When i try to do something like that:

Button On Click->Call ShowInfo();

class testing
{
private Object locker= new Object();

public void ShowInfo()
{
Console.WriteLine("Begin");

using(locker)
{
for (int j = 0; j < 10; j++)
{
Console.WriteLine("j=" + j);
}
}

Console.WriteLine("End");
}
}

then when i click the button very quick, the result will be like that:

Begin
j=0;
j=1;
Begin
j=0;
Begin

but the what the result i wanted is
Begin
j=0;
j=1;
j=2;
j=3;
j=4;
j=5;
j=6;
j=7;
j=8;
j=9;
End
Begin
j=0
.
.

What is wrong with my code? and how to do the same thing?
How to ensure some codes are completed before another thread is trying
to enter the code?Thanks