Synchronous Multiple User Reuest in ASP.NET

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synchronize

Hi,

I am creating a web application, in which a form update the XML file. I am getting an error if a user wants to write XML which is all ready writing by another user. How to synchronous this request one by one.
I used Semaphore and Lock, but either of them not working.
I used Lock
public static object obj_Lock = new object();
private void Button_Click(object sender, EventArgs e)
{
lock(obj_Lock)
{
Update_XML()
}
}
It is working well for two users, when multiple users access this method it raise a error, file used by another process.

Semaphore
public static Semaphore sem_Dest_XML = new Semaphore(1, 1);

private void Button_Click(object sender, EventArgs e)
{
try
{
sem_Dest_XML.WaitOne();
XML_Record obj_Record = new XML_Record();
Update_XML();
}

finally
{
sem_Dest_XML.Release();
}
}

In Samaphore I am not getting the sequence, suppose user_1 click first, user_2 click Second and User_3 Click Three. But it is executed in User_1 as First, User_3 as Second and User_2 as Last.
How to solve This.