this?

Answer 1

I don't have proof, but it could be telling the truth. If any other process opens the file without sharing it, with 500 or more threads attempting to open it, it is likely the file will be locked for one of them...